From f152ea700419b8244677e10adf02aec2a9dc7f9e Mon Sep 17 00:00:00 2001
From: "N. Taylor Mullen" <nimullen@microsoft.com>
Date: Thu, 23 Apr 2015 16:02:07 -0700
Subject: [PATCH] React to aspnet/Razor#279 changes.

- Updated all tests and TagHelpers to utilize the new TagHelperOutput.Attributes and TagHelperContext.AllAttribute types.

aspnet/Razor#279
---
 src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs   |   2 +-
 .../AnchorTagHelper.cs                        |   6 +-
 .../FormTagHelper.cs                          |   6 +-
 .../InputTagHelper.cs                         |  15 +-
 .../Internal/AttributeMatcher.cs              |   6 +-
 .../LinkTagHelper.cs                          |  10 +-
 .../OptionTagHelper.cs                        |   2 +-
 .../ScriptTagHelper.cs                        |  22 +-
 .../TagHelperOutputExtensions.cs              |  35 +-
 ...ite.MvcTagHelper_Home.Product.Encoded.html |   2 +-
 ...persWebSite.MvcTagHelper_Home.Product.html |   2 +-
 .../RazorPageTest.cs                          |  66 ++--
 .../AnchorTagHelperTest.cs                    |  23 +-
 .../CacheTagHelperTest.cs                     |  40 +-
 .../EnvironmentTagHelperTest.cs               |  12 +-
 .../FormTagHelperTest.cs                      |  48 +--
 .../InputTagHelperTest.cs                     | 141 +++++--
 .../Internal/AttributeMatcherTest.cs          |  11 +-
 .../LabelTagHelperTest.cs                     |   8 +-
 .../LinkTagHelperTest.cs                      | 222 ++++++++---
 .../OptionTagHelperTest.cs                    |  78 ++--
 .../ScriptTagHelperTest.cs                    | 130 +++++--
 .../SelectTagHelperTest.cs                    |  30 +-
 .../TagHelperOutputExtensionsTest.cs          | 353 ++++++++++++++++--
 .../TextAreaTagHelperTest.cs                  |   8 +-
 .../ValidationMessageTagHelperTest.cs         |  38 +-
 .../ValidationSummaryTagHelperTest.cs         |  24 +-
 .../TagHelpers/ATagHelper.cs                  |   4 +-
 28 files changed, 956 insertions(+), 388 deletions(-)

diff --git a/src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs b/src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs
index 5d85e48c37..463b2a32bb 100644
--- a/src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs
+++ b/src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs
@@ -288,7 +288,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                 foreach (var attribute in tagHelperOutput.Attributes)
                 {
                     writer.Write(' ');
-                    writer.Write(attribute.Key);
+                    writer.Write(attribute.Name);
                     writer.Write("=\"");
                     WriteTo(writer, HtmlEncoder, attribute.Value, escapeQuotes: true);
                     writer.Write('"');
diff --git a/src/Microsoft.AspNet.Mvc.TagHelpers/AnchorTagHelper.cs b/src/Microsoft.AspNet.Mvc.TagHelpers/AnchorTagHelper.cs
index 1aa66bc63c..ebe6252eab 100644
--- a/src/Microsoft.AspNet.Mvc.TagHelpers/AnchorTagHelper.cs
+++ b/src/Microsoft.AspNet.Mvc.TagHelpers/AnchorTagHelper.cs
@@ -82,7 +82,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var routePrefixedAttributes = output.FindPrefixedAttributes(RouteAttributePrefix);
 
             // If "href" is already set, it means the user is attempting to use a normal anchor.
-            if (output.Attributes.ContainsKey(Href))
+            if (output.Attributes.ContainsName(Href))
             {
                 if (Action != null ||
                     Controller != null ||
@@ -154,7 +154,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         // TODO: https://github.com/aspnet/Razor/issues/89 - We will not need this method once #89 is completed.
         private static Dictionary<string, object> GetRouteValues(
             TagHelperOutput output,
-            IEnumerable<KeyValuePair<string, object>> routePrefixedAttributes)
+            IEnumerable<TagHelperAttribute> routePrefixedAttributes)
         {
             Dictionary<string, object> routeValues = null;
             if (routePrefixedAttributes.Any())
@@ -165,7 +165,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 // Remove prefix from keys and convert all values to strings. HtmlString and similar classes are not
                 // meaningful to routing.
                 routeValues = routePrefixedAttributes.ToDictionary(
-                    attribute => attribute.Key.Substring(RouteAttributePrefix.Length),
+                    attribute => attribute.Name.Substring(RouteAttributePrefix.Length),
                     attribute => (object)attribute.Value.ToString());
             }
 
diff --git a/src/Microsoft.AspNet.Mvc.TagHelpers/FormTagHelper.cs b/src/Microsoft.AspNet.Mvc.TagHelpers/FormTagHelper.cs
index 02db12682f..cce5aa97ff 100644
--- a/src/Microsoft.AspNet.Mvc.TagHelpers/FormTagHelper.cs
+++ b/src/Microsoft.AspNet.Mvc.TagHelpers/FormTagHelper.cs
@@ -72,7 +72,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var routePrefixedAttributes = output.FindPrefixedAttributes(RouteAttributePrefix);
 
             // If "action" is already set, it means the user is attempting to use a normal <form>.
-            if (output.Attributes.ContainsKey(HtmlActionAttributeName))
+            if (output.Attributes.ContainsName(HtmlActionAttributeName))
             {
                 if (Action != null || Controller != null || Route != null || routePrefixedAttributes.Any())
                 {
@@ -146,7 +146,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         // TODO: https://github.com/aspnet/Razor/issues/89 - We will not need this method once #89 is completed.
         private static Dictionary<string, object> GetRouteValues(
             TagHelperOutput output,
-            IEnumerable<KeyValuePair<string, object>> routePrefixedAttributes)
+            IEnumerable<TagHelperAttribute> routePrefixedAttributes)
         {
             Dictionary<string, object> routeValues = null;
             if (routePrefixedAttributes.Any())
@@ -157,7 +157,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 // Remove prefix from keys and convert all values to strings. HtmlString and similar classes are not
                 // meaningful to routing.
                 routeValues = routePrefixedAttributes.ToDictionary(
-                    attribute => attribute.Key.Substring(RouteAttributePrefix.Length),
+                    attribute => attribute.Name.Substring(RouteAttributePrefix.Length),
                     attribute => (object)attribute.Value.ToString());
             }
 
diff --git a/src/Microsoft.AspNet.Mvc.TagHelpers/InputTagHelper.cs b/src/Microsoft.AspNet.Mvc.TagHelpers/InputTagHelper.cs
index 09ebb5b702..12e5aee175 100644
--- a/src/Microsoft.AspNet.Mvc.TagHelpers/InputTagHelper.cs
+++ b/src/Microsoft.AspNet.Mvc.TagHelpers/InputTagHelper.cs
@@ -155,7 +155,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
 
             // inputType may be more specific than default the generator chooses below.
-            if (!output.Attributes.ContainsKey("type"))
+            if (!output.Attributes.ContainsName("type"))
             {
                 output.Attributes["type"] = inputType;
             }
@@ -218,9 +218,16 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
 
             // Prepare to move attributes from current element to <input type="checkbox"/> generated just below.
-            var htmlAttributes = output.Attributes.ToDictionary(
-                attribute => attribute.Key,
-                attribute => (object)attribute.Value);
+            var htmlAttributes = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
+
+            // Construct attributes correctly (first attribute wins).
+            foreach (var attribute in output.Attributes)
+            {
+                if (!htmlAttributes.ContainsKey(attribute.Name))
+                {
+                    htmlAttributes.Add(attribute.Name, attribute.Value);
+                }
+            }
 
             var tagBuilder = Generator.GenerateCheckBox(
                 ViewContext,
diff --git a/src/Microsoft.AspNet.Mvc.TagHelpers/Internal/AttributeMatcher.cs b/src/Microsoft.AspNet.Mvc.TagHelpers/Internal/AttributeMatcher.cs
index 4152f2fa21..08953a1b7d 100644
--- a/src/Microsoft.AspNet.Mvc.TagHelpers/Internal/AttributeMatcher.cs
+++ b/src/Microsoft.AspNet.Mvc.TagHelpers/Internal/AttributeMatcher.cs
@@ -86,10 +86,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
 
             foreach (var attribute in requiredAttributes)
             {
-                if (!context.AllAttributes.ContainsKey(attribute) ||
+                if (!context.AllAttributes.ContainsName(attribute) ||
                     context.AllAttributes[attribute] == null ||
-                    (typeof(string).IsAssignableFrom(context.AllAttributes[attribute].GetType()) &&
-                    string.IsNullOrWhiteSpace(context.AllAttributes[attribute] as string)))
+                    (typeof(string).IsAssignableFrom(context.AllAttributes[attribute].Value.GetType()) &&
+                    string.IsNullOrWhiteSpace(context.AllAttributes[attribute].Value as string)))
                 {
                     // Missing attribute!
                     missingAttributes.Add(attribute);
diff --git a/src/Microsoft.AspNet.Mvc.TagHelpers/LinkTagHelper.cs b/src/Microsoft.AspNet.Mvc.TagHelpers/LinkTagHelper.cs
index 0bc7c12cb2..caf69579a7 100644
--- a/src/Microsoft.AspNet.Mvc.TagHelpers/LinkTagHelper.cs
+++ b/src/Microsoft.AspNet.Mvc.TagHelpers/LinkTagHelper.cs
@@ -236,7 +236,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var mode = modeResult.FullMatches.Select(match => match.Mode).Max();
 
             // NOTE: Values in TagHelperOutput.Attributes may already be HTML-encoded.
-            var attributes = new Dictionary<string, object>(output.Attributes);
+            var attributes = new TagHelperAttributeList(output.Attributes);
 
             var builder = new DefaultTagHelperContent();
 
@@ -261,7 +261,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             output.Content.SetContent(builder);
         }
 
-        private void BuildGlobbedLinkTags(IDictionary<string, object> attributes, TagHelperContent builder)
+        private void BuildGlobbedLinkTags(TagHelperAttributeList attributes, TagHelperContent builder)
         {
             EnsureGlobbingUrlBuilder();
 
@@ -341,7 +341,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
         }
 
-        private void BuildLinkTag(IDictionary<string, object> attributes, TagHelperContent builder)
+        private void BuildLinkTag(TagHelperAttributeList attributes, TagHelperContent builder)
         {
             EnsureFileVersionProvider();
             builder.Append("<link ");
@@ -350,7 +350,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             {
                 var attributeValue = attribute.Value;
                 if (FileVersion == true &&
-                    string.Equals(attribute.Key, HrefAttributeName, StringComparison.OrdinalIgnoreCase))
+                    string.Equals(attribute.Name, HrefAttributeName, StringComparison.OrdinalIgnoreCase))
                 {
                     // "href" values come from bound attributes and globbing. So anything but a non-null string is
                     // unexpected but could happen if another helper targeting the same element does something odd.
@@ -363,7 +363,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 }
 
                 builder
-                    .Append(attribute.Key)
+                    .Append(attribute.Name)
                     .Append("=\"")
                     .Append(HtmlEncoder, ViewContext.Writer.Encoding, attributeValue)
                     .Append("\" ");
diff --git a/src/Microsoft.AspNet.Mvc.TagHelpers/OptionTagHelper.cs b/src/Microsoft.AspNet.Mvc.TagHelpers/OptionTagHelper.cs
index dd2c758cbd..b6c175bb96 100644
--- a/src/Microsoft.AspNet.Mvc.TagHelpers/OptionTagHelper.cs
+++ b/src/Microsoft.AspNet.Mvc.TagHelpers/OptionTagHelper.cs
@@ -51,7 +51,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
 
             // Nothing to do if this <option/> is already selected.
-            if (!output.Attributes.ContainsKey("selected"))
+            if (!output.Attributes.ContainsName("selected"))
             {
                 // Is this <option/> element a child of a <select/> element the SelectTagHelper targeted?
                 object formDataEntry;
diff --git a/src/Microsoft.AspNet.Mvc.TagHelpers/ScriptTagHelper.cs b/src/Microsoft.AspNet.Mvc.TagHelpers/ScriptTagHelper.cs
index 4ab93099a2..d6e767285d 100644
--- a/src/Microsoft.AspNet.Mvc.TagHelpers/ScriptTagHelper.cs
+++ b/src/Microsoft.AspNet.Mvc.TagHelpers/ScriptTagHelper.cs
@@ -205,7 +205,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var mode = modeResult.FullMatches.Select(match => match.Mode).Max();
 
             // NOTE: Values in TagHelperOutput.Attributes may already be HTML-encoded.
-            var attributes = new Dictionary<string, object>(output.Attributes);
+            var attributes = new TagHelperAttributeList(output.Attributes);
 
             var builder = new DefaultTagHelperContent();
             var originalContent = await context.GetChildContentAsync();
@@ -233,7 +233,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
         private void BuildGlobbedScriptTags(
             TagHelperContent originalContent,
-            IDictionary<string, object> attributes,
+            TagHelperAttributeList attributes,
             TagHelperContent builder)
         {
             EnsureGlobbingUrlBuilder();
@@ -257,7 +257,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
         }
 
-        private void BuildFallbackBlock(IDictionary<string, object> attributes, DefaultTagHelperContent builder)
+        private void BuildFallbackBlock(TagHelperAttributeList attributes, DefaultTagHelperContent builder)
         {
             EnsureGlobbingUrlBuilder();
             EnsureFileVersionProvider();
@@ -272,10 +272,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                        .Append("||document.write(\"");
 
                 // May have no "src" attribute in the dictionary e.g. if Src and SrcInclude were not bound.
-                if (!attributes.ContainsKey(SrcAttributeName))
+                if (!attributes.ContainsName(SrcAttributeName))
                 {
                     // Need this entry to place each fallback source.
-                    attributes.Add(SrcAttributeName, null);
+                    attributes.Add(new TagHelperAttribute(SrcAttributeName, value: null));
                 }
 
                 foreach (var src in fallbackSrcs)
@@ -287,9 +287,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
                     foreach (var attribute in attributes)
                     {
-                        if (!attribute.Key.Equals(SrcAttributeName, StringComparison.OrdinalIgnoreCase))
+                        if (!attribute.Name.Equals(SrcAttributeName, StringComparison.OrdinalIgnoreCase))
                         {
-                            var encodedKey = JavaScriptEncoder.JavaScriptStringEncode(attribute.Key);
+                            var encodedKey = JavaScriptEncoder.JavaScriptStringEncode(attribute.Name);
                             var attributeValue = attribute.Value.ToString();
                             var encodedValue = JavaScriptEncoder.JavaScriptStringEncode(attributeValue);
 
@@ -307,7 +307,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                             // attribute.Key ("src") does not need to be JavaScript-encoded.
                             var encodedValue = JavaScriptEncoder.JavaScriptStringEncode(attributeValue);
 
-                            AppendAttribute(builder, attribute.Key, encodedValue, escapeQuotes: true);
+                            AppendAttribute(builder, attribute.Name, encodedValue, escapeQuotes: true);
                         }
                     }
 
@@ -342,7 +342,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
         private void BuildScriptTag(
             TagHelperContent content,
-            IDictionary<string, object> attributes,
+            TagHelperAttributeList attributes,
             TagHelperContent builder)
         {
             EnsureFileVersionProvider();
@@ -352,7 +352,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             {
                 var attributeValue = attribute.Value;
                 if (FileVersion == true &&
-                    string.Equals(attribute.Key, SrcAttributeName, StringComparison.OrdinalIgnoreCase))
+                    string.Equals(attribute.Name, SrcAttributeName, StringComparison.OrdinalIgnoreCase))
                 {
                     // "src" values come from bound attributes and globbing. So anything but a non-null string is
                     // unexpected but could happen if another helper targeting the same element does something odd.
@@ -364,7 +364,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     }
                 }
 
-                AppendAttribute(builder, attribute.Key, attributeValue, escapeQuotes: false);
+                AppendAttribute(builder, attribute.Name, attributeValue, escapeQuotes: false);
             }
 
             builder.Append(">")
diff --git a/src/Microsoft.AspNet.Mvc.TagHelpers/TagHelperOutputExtensions.cs b/src/Microsoft.AspNet.Mvc.TagHelpers/TagHelperOutputExtensions.cs
index e6a25d408a..1bdcb233a4 100644
--- a/src/Microsoft.AspNet.Mvc.TagHelpers/TagHelperOutputExtensions.cs
+++ b/src/Microsoft.AspNet.Mvc.TagHelpers/TagHelperOutputExtensions.cs
@@ -31,21 +31,23 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             [NotNull] string attributeName,
             [NotNull] TagHelperContext context)
         {
-            if (!tagHelperOutput.Attributes.ContainsKey(attributeName))
+            if (!tagHelperOutput.Attributes.ContainsName(attributeName))
             {
+                IEnumerable<IReadOnlyTagHelperAttribute> entries;
+
                 // We look for the original attribute so we can restore the exact attribute name the user typed.
                 // Approach also ignores changes made to tagHelperOutput[attributeName].
-                var entry = context.AllAttributes.FirstOrDefault(
-                    attribute => attribute.Key.Equals(attributeName, StringComparison.OrdinalIgnoreCase));
-
-                if (entry.Equals(default(KeyValuePair<string, object>)))
+                if (!context.AllAttributes.TryGetAttributes(attributeName, out entries))
                 {
                     throw new ArgumentException(
                         Resources.FormatTagHelperOutput_AttributeDoesNotExist(attributeName, nameof(TagHelperContext)),
                         nameof(attributeName));
                 }
-                
-                tagHelperOutput.Attributes.Add(entry.Key, entry.Value);
+
+                foreach (var entry in entries)
+                {
+                    tagHelperOutput.Attributes.Add(entry.Name, entry.Value);
+                }
             }
         }
 
@@ -57,7 +59,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         /// <param name="prefix">A prefix to look for.</param>
         /// <returns><see cref="KeyValuePair{string, string}"/>s with <see cref="KeyValuePair{string, string}.Key"/>
         /// starting with the given <paramref name="prefix"/>.</returns>
-        public static IEnumerable<KeyValuePair<string, object>> FindPrefixedAttributes(
+        public static IEnumerable<TagHelperAttribute> FindPrefixedAttributes(
             [NotNull] this TagHelperOutput tagHelperOutput,
             [NotNull] string prefix)
         {
@@ -65,7 +67,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // We're only interested in HTML attributes that have the desired prefix.
             var prefixedAttributes = tagHelperOutput.Attributes
-                .Where(attribute => attribute.Key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
+                .Where(attribute => attribute.Name.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
                 .ToArray();
 
             return prefixedAttributes;
@@ -85,13 +87,22 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             foreach (var attribute in tagBuilder.Attributes)
             {
-                if (!tagHelperOutput.Attributes.ContainsKey(attribute.Key))
+                if (!tagHelperOutput.Attributes.ContainsName(attribute.Key))
                 {
                     tagHelperOutput.Attributes.Add(attribute.Key, attribute.Value);
                 }
                 else if (attribute.Key.Equals("class", StringComparison.OrdinalIgnoreCase))
                 {
-                    tagHelperOutput.Attributes["class"] += " " + attribute.Value;
+                    TagHelperAttribute classAttribute;
+
+                    if (tagHelperOutput.Attributes.TryGetAttribute("class", out classAttribute))
+                    {
+                        tagHelperOutput.Attributes["class"] = classAttribute.Value + " " + attribute.Value;
+                    }
+                    else
+                    {
+                        tagHelperOutput.Attributes.Add("class", attribute.Value);
+                    }
                 }
             }
         }
@@ -104,7 +115,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         /// <param name="attributes">Attributes to remove.</param>
         public static void RemoveRange(
             [NotNull] this TagHelperOutput tagHelperOutput,
-            [NotNull] IEnumerable<KeyValuePair<string, object>> attributes)
+            [NotNull] IEnumerable<TagHelperAttribute> attributes)
         {
             foreach (var attribute in attributes)
             {
diff --git a/test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/MvcTagHelpersWebSite.MvcTagHelper_Home.Product.Encoded.html b/test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/MvcTagHelpersWebSite.MvcTagHelper_Home.Product.Encoded.html
index 6356668cc9..5bd80b6e84 100644
--- a/test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/MvcTagHelpersWebSite.MvcTagHelper_Home.Product.Encoded.html
+++ b/test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/MvcTagHelpersWebSite.MvcTagHelper_Home.Product.Encoded.html
@@ -7,7 +7,7 @@
     <title></title>
 </head>
 <body>
-    <form action="HtmlEncode[[/MvcTagHelper_Home/ProductSubmit]]" method="get">
+    <form method="get" action="HtmlEncode[[/MvcTagHelper_Home/ProductSubmit]]">
         <div>
             <label class="product" for="HtmlEncode[[HomePage]]">HtmlEncode[[HomePage]]</label>
             <input size="50" type="HtmlEncode[[url]]" id="HtmlEncode[[HomePage]]" name="HtmlEncode[[HomePage]]" value="HtmlEncode[[http://www.contoso.com/]]" />
diff --git a/test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/MvcTagHelpersWebSite.MvcTagHelper_Home.Product.html b/test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/MvcTagHelpersWebSite.MvcTagHelper_Home.Product.html
index 82cbb7839d..805c8d10ed 100644
--- a/test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/MvcTagHelpersWebSite.MvcTagHelper_Home.Product.html
+++ b/test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/MvcTagHelpersWebSite.MvcTagHelper_Home.Product.html
@@ -7,7 +7,7 @@
     <title></title>
 </head>
 <body>
-    <form action="/MvcTagHelper_Home/ProductSubmit" method="get">
+    <form method="get" action="/MvcTagHelper_Home/ProductSubmit">
         <div>
             <label class="product" for="HomePage">HomePage</label>
             <input size="50" type="url" id="HomePage" name="HomePage" value="http://www.contoso.com/" />
diff --git a/test/Microsoft.AspNet.Mvc.Razor.Test/RazorPageTest.cs b/test/Microsoft.AspNet.Mvc.Razor.Test/RazorPageTest.cs
index 1adccac03e..acec8dfb58 100644
--- a/test/Microsoft.AspNet.Mvc.Razor.Test/RazorPageTest.cs
+++ b/test/Microsoft.AspNet.Mvc.Razor.Test/RazorPageTest.cs
@@ -747,7 +747,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                         // parameters: TagName, Attributes, SelfClosing, PreContent, Content, PostContent
                         GetTagHelperOutput(
                             tagName:     "div",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  null,
@@ -759,7 +759,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     null,
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  null,
@@ -771,7 +771,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "  ",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  null,
@@ -783,7 +783,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "p",
-                            attributes:  new Dictionary<string, object>() { { "test", "testVal" } },
+                            attributes:  new TagHelperAttributeList() { { "test", "testVal" } },
                             selfClosing: false,
                             preElement:  null,
                             preContent:  null,
@@ -795,7 +795,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "p",
-                            attributes:  new Dictionary<string, object>() { { "test", "testVal" }, { "something", "  spaced  " } },
+                            attributes:  new TagHelperAttributeList() { { "test", "testVal" }, { "something", "  spaced  " } },
                             selfClosing: false,
                             preElement:  null,
                             preContent:  null,
@@ -807,7 +807,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "p",
-                            attributes:  new Dictionary<string, object>() { { "test", "testVal" } },
+                            attributes:  new TagHelperAttributeList() { { "test", "testVal" } },
                             selfClosing: true,
                             preElement:  null,
                             preContent:  null,
@@ -819,7 +819,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "p",
-                            attributes:  new Dictionary<string, object>() { { "test", "testVal" }, { "something", "  spaced  " } },
+                            attributes:  new TagHelperAttributeList() { { "test", "testVal" }, { "something", "  spaced  " } },
                             selfClosing: true,
                             preElement:  null,
                             preContent:  null,
@@ -831,7 +831,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "p",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  "Hello World!",
@@ -843,7 +843,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "p",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  null,
@@ -855,7 +855,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "p",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  null,
@@ -867,7 +867,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "p",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  "Hello",
@@ -879,7 +879,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "p",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: true,
                             preElement:  null,
                             preContent:  "Hello",
@@ -891,7 +891,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  "Hello",
@@ -903,7 +903,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "random",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: true,
                             preElement:  null,
                             preContent:  "Hello",
@@ -915,7 +915,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  "Before",
                             preContent:  null,
@@ -927,7 +927,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     null,
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  "Before",
                             preContent:  null,
@@ -939,7 +939,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     null,
-                            attributes:  new Dictionary<string, object> { { "test", "testVal" } },
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
                             selfClosing: true,
                             preElement:  "Before",
                             preContent:  null,
@@ -951,7 +951,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object> { { "test", "testVal" } },
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
                             selfClosing: true,
                             preElement:  "Before",
                             preContent:  null,
@@ -963,7 +963,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: true,
                             preElement:  "Before",
                             preContent:  null,
@@ -975,7 +975,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  null,
@@ -987,7 +987,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     null,
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  null,
                             preContent:  null,
@@ -999,7 +999,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     null,
-                            attributes:  new Dictionary<string, object> { { "test", "testVal" } },
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
                             selfClosing: true,
                             preElement:  null,
                             preContent:  null,
@@ -1011,7 +1011,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object> { { "test", "testVal" } },
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
                             selfClosing: true,
                             preElement:  null,
                             preContent:  null,
@@ -1023,7 +1023,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: true,
                             preElement:  null,
                             preContent:  null,
@@ -1035,7 +1035,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  "Before",
                             preContent:  "Hello",
@@ -1047,7 +1047,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object> { { "test", "testVal" } },
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
                             selfClosing: false,
                             preElement:  "Before",
                             preContent:  "Hello",
@@ -1059,7 +1059,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     "custom",
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: true,
                             preElement:  "Before",
                             preContent:  "Hello",
@@ -1071,7 +1071,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     null,
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: true,
                             preElement:  "Before",
                             preContent:  "Hello",
@@ -1083,7 +1083,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     null,
-                            attributes:  new Dictionary<string, object>(),
+                            attributes:  new TagHelperAttributeList(),
                             selfClosing: false,
                             preElement:  "Before",
                             preContent:  "Hello",
@@ -1095,7 +1095,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     {
                         GetTagHelperOutput(
                             tagName:     null,
-                            attributes:  new Dictionary<string, object> { { "test", "testVal" } },
+                            attributes:  new TagHelperAttributeList { { "test", "testVal" } },
                             selfClosing: false,
                             preElement:  "Before",
                             preContent:  "Hello",
@@ -1161,7 +1161,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                 },
                 startTagHelperWritingScope: () => { },
                 endTagHelperWritingScope: () => defaultTagHelperContent);
-            tagHelperExecutionContext.Output = new TagHelperOutput("p", new Dictionary<string, object>());
+            tagHelperExecutionContext.Output = new TagHelperOutput("p", new TagHelperAttributeList());
             if (childContentRetrieved)
             {
                 await tagHelperExecutionContext.GetChildContentAsync();
@@ -1193,7 +1193,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                 executeChildContentAsync: () => { return Task.FromResult(result: true); },
                 startTagHelperWritingScope: () => { },
                 endTagHelperWritingScope: () => new DefaultTagHelperContent());
-            tagHelperExecutionContext.Output = new TagHelperOutput("p", new Dictionary<string, object>());
+            tagHelperExecutionContext.Output = new TagHelperOutput("p", new TagHelperAttributeList());
             tagHelperExecutionContext.Output.Content.SetContent("Hello World!");
 
             // Act
@@ -1238,7 +1238,7 @@ namespace Microsoft.AspNet.Mvc.Razor
 
         private static TagHelperOutput GetTagHelperOutput(
             string tagName,
-            IDictionary<string, object> attributes,
+            TagHelperAttributeList attributes,
             bool selfClosing,
             string preElement,
             string preContent,
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/AnchorTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/AnchorTagHelperTest.cs
index 37b20fd90f..27674e119b 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/AnchorTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/AnchorTagHelperTest.cs
@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Mvc.ModelBinding;
 using Microsoft.AspNet.Mvc.Rendering;
@@ -24,7 +25,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var metadataProvider = new TestModelMetadataProvider();
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>
+                allAttributes: new TagHelperAttributeList
                 {
                     { "id", "myanchor" },
                     { "asp-route-foo", "bar" },
@@ -44,7 +45,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 expectedTagName,
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     { "id", "myanchor" },
                     { "asp-route-foo", "bar" },
@@ -74,9 +75,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             Assert.Equal(2, output.Attributes.Count);
-            var attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("id"));
+            var attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("id"));
             Assert.Equal("myanchor", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("href"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("href"));
             Assert.Equal("home/index", attribute.Value);
             Assert.Equal("Something", output.Content.GetContent());
             Assert.Equal(expectedTagName, output.TagName);
@@ -87,7 +88,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -98,7 +100,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 "a",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             output.Content.SetContent(string.Empty);
 
             var generator = new Mock<IHtmlGenerator>(MockBehavior.Strict);
@@ -129,7 +131,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -140,7 +143,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 "a",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             output.Content.SetContent(string.Empty);
 
             var generator = new Mock<IHtmlGenerator>();
@@ -181,7 +184,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var anchorTagHelper = new AnchorTagHelper();
             var output = new TagHelperOutput(
                 "a",
-                attributes: new Dictionary<string, object>()
+                attributes: new TagHelperAttributeList
                 {
                     { "href", "http://www.contoso.com" }
                 });
@@ -219,7 +222,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             typeof(AnchorTagHelper).GetProperty(propertyName).SetValue(anchorTagHelper, "Home");
             var output = new TagHelperOutput(
                 "a",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             var expectedErrorMessage = "Cannot determine an 'href' attribute for <a>. An <a> with a specified " +
                 "'asp-route' must not have an 'asp-action' or 'asp-controller' attribute.";
 
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/CacheTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/CacheTagHelperTest.cs
index 09c29bf98d..6774a68b31 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/CacheTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/CacheTagHelperTest.cs
@@ -255,7 +255,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             cache.Setup(c => c.TryGetValue(It.IsAny<string>(), It.IsAny<IEntryLink>(), out cacheResult))
                 .Returns(false);
             var tagHelperContext = GetTagHelperContext(id, childContent);
-            var tagHelperOutput = new TagHelperOutput("cache", new Dictionary<string, object>());
+            var tagHelperOutput = new TagHelperOutput("cache", new TagHelperAttributeList());
             var cacheTagHelper = new CacheTagHelper
             {
                 ViewContext = GetViewContext(),
@@ -295,7 +295,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             cache.Setup(c => c.TryGetValue(It.IsAny<string>(), It.IsAny<IEntryLink>(), out cacheResult))
                 .Returns(false);
             var tagHelperContext = GetTagHelperContext(id, childContent);
-            var tagHelperOutput = new TagHelperOutput("cache", new Dictionary<string, object>());
+            var tagHelperOutput = new TagHelperOutput("cache", new TagHelperAttributeList());
             var cacheTagHelper = new CacheTagHelper
             {
                 ViewContext = GetViewContext(),
@@ -327,7 +327,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var childContent = "original-child-content";
             var cache = new MemoryCache(new MemoryCacheOptions());
             var tagHelperContext1 = GetTagHelperContext(id, childContent);
-            var tagHelperOutput1 = new TagHelperOutput("cache", new Dictionary<string, object>());
+            var tagHelperOutput1 = new TagHelperOutput("cache", new TagHelperAttributeList());
             var cacheTagHelper1 = new CacheTagHelper
             {
                 VaryByQuery = "key1,key2",
@@ -348,7 +348,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Arrange - 2
             var tagHelperContext2 = GetTagHelperContext(id, "different-content");
-            var tagHelperOutput2 = new TagHelperOutput("cache", new Dictionary<string, object>());
+            var tagHelperOutput2 = new TagHelperOutput("cache", new TagHelperAttributeList());
             var cacheTagHelper2 = new CacheTagHelper
             {
                 VaryByQuery = "key1,key2",
@@ -376,8 +376,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var childContent1 = "original-child-content";
             var cache = new MemoryCache(new MemoryCacheOptions());
             var tagHelperContext1 = GetTagHelperContext(id, childContent1);
-            var tagHelperOutput1 = new TagHelperOutput("cache",
-                                                       new Dictionary<string, object> { { "attr", "value" } });
+            var tagHelperOutput1 = new TagHelperOutput("cache", new TagHelperAttributeList { { "attr", "value" } });
             tagHelperOutput1.PreContent.Append("<cache>");
             tagHelperOutput1.PostContent.SetContent("</cache>");
             var cacheTagHelper1 = new CacheTagHelper
@@ -400,9 +399,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange - 2
             var childContent2 = "different-content";
             var tagHelperContext2 = GetTagHelperContext(id, childContent2);
-            var tagHelperOutput2 = new TagHelperOutput(
-                "cache",
-                new Dictionary<string, object> { { "attr", "value" } });
+            var tagHelperOutput2 = new TagHelperOutput("cache", new TagHelperAttributeList { { "attr", "value" } });
             tagHelperOutput2.PreContent.SetContent("<cache>");
             tagHelperOutput2.PostContent.SetContent("</cache>");
             var cacheTagHelper2 = new CacheTagHelper
@@ -611,8 +608,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                  .Returns(() => currentTime);
             var cache = new MemoryCache(new MemoryCacheOptions { Clock = clock.Object });
             var tagHelperContext1 = GetTagHelperContext(id, childContent1);
-            var tagHelperOutput1 = new TagHelperOutput("cache",
-                                                       new Dictionary<string, object> { { "attr", "value" } });
+            var tagHelperOutput1 = new TagHelperOutput("cache", new TagHelperAttributeList { { "attr", "value" } });
             tagHelperOutput1.PreContent.SetContent("<cache>");
             tagHelperOutput1.PostContent.SetContent("</cache>");
             var cacheTagHelper1 = new CacheTagHelper
@@ -634,8 +630,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange - 2
             var childContent2 = "different-content";
             var tagHelperContext2 = GetTagHelperContext(id, childContent2);
-            var tagHelperOutput2 = new TagHelperOutput("cache",
-                                                       new Dictionary<string, object> { { "attr", "value" } });
+            var tagHelperOutput2 = new TagHelperOutput("cache", new TagHelperAttributeList { { "attr", "value" } });
             tagHelperOutput2.PreContent.SetContent("<cache>");
             tagHelperOutput2.PostContent.SetContent("</cache>");
             var cacheTagHelper2 = new CacheTagHelper
@@ -668,8 +663,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                  .Returns(() => currentTime);
             var cache = new MemoryCache(new MemoryCacheOptions { Clock = clock.Object });
             var tagHelperContext1 = GetTagHelperContext(id, childContent1);
-            var tagHelperOutput1 = new TagHelperOutput("cache",
-                                                       new Dictionary<string, object> { { "attr", "value" } });
+            var tagHelperOutput1 = new TagHelperOutput("cache", new TagHelperAttributeList { { "attr", "value" } });
             tagHelperOutput1.PreContent.SetContent("<cache>");
             tagHelperOutput1.PostContent.SetContent("</cache>");
             var cacheTagHelper1 = new CacheTagHelper
@@ -692,8 +686,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             currentTime = currentTime.AddMinutes(5).AddSeconds(2);
             var childContent2 = "different-content";
             var tagHelperContext2 = GetTagHelperContext(id, childContent2);
-            var tagHelperOutput2 = new TagHelperOutput("cache",
-                                                       new Dictionary<string, object> { { "attr", "value" } });
+            var tagHelperOutput2 = new TagHelperOutput("cache", new TagHelperAttributeList { { "attr", "value" } });
             tagHelperOutput2.PreContent.SetContent("<cache>");
             tagHelperOutput2.PostContent.SetContent("</cache>");
             var cacheTagHelper2 = new CacheTagHelper
@@ -725,8 +718,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                  .Returns(() => currentTime);
             var cache = new MemoryCache(new MemoryCacheOptions { Clock = clock.Object });
             var tagHelperContext1 = GetTagHelperContext(id, childContent1);
-            var tagHelperOutput1 = new TagHelperOutput("cache",
-                                                       new Dictionary<string, object> { { "attr", "value" } });
+            var tagHelperOutput1 = new TagHelperOutput("cache", new TagHelperAttributeList { { "attr", "value" } });
             tagHelperOutput1.PreContent.SetContent("<cache>");
             tagHelperOutput1.PostContent.SetContent("</cache>");
             var cacheTagHelper1 = new CacheTagHelper
@@ -749,8 +741,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             currentTime = currentTime.AddSeconds(35);
             var childContent2 = "different-content";
             var tagHelperContext2 = GetTagHelperContext(id, childContent2);
-            var tagHelperOutput2 = new TagHelperOutput("cache",
-                                                       new Dictionary<string, object> { { "attr", "value" } });
+            var tagHelperOutput2 = new TagHelperOutput("cache", new TagHelperAttributeList { { "attr", "value" } });
             tagHelperOutput2.PreContent.SetContent("<cache>");
             tagHelperOutput2.PostContent.SetContent("</cache>");
             var cacheTagHelper2 = new CacheTagHelper
@@ -780,7 +771,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var tokenSource = new CancellationTokenSource();
             var cache = new MemoryCache(new MemoryCacheOptions());
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new TagHelperAttributeList(),
                 items: new Dictionary<object, object>(),
                 uniqueId: id,
                 getChildContentAsync: () =>
@@ -793,8 +784,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     });
                     return Task.FromResult<TagHelperContent>(expectedContent);
                 });
-            var tagHelperOutput = new TagHelperOutput("cache",
-                                                       new Dictionary<string, object> { { "attr", "value" } });
+            var tagHelperOutput = new TagHelperOutput("cache", new TagHelperAttributeList { { "attr", "value" } });
             tagHelperOutput.PreContent.SetContent("<cache>");
             tagHelperOutput.PostContent.SetContent("</cache>");
             var cacheTagHelper = new CacheTagHelper
@@ -837,7 +827,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                                                             string childContent = "some child content")
         {
             return new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new TagHelperAttributeList(),
                 items: new Dictionary<object, object>(),
                 uniqueId: id,
                 getChildContentAsync: () =>
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/EnvironmentTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/EnvironmentTagHelperTest.cs
index 148c22577c..ee2b69366b 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/EnvironmentTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/EnvironmentTagHelperTest.cs
@@ -80,7 +80,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Test
             // Arrange
             var content = "content";
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object> { { "names", namesAttribute } },
+                attributes: new TagHelperAttributeList { { "names", namesAttribute } },
                 content: content);
             var output = MakeTagHelperOutput("environment");
             var hostingEnvironment = new Mock<IHostingEnvironment>();
@@ -108,7 +108,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Test
             // Arrange
             var content = "content";
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object> { { "names", namesAttribute } },
+                attributes: new TagHelperAttributeList { { "names", namesAttribute } },
                 content: content);
             var output = MakeTagHelperOutput("environment");
             var hostingEnvironment = new Mock<IHostingEnvironment>();
@@ -129,10 +129,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Test
         }
 
         private TagHelperContext MakeTagHelperContext(
-            IDictionary<string, object> attributes = null,
+            TagHelperAttributeList attributes = null,
             string content = null)
         {
-            attributes = attributes ?? new Dictionary<string, object>();
+            attributes = attributes ?? new TagHelperAttributeList();
 
             return new TagHelperContext(
                 attributes,
@@ -146,9 +146,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Test
                 });
         }
 
-        private TagHelperOutput MakeTagHelperOutput(string tagName, IDictionary<string, object> attributes = null)
+        private TagHelperOutput MakeTagHelperOutput(string tagName, TagHelperAttributeList attributes = null)
         {
-            attributes = attributes ?? new Dictionary<string, object>();
+            attributes = attributes ?? new TagHelperAttributeList();
 
             return new TagHelperOutput(tagName, attributes);
         }
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/FormTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/FormTagHelperTest.cs
index c7b239acef..a819bbffbb 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/FormTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/FormTagHelperTest.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.ModelBinding;
@@ -25,7 +26,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedTagName = "not-form";
             var metadataProvider = new TestModelMetadataProvider();
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>
+                allAttributes: new TagHelperAttributeList
                 {
                     { "id", "myform" },
                     { "asp-route-foo", "bar" },
@@ -44,7 +45,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 expectedTagName,
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     { "id", "myform" },
                     { "asp-route-foo", "bar" },
@@ -74,11 +75,11 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             Assert.Equal(3, output.Attributes.Count);
-            var attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("id"));
+            var attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("id"));
             Assert.Equal("myform", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("method"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("method"));
             Assert.Equal("post", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("action"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("action"));
             Assert.Equal("home/index", attribute.Value);
             Assert.Empty(output.PreContent.GetContent());
             Assert.True(output.Content.IsEmpty);
@@ -97,7 +98,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var viewContext = CreateViewContext();
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -108,7 +110,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 "form",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             var generator = new Mock<IHtmlGenerator>(MockBehavior.Strict);
             generator
                 .Setup(mock => mock.GenerateForm(
@@ -148,7 +150,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var testViewContext = CreateViewContext();
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -157,10 +160,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.SetContent("Something");
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var expectedAttribute = new KeyValuePair<string, object>("asp-ROUTEE-NotRoute", "something");
+            var expectedAttribute = new TagHelperAttribute("asp-ROUTEE-NotRoute", "something");
             var output = new TagHelperOutput(
                 "form",
-                attributes: new Dictionary<string, object>()
+                attributes: new TagHelperAttributeList()
                 {
                     { "asp-route-val", "hello" },
                     { "asp-roUte--Foo", "bar" }
@@ -185,9 +188,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         var routeValueDictionary = (Dictionary<string, object>)routeValues;
 
                         Assert.Equal(2, routeValueDictionary.Count);
-                        var routeValue = Assert.Single(routeValueDictionary, kvp => kvp.Key.Equals("val"));
+                        var routeValue = Assert.Single(routeValueDictionary, attr => attr.Key.Equals("val"));
                         Assert.Equal("hello", routeValue.Value);
-                        routeValue = Assert.Single(routeValueDictionary, kvp => kvp.Key.Equals("-Foo"));
+                        routeValue = Assert.Single(routeValueDictionary, attr => attr.Key.Equals("-Foo"));
                         Assert.Equal("bar", routeValue.Value);
                     })
                 .Returns(new TagBuilder("form", new CommonTestEncoder()))
@@ -219,7 +222,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var viewContext = CreateViewContext();
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -230,7 +234,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 "form",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             var generator = new Mock<IHtmlGenerator>(MockBehavior.Strict);
             generator
                 .Setup(mock => mock.GenerateForm(viewContext, "Index", "Home", null, null, null))
@@ -264,7 +268,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var viewContext = CreateViewContext();
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -275,7 +280,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 "form",
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     { "asp-route-foo", "bar" }
                 });
@@ -333,12 +338,13 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             };
 
             var output = new TagHelperOutput("form",
-                                             attributes: new Dictionary<string, object>
+                                             attributes: new TagHelperAttributeList
                                              {
                                                  { "aCTiON", "my-action" },
                                              });
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -356,7 +362,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Assert.Equal("form", output.TagName);
             Assert.False(output.SelfClosing);
             var attribute = Assert.Single(output.Attributes);
-            Assert.Equal(new KeyValuePair<string, object>("aCTiON", "my-action"), attribute);
+            Assert.Equal(new TagHelperAttribute("aCTiON", "my-action"), attribute);
             Assert.Empty(output.PreContent.GetContent());
             Assert.True(output.Content.IsEmpty);
             Assert.Equal(expectedPostContent, output.PostContent.GetContent());
@@ -372,7 +378,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var formTagHelper = new FormTagHelper();
             var tagHelperOutput = new TagHelperOutput(
                 "form",
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     { "action", "my-action" },
                 });
@@ -409,7 +415,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             typeof(FormTagHelper).GetProperty(propertyName).SetValue(formTagHelper, "Home");
             var output = new TagHelperOutput(
                 "form",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             var expectedErrorMessage = "Cannot determine an 'action' attribute for <form>. A <form> with a specified " +
                 "'asp-route' must not have an 'asp-action' or 'asp-controller' attribute.";
 
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/InputTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/InputTagHelperTest.cs
index cd5955a59b..ced7f834ac 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/InputTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/InputTagHelperTest.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.ModelBinding;
@@ -18,6 +19,96 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 {
     public class InputTagHelperTest
     {
+        public static TheoryData MultiAttributeCheckBoxData
+        {
+            get
+            {
+                // outputAttributes, expectedAttributeString
+                return new TheoryData<TagHelperAttributeList, string>
+                {
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "hello", "world" },
+                            { "hello", "world2" }
+                        },
+                        "hello=\"HtmlEncode[[world]]\""
+                    },
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "hello", "world" },
+                            { "hello", "world2" },
+                            { "hello", "world3" }
+                        },
+                        "hello=\"HtmlEncode[[world]]\""
+                    },
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "HelLO", "world" },
+                            { "HELLO", "world2" }
+                        },
+                        "HelLO=\"HtmlEncode[[world]]\""
+                    },
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "Hello", "world" },
+                            { "HELLO", "world2" },
+                            { "hello", "world3" }
+                        },
+                        "Hello=\"HtmlEncode[[world]]\""
+                    },
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "HeLlO", "world" },
+                            { "hello", "world2" }
+                        },
+                        "HeLlO=\"HtmlEncode[[world]]\""
+                    },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(MultiAttributeCheckBoxData))]
+        public async Task CheckBoxHandlesMultipleAttributesSameNameCorrectly(
+            TagHelperAttributeList outputAttributes,
+            string expectedAttributeString)
+        {
+            // Arrange
+            var originalContent = "original content";
+            var originalTagName = "not-input";
+            var expectedContent = $"{originalContent}<input {expectedAttributeString} id=\"HtmlEncode[[IsACar]]\" " +
+                "name=\"HtmlEncode[[IsACar]]\" type=\"HtmlEncode[[checkbox]]\" value=\"HtmlEncode[[true]]\" />" +
+                "<input name=\"HtmlEncode[[IsACar]]\" type=\"HtmlEncode[[hidden]]\" value=\"HtmlEncode[[false]]\" />";
+
+            var context = new TagHelperContext(
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                items: new Dictionary<object, object>(),
+                uniqueId: "test",
+                getChildContentAsync: () => Task.FromResult<TagHelperContent>(result: null));
+            var output = new TagHelperOutput(originalTagName, outputAttributes)
+            {
+                SelfClosing = true,
+            };
+            output.Content.SetContent(originalContent);
+            var htmlGenerator = new TestableHtmlGenerator(new EmptyModelMetadataProvider());
+            var tagHelper = GetTagHelper(htmlGenerator, model: false, propertyName: nameof(Model.IsACar));
+
+            // Act
+            await tagHelper.ProcessAsync(context, output);
+
+            // Assert
+            Assert.Empty(output.Attributes); // Moved to Content and cleared
+            Assert.Equal(expectedContent, output.Content.GetContent());
+            Assert.True(output.SelfClosing);
+            Assert.Null(output.TagName); // Cleared
+        }
+
         // Top-level container (List<Model> or Model instance), immediate container type (Model or NestModel),
         // model accessor, expression path / id, expected value.
         public static TheoryData<object, Type, object, NameAndId, string> TestDataSet
@@ -84,7 +175,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string expectedValue)
         {
             // Arrange
-            var expectedAttributes = new Dictionary<string, object>
+            var expectedAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
                 { "type", "text" },
@@ -99,7 +190,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedTagName = "not-input";
 
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -108,7 +200,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.SetContent("Something");
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
@@ -160,7 +252,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedPostContent = "original post-content";
 
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -169,7 +262,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.SetContent("Something");
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
@@ -236,7 +329,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string model)
         {
             // Arrange
-            var contextAttributes = new Dictionary<string, object>
+            var contextAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
@@ -245,7 +338,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 contextAttributes["type"] = inputTypeName;  // Support restoration of type attribute, if any.
             }
 
-            var expectedAttributes = new Dictionary<string, object>
+            var expectedAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control hidden-control" },
                 { "type", inputTypeName ?? "hidden" },      // Generator restores type attribute; adds "hidden" if none.
@@ -265,7 +358,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.SetContent("Something");
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
@@ -335,7 +428,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string model)
         {
             // Arrange
-            var contextAttributes = new Dictionary<string, object>
+            var contextAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
@@ -344,7 +437,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 contextAttributes["type"] = inputTypeName;  // Support restoration of type attribute, if any.
             }
 
-            var expectedAttributes = new Dictionary<string, object>
+            var expectedAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control password-control" },
                 { "type", inputTypeName ?? "password" },    // Generator restores type attribute; adds "password" if none.
@@ -364,7 +457,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.SetContent("Something");
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
@@ -429,7 +522,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var value = "match";            // Real generator would use this for comparison with For.Metadata.Model.
-            var contextAttributes = new Dictionary<string, object>
+            var contextAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
                 { "value", value },
@@ -439,7 +532,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 contextAttributes["type"] = inputTypeName;  // Support restoration of type attribute, if any.
             }
 
-            var expectedAttributes = new Dictionary<string, object>
+            var expectedAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control radio-control" },
                 { "type", inputTypeName ?? "radio" },       // Generator restores type attribute; adds "radio" if none.
@@ -460,7 +553,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.SetContent("Something");
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
@@ -536,7 +629,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string model)
         {
             // Arrange
-            var contextAttributes = new Dictionary<string, object>
+            var contextAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
@@ -545,7 +638,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 contextAttributes["type"] = inputTypeName;  // Support restoration of type attribute, if any.
             }
 
-            var expectedAttributes = new Dictionary<string, object>
+            var expectedAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control text-control" },
                 { "type", inputTypeName ?? "text" },        // Generator restores type attribute; adds "text" if none.
@@ -565,7 +658,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.SetContent("Something");
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
@@ -668,19 +761,20 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string expectedType)
         {
             // Arrange
-            var expectedAttributes = new Dictionary<string, object>
+            var expectedAttributes = new TagHelperAttributeList
             {
                 { "type", expectedType },                   // Calculated; not passed to HtmlGenerator.
             };
             var expectedTagName = "not-input";
 
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()));
 
-            var output = new TagHelperOutput(expectedTagName, attributes: new Dictionary<string, object>())
+            var output = new TagHelperOutput(expectedTagName, attributes: new TagHelperAttributeList())
             {
                 SelfClosing = true,
             };
@@ -748,19 +842,20 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string expectedType)
         {
             // Arrange
-            var expectedAttributes = new Dictionary<string, object>
+            var expectedAttributes = new TagHelperAttributeList
             {
                 { "type", expectedType },                   // Calculated; not passed to HtmlGenerator.
             };
             var expectedTagName = "not-input";
 
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()));
 
-            var output = new TagHelperOutput(expectedTagName, attributes: new Dictionary<string, object>())
+            var output = new TagHelperOutput(expectedTagName, attributes: new TagHelperAttributeList())
             {
                 SelfClosing = true,
             };
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/Internal/AttributeMatcherTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/Internal/AttributeMatcherTest.cs
index 6853e0ddf7..15fa7fe3cd 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/Internal/AttributeMatcherTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/Internal/AttributeMatcherTest.cs
@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Razor.Runtime.TagHelpers;
 using Xunit;
@@ -19,7 +20,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
             {
                 ModeAttributes.Create("mode0", new [] { "first-attr" })
             };
-            var attributes = new Dictionary<string, object>
+            var attributes = new TagHelperAttributeList
             {
                 ["first-attr"] = "value",
                 ["not-in-any-mode"] = "value"
@@ -47,7 +48,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
             {
                 ModeAttributes.Create("mode0", new [] { "first-attr", "second-attr" })
             };
-            var attributes = new Dictionary<string, object>
+            var attributes = new TagHelperAttributeList
             {
                 ["first-attr"] = "value",
                 ["second-attr"] = "value",
@@ -82,7 +83,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
                 ModeAttributes.Create("mode2", new [] { "first-attr", "second-attr", "third-attr" }),
                 ModeAttributes.Create("mode3", new [] { "fourth-attr" })
             };
-            var attributes = new Dictionary<string, object>
+            var attributes = new TagHelperAttributeList
             {
                 ["second-attr"] = "value",
                 ["third-attr"] = "value",
@@ -119,10 +120,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
         }
 
         private static TagHelperContext MakeTagHelperContext(
-            IDictionary<string, object> attributes = null,
+            TagHelperAttributeList attributes = null,
             string content = null)
         {
-            attributes = attributes ?? new Dictionary<string, object>();
+            attributes = attributes ?? new TagHelperAttributeList();
 
             return new TagHelperContext(
                 attributes,
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/LabelTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/LabelTagHelperTest.cs
index 3eb6e69872..d1b60fcaf8 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/LabelTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/LabelTagHelperTest.cs
@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Mvc.ModelBinding;
 using Microsoft.AspNet.Mvc.Rendering;
@@ -164,7 +165,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var expectedTagName = "not-label";
-            var expectedAttributes = new Dictionary<string, object>
+            var expectedAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
                 { "for", tagHelperOutputContent.ExpectedId }
@@ -186,7 +187,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedPostContent = "original post-content";
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -195,7 +197,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.SetContent(tagHelperOutputContent.OriginalChildContent);
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var htmlAttributes = new Dictionary<string, object>
+            var htmlAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/LinkTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/LinkTagHelperTest.cs
index 3dbf268f4a..2d7afff351 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/LinkTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/LinkTagHelperTest.cs
@@ -28,14 +28,118 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 {
     public class LinkTagHelperTest
     {
+        public static TheoryData MultiAttributeSameNameData
+        {
+            get
+            {
+                // outputAttributes, expectedAttributeString
+                return new TheoryData<TagHelperAttributeList, string>
+                {
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "hello", "world" },
+                            { "hello", "world2" }
+                        },
+                        "hello=\"HtmlEncode[[world]]\" hello=\"HtmlEncode[[world2]]\""
+                    },
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "hello", "world" },
+                            { "hello", "world2" },
+                            { "hello", "world3" }
+                        },
+                        "hello=\"HtmlEncode[[world]]\" hello=\"HtmlEncode[[world2]]\" hello=\"HtmlEncode[[world3]]\""
+                    },
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "HelLO", "world" },
+                            { "HELLO", "world2" }
+                        },
+                        "HelLO=\"HtmlEncode[[world]]\" HELLO=\"HtmlEncode[[world2]]\""
+                    },
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "Hello", "world" },
+                            { "HELLO", "world2" },
+                            { "hello", "world3" }
+                        },
+                        "Hello=\"HtmlEncode[[world]]\" HELLO=\"HtmlEncode[[world2]]\" hello=\"HtmlEncode[[world3]]\""
+                    },
+                    {
+                        new TagHelperAttributeList
+                        {
+                            { "HeLlO", "world" },
+                            { "hello", "world2" }
+                        },
+                        "HeLlO=\"HtmlEncode[[world]]\" hello=\"HtmlEncode[[world2]]\""
+                    },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(MultiAttributeSameNameData))]
+        public void HandlesMultipleAttributesSameNameCorrectly(
+            TagHelperAttributeList outputAttributes,
+            string expectedAttributeString)
+        {
+            // Arrange
+            var allAttributes = new TagHelperAttributeList(
+                outputAttributes.Concat(
+                    new TagHelperAttributeList
+                    {
+                        { "rel", new HtmlString("stylesheet") },
+                        { "href", "test.css" },
+                        { "asp-fallback-href", "test.css" },
+                        { "asp-fallback-test-class", "hidden" },
+                        { "asp-fallback-test-property", "visibility" },
+                        { "asp-fallback-test-value", "hidden" },
+                    }));
+            var context = MakeTagHelperContext(allAttributes);
+            var combinedOutputAttributes = new TagHelperAttributeList(
+                outputAttributes.Concat(
+                    new[]
+                    {
+                        new TagHelperAttribute("rel", new HtmlString("stylesheet"))
+                    }));
+            var output = MakeTagHelperOutput("link", combinedOutputAttributes);
+            var logger = new Mock<ILogger<LinkTagHelper>>();
+            var hostingEnvironment = MakeHostingEnvironment();
+            var viewContext = MakeViewContext();
+            var helper = new LinkTagHelper
+            {
+                HtmlEncoder = new CommonTestEncoder(),
+                JavaScriptEncoder = new CommonTestEncoder(),
+                Logger = logger.Object,
+                HostingEnvironment = hostingEnvironment,
+                ViewContext = viewContext,
+                FallbackHref = "test.css",
+                FallbackTestClass = "hidden",
+                FallbackTestProperty = "visibility",
+                FallbackTestValue = "hidden",
+                Href = "test.css",
+                Cache = MakeCache(),
+            };
+
+            // Act
+            helper.Process(context, output);
+
+            // Assert
+            Assert.StartsWith("<link " + expectedAttributeString + " rel=\"stylesheet\"", output.Content.GetContent());
+        }
+
         public static TheoryData RunsWhenRequiredAttributesArePresent_Data
         {
             get
             {
-                return new TheoryData<IDictionary<string, object>, Action<LinkTagHelper>>
+                return new TheoryData<TagHelperAttributeList, Action<LinkTagHelper>>
                 {
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-href-include"] = "*.css"
                         },
@@ -45,7 +149,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-href-include"] = "*.css",
                             ["asp-href-exclude"] = "*.min.css"
@@ -57,7 +161,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-href"] = "test.css",
                             ["asp-fallback-test-class"] = "hidden",
@@ -73,7 +177,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-href-include"] = "*.css",
                             ["asp-fallback-test-class"] = "hidden",
@@ -90,7 +194,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     },
                     // File Version
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-file-version"] = "true"
                         },
@@ -100,7 +204,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-href-include"] = "*.css",
                             ["asp-file-version"] = "true"
@@ -112,7 +216,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-href-include"] = "*.css",
                             ["asp-href-exclude"] = "*.min.css",
@@ -126,7 +230,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-href"] = "test.css",
                             ["asp-fallback-test-class"] = "hidden",
@@ -144,7 +248,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-href-include"] = "*.css",
                             ["asp-fallback-test-class"] = "hidden",
@@ -168,7 +272,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         [Theory]
         [MemberData(nameof(RunsWhenRequiredAttributesArePresent_Data))]
         public void RunsWhenRequiredAttributesArePresent(
-            IDictionary<string, object> attributes,
+            TagHelperAttributeList attributes,
             Action<LinkTagHelper> setProperties)
         {
             // Arrange
@@ -202,21 +306,21 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
-                    ["rel"] = new HtmlString("stylesheet"),
-                    ["data-extra"] = new HtmlString("something"),
-                    ["href"] = "test.css",
-                    ["asp-fallback-href"] = "test.css",
-                    ["asp-fallback-test-class"] = "hidden",
-                    ["asp-fallback-test-property"] = "visibility",
-                    ["asp-fallback-test-value"] = "hidden"
+                    { "rel", new HtmlString("stylesheet") },
+                    { "data-extra", new HtmlString("something") },
+                    { "href", "test.css" },
+                    { "asp-fallback-href", "test.css" },
+                    { "asp-fallback-test-class", "hidden" },
+                    { "asp-fallback-test-property", "visibility" },
+                    { "asp-fallback-test-value", "hidden" },
                 });
             var output = MakeTagHelperOutput("link",
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
-                    ["rel"] = new HtmlString("stylesheet"),
-                    ["data-extra"] = new HtmlString("something"),
+                    { "rel", new HtmlString("stylesheet") },
+                    { "data-extra", new HtmlString("something") },
                 });
             var logger = new Mock<ILogger<LinkTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
@@ -248,10 +352,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             get
             {
-                return new TheoryData<IDictionary<string, object>, Action<LinkTagHelper>>
+                return new TheoryData<TagHelperAttributeList, Action<LinkTagHelper>>
                 {
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             // This is commented out on purpose: ["asp-href-include"] = "*.css",
                             ["asp-href-exclude"] = "*.min.css"
@@ -263,7 +367,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             // This is commented out on purpose: ["asp-fallback-href"] = "test.css",
                             ["asp-fallback-test-class"] = "hidden",
@@ -279,7 +383,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-href"] = "test.css",
                             ["asp-fallback-test-class"] = "hidden",
@@ -295,7 +399,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             // This is commented out on purpose: ["asp-fallback-href-include"] = "test.css",
                             ["asp-fallback-href-exclude"] = "**/*.min.css",
@@ -319,7 +423,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         [Theory]
         [MemberData(nameof(DoesNotRunWhenARequiredAttributeIsMissing_Data))]
         public void DoesNotRunWhenARequiredAttributeIsMissing(
-            IDictionary<string, object> attributes,
+            TagHelperAttributeList attributes,
             Action<LinkTagHelper> setProperties)
         {
             // Arrange
@@ -375,15 +479,15 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
-                    ["rel"] = new HtmlString("stylesheet"),
-                    ["href"] = "/css/site.css",
-                    ["asp-href-include"] = "**/*.css"
+                    { "rel", new HtmlString("stylesheet") },
+                    { "href", "/css/site.css" },
+                    { "asp-href-include", "**/*.css" },
                 });
-            var output = MakeTagHelperOutput("link", attributes: new Dictionary<string, object>
+            var output = MakeTagHelperOutput("link", attributes: new TagHelperAttributeList
             {
-                ["rel"] = new HtmlString("stylesheet"),
+                { "rel", new HtmlString("stylesheet") },
             });
             var logger = new Mock<ILogger<LinkTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
@@ -418,13 +522,13 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     ["rel"] = "stylesheet",
                     ["href"] = "/css/site.css",
                     ["asp-href-include"] = "**/*.css"
                 });
-            var output = MakeTagHelperOutput("link", attributes: new Dictionary<string, object>
+            var output = MakeTagHelperOutput("link", attributes: new TagHelperAttributeList
             {
                 ["rel"] = "stylesheet",
             });
@@ -461,15 +565,15 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
-                    ["rel"] = new HtmlString("stylesheet"),
-                    ["href"] = "/css/site.css",
-                    ["asp-file-version"] = "true"
+                    { "rel", new HtmlString("stylesheet") },
+                    { "href", "/css/site.css" },
+                    { "asp-file-version", "true" }
                 });
-            var output = MakeTagHelperOutput("link", attributes: new Dictionary<string, object>
+            var output = MakeTagHelperOutput("link", attributes: new TagHelperAttributeList
             {
-                ["rel"] = new HtmlString("stylesheet"),
+                { "rel", new HtmlString("stylesheet") },
             });
             var logger = new Mock<ILogger<LinkTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
@@ -500,15 +604,15 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
-                    ["rel"] = new HtmlString("stylesheet"),
-                    ["href"] = "/bar/css/site.css",
-                    ["asp-file-version"] = "true"
+                    { "rel", new HtmlString("stylesheet") },
+                    { "href", "/bar/css/site.css" },
+                    { "asp-file-version", "true" },
                 });
-            var output = MakeTagHelperOutput("link", attributes: new Dictionary<string, object>
+            var output = MakeTagHelperOutput("link", attributes: new TagHelperAttributeList
             {
-                ["rel"] = new HtmlString("stylesheet"),
+                { "rel", new HtmlString("stylesheet") },
             });
             var logger = new Mock<ILogger<LinkTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
@@ -539,16 +643,16 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
-                    ["rel"] = new HtmlString("stylesheet"),
-                    ["href"] = "/css/site.css",
-                    ["asp-href-include"] = "**/*.css",
-                    ["asp-file-version"] = "true"
+                    { "rel", new HtmlString("stylesheet") },
+                    { "href", "/css/site.css" },
+                    { "asp-href-include", "**/*.css" },
+                    { "asp-file-version", "true" },
                 });
-            var output = MakeTagHelperOutput("link", attributes: new Dictionary<string, object>
+            var output = MakeTagHelperOutput("link", attributes: new TagHelperAttributeList
             {
-                ["rel"] = new HtmlString("stylesheet"),
+                { "rel", new HtmlString("stylesheet") },
             });
             var logger = new Mock<ILogger<LinkTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
@@ -600,10 +704,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         }
 
         private static TagHelperContext MakeTagHelperContext(
-            IDictionary<string, object> attributes = null,
+            TagHelperAttributeList attributes = null,
             string content = null)
         {
-            attributes = attributes ?? new Dictionary<string, object>();
+            attributes = attributes ?? new TagHelperAttributeList();
 
             return new TagHelperContext(
                 attributes,
@@ -617,9 +721,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
         }
 
-        private static TagHelperOutput MakeTagHelperOutput(string tagName, IDictionary<string, object> attributes = null)
+        private static TagHelperOutput MakeTagHelperOutput(string tagName, TagHelperAttributeList attributes = null)
         {
-            attributes = attributes ?? new Dictionary<string, object>();
+            attributes = attributes ?? new TagHelperAttributeList();
 
             return new TagHelperOutput(tagName, attributes);
         }
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/OptionTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/OptionTagHelperTest.cs
index 7696f26312..e71bcf0137 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/OptionTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/OptionTagHelperTest.cs
@@ -26,7 +26,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         null, null, null, null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }
                             },
@@ -36,7 +36,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         null, string.Empty, "value", null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "" }
                             },
@@ -46,7 +46,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         null, "selected", "value", null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "selected" }
                             },
@@ -56,7 +56,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         null, null, "value", Enumerable.Empty<string>(),
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }
                             },
@@ -66,7 +66,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         null, null, "value", new [] { string.Empty, },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }
                             },
@@ -76,7 +76,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         null, string.Empty, "value", new [] { string.Empty, },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "" }
                             },
@@ -86,7 +86,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         null, null, "value", new [] { "value", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "selected" }
                             },
@@ -96,7 +96,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         null, null, "value", new [] { string.Empty, "value", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "selected" }
                             },
@@ -106,7 +106,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         string.Empty, null, null, null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }
                             },
@@ -116,7 +116,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         string.Empty, string.Empty, null, null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "" }
                             },
@@ -126,7 +126,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         string.Empty, "selected", null, null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "selected" }
                             },
@@ -136,7 +136,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         string.Empty, null, null, Enumerable.Empty<string>(),
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }
                             },
@@ -146,7 +146,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         string.Empty, null, null, new [] { string.Empty, },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "selected" }
                             },
@@ -156,7 +156,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         string.Empty, string.Empty, null, new [] { string.Empty, },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "" }
                             },
@@ -166,7 +166,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         string.Empty, null, null, new [] { "text", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }
                             },
@@ -176,7 +176,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         string.Empty, null, null, new [] { string.Empty, "text", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "selected" }
                             },
@@ -186,7 +186,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", null, null, null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }
                             },
@@ -196,7 +196,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", string.Empty, null, null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "" }
                             },
@@ -206,7 +206,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", "selected", null, null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "selected" }
                             },
@@ -216,7 +216,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", null, null, Enumerable.Empty<string>(),
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }
                             },
@@ -226,7 +226,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", null, null, new [] { string.Empty, },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }
                             },
@@ -236,7 +236,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "HtmlEncode[[text]]", null, null, new [] { "text", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "selected" }
                             },
@@ -246,7 +246,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", string.Empty, null, new [] { "text", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "" }
                             },
@@ -256,7 +256,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "HtmlEncode[[text]]", null, null, new [] { string.Empty, "text", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "selected", "selected" }
                             },
@@ -266,7 +266,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", string.Empty, "value", null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "" }
                             },
@@ -276,7 +276,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", "selected", "value", null,
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "selected" }
                             },
@@ -286,7 +286,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", null, "value", Enumerable.Empty<string>(),
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }
                             },
@@ -296,7 +296,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", null, "value", new [] { string.Empty, },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }
                             },
@@ -306,7 +306,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", string.Empty, "value", new [] { string.Empty, },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "" }
                             },
@@ -316,7 +316,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", null, "value", new [] { "text", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }
                             },
@@ -326,7 +326,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", null, "value", new [] { "value", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "selected" }
                             },
@@ -336,7 +336,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         "text", null, "value", new [] { string.Empty, "value", },
                         GetTagHelperOutput(
                             "not-option",
-                            new Dictionary<string, object>
+                            new TagHelperAttributeList
                             {
                                 { "label", "my-label" }, { "value", "value" }, { "selected", "selected" }
                             },
@@ -379,7 +379,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             TagHelperOutput expectedTagHelperOutput)
         {
             // Arrange
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "label", "my-label" },
             };
@@ -388,7 +388,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 originalAttributes.Add("selected", selected);
             }
 
-            var contextAttributes = new Dictionary<string, object>(originalAttributes);
+            var contextAttributes = new TagHelperAttributeList(originalAttributes);
             if (value != null)
             {
                 contextAttributes.Add("value", value);
@@ -449,14 +449,14 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             TagHelperOutput ignored)
         {
             // Arrange
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "label", "my-label" },
                 { "selected", selected },
             };
             var originalTagName = "not-option";
 
-            var contextAttributes = new Dictionary<string, object>
+            var contextAttributes = new TagHelperAttributeList
             {
                 { "label", "my-label" },
                 { "selected", selected },
@@ -510,14 +510,14 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             TagHelperOutput ignoredOutput)
         {
             // Arrange
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "label", "my-label" },
                 { "selected", selected },
             };
             var originalTagName = "not-option";
 
-            var contextAttributes = new Dictionary<string, object>
+            var contextAttributes = new TagHelperAttributeList
             {
                 { "label", "my-label" },
                 { "selected", selected },
@@ -554,7 +554,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         }
 
         private static TagHelperOutput GetTagHelperOutput(
-            string tagName, IDictionary<string, object> attributes, string content)
+            string tagName, TagHelperAttributeList attributes, string content)
         {
             var tagHelperOutput = new TagHelperOutput(tagName, attributes);
             tagHelperOutput.Content.SetContent(content);
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ScriptTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ScriptTagHelperTest.cs
index 514836b939..a5423a1a0a 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ScriptTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ScriptTagHelperTest.cs
@@ -28,14 +28,64 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 {
     public class ScriptTagHelperTest
     {
+        [Theory]
+        [MemberData(nameof(LinkTagHelperTest.MultiAttributeSameNameData), MemberType = typeof(LinkTagHelperTest))]
+        public async Task HandlesMultipleAttributesSameNameCorrectly(
+            TagHelperAttributeList outputAttributes,
+            string expectedAttributeString)
+        {
+            // Arrange
+            var allAttributes = new TagHelperAttributeList(
+                outputAttributes.Concat(
+                    new TagHelperAttributeList
+                    {
+                        ["data-extra"] = "something",
+                        ["src"] = "/blank.js",
+                        ["asp-fallback-src"] = "http://www.example.com/blank.js",
+                        ["asp-fallback-test"] = "isavailable()",
+                    }));
+            var tagHelperContext = MakeTagHelperContext(allAttributes);
+            var viewContext = MakeViewContext();
+            var combinedOutputAttributes = new TagHelperAttributeList(
+                outputAttributes.Concat(
+                    new[]
+                    {
+                        new TagHelperAttribute("data-extra", new HtmlString("something"))
+                    }));
+            var output = MakeTagHelperOutput("script", combinedOutputAttributes);
+            var hostingEnvironment = MakeHostingEnvironment();
+
+            var helper = new ScriptTagHelper
+            {
+                HtmlEncoder = new CommonTestEncoder(),
+                JavaScriptEncoder = new CommonTestEncoder(),
+                ViewContext = viewContext,
+                HostingEnvironment = hostingEnvironment,
+                FallbackSrc = "~/blank.js",
+                FallbackTestExpression = "http://www.example.com/blank.js",
+                Src = "/blank.js",
+                Cache = MakeCache(),
+                Logger = CreateLogger()
+            };
+
+            // Act
+            await helper.ProcessAsync(tagHelperContext, output);
+
+            // Assert
+            Assert.StartsWith(
+                "<script " + expectedAttributeString + " data-extra=\"something\" " +
+                "src=\"HtmlEncode[[/blank.js]]\"",
+                output.Content.GetContent());
+        }
+
         public static TheoryData RunsWhenRequiredAttributesArePresent_Data
         {
             get
             {
-                return new TheoryData<IDictionary<string, object>, Action<ScriptTagHelper>>
+                return new TheoryData<TagHelperAttributeList, Action<ScriptTagHelper>>
                 {
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-src-include"] = "*.js"
                         },
@@ -45,7 +95,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-src-include"] = "*.js",
                             ["asp-src-exclude"] = "*.min.js"
@@ -57,7 +107,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-src"] = "test.js",
                             ["asp-fallback-test"] = "isavailable()"
@@ -69,7 +119,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-src-include"] = "*.js",
                             ["asp-fallback-test"] = "isavailable()"
@@ -81,7 +131,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-src"] = "test.js",
                             ["asp-fallback-src-include"] = "*.js",
@@ -95,7 +145,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-src-include"] = "*.js",
                             ["asp-fallback-src-exclude"] = "*.min.js",
@@ -110,7 +160,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     },
                     // File Version
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-file-version"] = "true"
                         },
@@ -120,7 +170,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-src-include"] = "*.js",
                             ["asp-file-version"] = "true"
@@ -132,7 +182,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-src-include"] = "*.js",
                             ["asp-src-exclude"] = "*.min.js",
@@ -146,7 +196,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-src"] = "test.js",
                             ["asp-fallback-test"] = "isavailable()",
@@ -160,7 +210,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-src-include"] = "*.js",
                             ["asp-fallback-test"] = "isavailable()",
@@ -174,7 +224,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-src"] = "test.js",
                             ["asp-fallback-src-include"] = "*.js",
@@ -190,7 +240,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-src-include"] = "*.js",
                             ["asp-fallback-src-exclude"] = "*.min.js",
@@ -212,7 +262,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         [Theory]
         [MemberData(nameof(RunsWhenRequiredAttributesArePresent_Data))]
         public async Task RunsWhenRequiredAttributesArePresent(
-            IDictionary<string, object> attributes,
+            TagHelperAttributeList attributes,
             Action<ScriptTagHelper> setProperties)
         {
             // Arrange
@@ -245,10 +295,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             get
             {
-                return new TheoryData<IDictionary<string, object>, Action<ScriptTagHelper>>
+                return new TheoryData<TagHelperAttributeList, Action<ScriptTagHelper>>
                 {
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             // This is commented out on purpose: ["asp-src-include"] = "*.js",
                             ["asp-src-exclude"] = "*.min.js"
@@ -260,7 +310,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             // This is commented out on purpose: ["asp-fallback-src"] = "test.js",
                             ["asp-fallback-test"] = "isavailable()",
@@ -272,7 +322,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             ["asp-fallback-src"] = "test.js",
                             // This is commented out on purpose: ["asp-fallback-test"] = "isavailable()"
@@ -284,7 +334,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                         }
                     },
                     {
-                        new Dictionary<string, object>
+                        new TagHelperAttributeList
                         {
                             // This is commented out on purpose: ["asp-fallback-src-include"] = "test.js",
                             ["asp-fallback-src-exclude"] = "**/*.min.js",
@@ -304,7 +354,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         [Theory]
         [MemberData(nameof(DoesNotRunWhenARequiredAttributeIsMissing_Data))]
         public void DoesNotRunWhenARequiredAttributeIsMissing(
-            IDictionary<string, object> attributes,
+            TagHelperAttributeList attributes,
             Action<ScriptTagHelper> setProperties)
         {
             // Arrange
@@ -333,7 +383,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         [Theory]
         [MemberData(nameof(DoesNotRunWhenARequiredAttributeIsMissing_Data))]
         public async Task LogsWhenARequiredAttributeIsMissing(
-            IDictionary<string, object> attributes,
+            TagHelperAttributeList attributes,
             Action<ScriptTagHelper> setProperties)
         {
             // Arrange
@@ -431,7 +481,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var tagHelperContext = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     ["data-extra"] = "something",
                     ["src"] = "/blank.js",
@@ -443,7 +493,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var viewContext = MakeViewContext();
 
             var output = MakeTagHelperOutput("src",
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     ["data-extra"] = "something",
                     ["data-more"] = "else",
@@ -480,12 +530,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     ["src"] = "/js/site.js",
                     ["asp-src-include"] = "**/*.js"
                 });
-            var output = MakeTagHelperOutput("script", attributes: new Dictionary<string, object>());
+            var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
             var logger = new Mock<ILogger<ScriptTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
             var viewContext = MakeViewContext();
@@ -517,12 +567,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     ["src"] = "/js/site.js",
                     ["asp-src-include"] = "**/*.js"
                 });
-            var output = MakeTagHelperOutput("script", attributes: new Dictionary<string, object>());
+            var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
             var logger = new Mock<ILogger<ScriptTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
             var viewContext = MakeViewContext();
@@ -555,12 +605,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     ["src"] = "/js/site.js",
                     ["asp-file-version"] = "true"
                 });
-            var output = MakeTagHelperOutput("script", attributes: new Dictionary<string, object>());
+            var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
 
             var logger = new Mock<ILogger<ScriptTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
@@ -592,12 +642,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     ["src"] = "/bar/js/site.js",
                     ["asp-file-version"] = "true"
                 });
-            var output = MakeTagHelperOutput("script", attributes: new Dictionary<string, object>());
+            var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
 
             var logger = new Mock<ILogger<ScriptTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
@@ -629,14 +679,14 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     ["src"] = "/js/site.js",
                     ["asp-fallback-src-include"] = "fallback.js",
                     ["asp-fallback-test"] = "isavailable()",
                     ["asp-file-version"] = "true"
                 });
-            var output = MakeTagHelperOutput("script", attributes: new Dictionary<string, object>());
+            var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
 
             var logger = new Mock<ILogger<ScriptTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
@@ -672,13 +722,13 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             // Arrange
             var context = MakeTagHelperContext(
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     ["src"] = "/js/site.js",
                     ["asp-src-include"] = "*.js",
                     ["asp-file-version"] = "true"
                 });
-            var output = MakeTagHelperOutput("script", attributes: new Dictionary<string, object>());
+            var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
             var logger = new Mock<ILogger<ScriptTagHelper>>();
             var hostingEnvironment = MakeHostingEnvironment();
             var viewContext = MakeViewContext();
@@ -709,10 +759,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         }
 
         private TagHelperContext MakeTagHelperContext(
-            IDictionary<string, object> attributes = null,
+            TagHelperAttributeList attributes = null,
             string content = null)
         {
-            attributes = attributes ?? new Dictionary<string, object>();
+            attributes = attributes ?? new TagHelperAttributeList();
 
             return new TagHelperContext(
                 attributes,
@@ -746,9 +796,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             return viewContext;
         }
 
-        private TagHelperOutput MakeTagHelperOutput(string tagName, IDictionary<string, object> attributes = null)
+        private TagHelperOutput MakeTagHelperOutput(string tagName, TagHelperAttributeList attributes = null)
         {
-            attributes = attributes ?? new Dictionary<string, object>();
+            attributes = attributes ?? new TagHelperAttributeList();
 
             return new TagHelperOutput(tagName, attributes);
         }
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/SelectTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/SelectTagHelperTest.cs
index c73f33777e..7a234bc09c 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/SelectTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/SelectTagHelperTest.cs
@@ -171,13 +171,13 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string ignored)
         {
             // Arrange
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
             var originalPostContent = "original content";
 
-            var expectedAttributes = new Dictionary<string, object>(originalAttributes)
+            var expectedAttributes = new TagHelperAttributeList(originalAttributes)
             {
                 { "id", nameAndId.Id },
                 { "name", nameAndId.Name },
@@ -198,7 +198,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var modelExpression = new ModelExpression(nameAndId.Name, modelExplorer);
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -257,13 +258,13 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string expectedOptions)
         {
             // Arrange
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
             var originalPostContent = "original content";
 
-            var expectedAttributes = new Dictionary<string, object>(originalAttributes)
+            var expectedAttributes = new TagHelperAttributeList(originalAttributes)
             {
                 { "id", nameAndId.Id },
                 { "name", nameAndId.Name },
@@ -285,7 +286,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var modelExpression = new ModelExpression(nameAndId.Name, modelExplorer);
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -358,13 +360,13 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string expectedOptions)
         {
             // Arrange
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
             var originalPostContent = "original content";
 
-            var expectedAttributes = new Dictionary<string, object>(originalAttributes)
+            var expectedAttributes = new TagHelperAttributeList(originalAttributes)
             {
                 { "id", nameAndId.Id },
                 { "name", nameAndId.Name },
@@ -386,7 +388,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var modelExpression = new ModelExpression(name: string.Empty, modelExplorer: modelExplorer);
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -459,12 +462,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             IEnumerable<SelectListItem> expectedItems)
         {
             // Arrange
-            var contextAttributes = new Dictionary<string, object>
+            var contextAttributes = new TagHelperAttributeList
             {
                 // Provided for completeness. Select tag helper does not confirm AllAttributes set is consistent.
                 { attributeName, attributeValue },
             };
-            var originalAttributes = new Dictionary<string, object>
+            var originalAttributes = new TagHelperAttributeList
             {
                 { attributeName, attributeValue },
             };
@@ -545,8 +548,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             bool allowMultiple)
         {
             // Arrange
-            var contextAttributes = new Dictionary<string, object>();
-            var originalAttributes = new Dictionary<string, object>();
+            var contextAttributes = new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>());
+            var originalAttributes = new TagHelperAttributeList();
             var propertyName = "Property1";
             var tagName = "select";
 
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/TagHelperOutputExtensionsTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/TagHelperOutputExtensionsTest.cs
index 2e3e6de56a..e46631f2e2 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/TagHelperOutputExtensionsTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/TagHelperOutputExtensionsTest.cs
@@ -8,12 +8,111 @@ using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Razor.Runtime.TagHelpers;
 using Microsoft.AspNet.Testing;
 using Microsoft.Framework.WebEncoders.Testing;
+using Microsoft.Internal.Web.Utils;
 using Xunit;
 
 namespace Microsoft.AspNet.Mvc.TagHelpers
 {
     public class TagHelperOutputExtensionsTest
     {
+        public static TheoryData CopyHtmlAttributeData_MultipleAttributesSameName
+        {
+            get
+            {
+                // attributeNameToCopy, allAttributes, expectedAttributes
+                return new TheoryData<string, TagHelperAttributeList, IEnumerable<TagHelperAttribute>>
+                {
+                    {
+                        "hello",
+                        new TagHelperAttributeList
+                        {
+                            { "hello", "world" },
+                            { "hello", "world2" }
+                        },
+                        new[]
+                        {
+                            new TagHelperAttribute("hello", "world"),
+                            new TagHelperAttribute("hello", "world2"),
+                        }
+                    },
+                    {
+                        "HELLO",
+                        new TagHelperAttributeList
+                        {
+                            { "hello", "world" },
+                            { "hello", "world2" }
+                        },
+                        new[]
+                        {
+                            new TagHelperAttribute("hello", "world"),
+                            new TagHelperAttribute("hello", "world2"),
+                        }
+                    },
+                    {
+                        "hello",
+                        new TagHelperAttributeList
+                        {
+                            { "HelLO", "world" },
+                            { "HELLO", "world2" }
+                        },
+                        new[]
+                        {
+                            new TagHelperAttribute("HelLO", "world"),
+                            new TagHelperAttribute("HELLO", "world2"),
+                        }
+                    },
+                    {
+                        "hello",
+                        new TagHelperAttributeList
+                        {
+                            { "hello", "world" },
+                            { "HELLO", "world2" }
+                        },
+                        new[]
+                        {
+                            new TagHelperAttribute("hello", "world"),
+                            new TagHelperAttribute("HELLO", "world2"),
+                        }
+                    },
+                    {
+                        "HELLO",
+                        new TagHelperAttributeList
+                        {
+                            { "HeLlO", "world" },
+                            { "heLLo", "world2" }
+                        },
+                        new[]
+                        {
+                            new TagHelperAttribute("HeLlO", "world"),
+                            new TagHelperAttribute("heLLo", "world2"),
+                        }
+                    },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(CopyHtmlAttributeData_MultipleAttributesSameName))]
+        public void CopyHtmlAttribute_CopiesAllOriginalAttributes(
+            string attributeNameToCopy,
+            TagHelperAttributeList allAttributes,
+            IEnumerable<TagHelperAttribute> expectedAttributes)
+        {
+            // Arrange
+            var output = new TagHelperOutput("p", attributes: new TagHelperAttributeList());
+            var context = new TagHelperContext(
+                allAttributes,
+                items: new Dictionary<object, object>(),
+                uniqueId: "test",
+                getChildContentAsync: () => Task.FromResult<TagHelperContent>(result: null));
+
+            // Act
+            output.CopyHtmlAttribute(attributeNameToCopy, context);
+
+            // Assert
+            Assert.Equal(expectedAttributes, output.Attributes, CaseSensitiveTagHelperAttributeComparer.Default);
+        }
+
         [Theory]
         [InlineData("hello", "world")]
         [InlineData("HeLlO", "wOrLd")]
@@ -22,9 +121,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(StringComparer.Ordinal)
+                allAttributes: new TagHelperAttributeList
                 {
                     { attributeName, attributeValue }
                 },
@@ -36,7 +135,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.Append("Something");
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var expectedAttribute = new KeyValuePair<string, object>(attributeName, attributeValue);
+            var expectedAttribute = new TagHelperAttribute(attributeName, attributeValue);
 
             // Act
             tagHelperOutput.CopyHtmlAttribute("hello", tagHelperContext);
@@ -53,13 +152,13 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var attributeName = "hello";
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>()
+                attributes: new TagHelperAttributeList()
                 {
                     { attributeName, "world2" }
                 });
-            var expectedAttribute = new KeyValuePair<string, object>(attributeName, "world2");
+            var expectedAttribute = new TagHelperAttribute(attributeName, "world2");
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(StringComparer.Ordinal)
+                allAttributes: new TagHelperAttributeList
                 {
                     { attributeName, "world" }
                 },
@@ -87,9 +186,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var invalidAttributeName = "hello2";
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(StringComparer.Ordinal)
+                allAttributes: new TagHelperAttributeList
                 {
                     { "hello", "world" }
                 },
@@ -115,12 +214,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>()
+                attributes: new TagHelperAttributeList()
                 {
                     { "route-Hello", "World" },
                     { "Route-I", "Am" }
                 });
-            var expectedAttribute = new KeyValuePair<string, object>("type", "btn");
+            var expectedAttribute = new TagHelperAttribute("type", "btn");
             tagHelperOutput.Attributes.Add(expectedAttribute);
             var attributes = tagHelperOutput.FindPrefixedAttributes("route-");
 
@@ -138,7 +237,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>()
+                attributes: new TagHelperAttributeList()
                 {
                     { "routeHello", "World" },
                     { "Routee-I", "Am" }
@@ -149,20 +248,178 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             Assert.Empty(attributes);
-            var attribute = Assert.Single(tagHelperOutput.Attributes, kvp => kvp.Key.Equals("routeHello"));
+            var attribute = Assert.Single(tagHelperOutput.Attributes, attr => attr.Name.Equals("routeHello"));
             Assert.Equal(attribute.Value, "World");
-            attribute = Assert.Single(tagHelperOutput.Attributes, kvp => kvp.Key.Equals("Routee-I"));
+            attribute = Assert.Single(tagHelperOutput.Attributes, attr => attr.Name.Equals("Routee-I"));
             Assert.Equal(attribute.Value, "Am");
         }
 
+        public static TheoryData MultipleAttributeSameNameData
+        {
+            get
+            {
+                // tagBuilderAttributes, outputAttributes, expectedAttributes
+                return new TheoryData<Dictionary<string, string>, TagHelperAttributeList, TagHelperAttributeList>
+                {
+                    {
+                        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+                        {
+                            { "class", "btn" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "class", "btn2" },
+                            { "class", "btn3" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "class", "btn2 btn" }
+                        }
+                    },
+                    {
+                        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+                        {
+                            { "ClAsS", "btn" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "class", "btn2" },
+                            { "class", "btn3" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "class", "btn2 btn" }
+                        }
+                    },
+                    {
+                        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+                        {
+                            { "class", "btn" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "clASS", "btn2" },
+                            { "class", "btn3" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "class", "btn2 btn" }
+                        }
+                    },
+                    {
+                        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+                        {
+                            { "class", "btn" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "clASS", "btn2" },
+                            { "CLass", "btn3" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "class", "btn2 btn" }
+                        }
+                    },
+                    {
+                        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+                        {
+                            { "CLASS", "btn" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "clASS", "btn2" },
+                            { "CLass", "btn3" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "class", "btn2 btn" }
+                        }
+                    },
+                    {
+                        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+                        {
+                            { "CLASS", "btn" }
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "before", "before value" },
+                            { "clASS", "btn2" },
+                            { "mid", "mid value" },
+                            { "CLass", "btn3" },
+                            { "after", "after value" },
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "before", "before value" },
+                            { "class", "btn2 btn" },
+                            { "mid", "mid value" },
+                            { "after", "after value" },
+                        }
+                    },
+                    {
+                        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+                        {
+                            { "A", "A Value" },
+                            { "CLASS", "btn" },
+                            { "B", "B Value" },
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "before", "before value" },
+                            { "clASS", "btn2" },
+                            { "mid", "mid value" },
+                            { "CLass", "btn3" },
+                            { "after", "after value" },
+                        },
+                        new TagHelperAttributeList
+                        {
+                            { "before", "before value" },
+                            { "class", "btn2 btn" },
+                            { "mid", "mid value" },
+                            { "after", "after value" },
+                            { "A", "A Value" },
+                            { "B", "B Value" },
+                        }
+                    }
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(MultipleAttributeSameNameData))]
+        public void MergeAttributes_ClearsDuplicateClassNameAttributes(
+            Dictionary<string, string> tagBuilderAttributes,
+            TagHelperAttributeList outputAttributes,
+            TagHelperAttributeList expectedAttributes)
+        {
+            // Arrange
+            var tagHelperOutput = new TagHelperOutput("p", outputAttributes);
+
+            var tagBuilder = new TagBuilder("p", new CommonTestEncoder());
+            foreach (var attr in tagBuilderAttributes)
+            {
+                tagBuilder.Attributes.Add(attr.Key, attr.Value);
+            }
+
+            // Act
+            tagHelperOutput.MergeAttributes(tagBuilder);
+
+            // Assert
+            Assert.Equal(
+                expectedAttributes,
+                tagHelperOutput.Attributes,
+                CaseSensitiveTagHelperAttributeComparer.Default);
+        }
+
         [Fact]
         public void MergeAttributes_DoesNotReplace_TagHelperOutputAttributeValues()
         {
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>());
-            var expectedAttribute = new KeyValuePair<string, object>("type", "btn");
+                attributes: new TagHelperAttributeList());
+            var expectedAttribute = new TagHelperAttribute("type", "btn");
             tagHelperOutput.Attributes.Add(expectedAttribute);
 
             var tagBuilder = new TagBuilder("p", new CommonTestEncoder());
@@ -182,13 +439,13 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             tagHelperOutput.Attributes.Add("class", "Hello");
 
             var tagBuilder = new TagBuilder("p", new CommonTestEncoder());
             tagBuilder.Attributes.Add("class", "btn");
 
-            var expectedAttribute = new KeyValuePair<string, object>("class", "Hello btn");
+            var expectedAttribute = new TagHelperAttribute("class", "Hello btn");
 
             // Act
             tagHelperOutput.MergeAttributes(tagBuilder);
@@ -208,7 +465,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             tagHelperOutput.Attributes.Add(originalName, "Hello");
 
             var tagBuilder = new TagBuilder("p", new CommonTestEncoder());
@@ -219,7 +476,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             var attribute = Assert.Single(tagHelperOutput.Attributes);
-            Assert.Equal(new KeyValuePair<string, object>(originalName, "Hello btn"), attribute);
+            Assert.Equal(new TagHelperAttribute(originalName, "Hello btn"), attribute);
         }
 
         [Fact]
@@ -228,10 +485,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
 
             var tagBuilder = new TagBuilder("p", new CommonTestEncoder());
-            var expectedAttribute = new KeyValuePair<string, object>("visible", "val < 3");
+            var expectedAttribute = new TagHelperAttribute("visible", "val < 3");
             tagBuilder.Attributes.Add("visible", "val < 3");
 
             // Act
@@ -248,11 +505,11 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
 
             var tagBuilder = new TagBuilder("p", new CommonTestEncoder());
-            var expectedAttribute1 = new KeyValuePair<string, object>("class", "btn");
-            var expectedAttribute2 = new KeyValuePair<string, object>("class2", "btn");
+            var expectedAttribute1 = new TagHelperAttribute("class", "btn");
+            var expectedAttribute2 = new TagHelperAttribute("class2", "btn");
             tagBuilder.Attributes.Add("class", "btn");
             tagBuilder.Attributes.Add("class2", "btn");
 
@@ -261,9 +518,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             Assert.Equal(2, tagHelperOutput.Attributes.Count);
-            var attribute = Assert.Single(tagHelperOutput.Attributes, kvp => kvp.Key.Equals("class"));
+            var attribute = Assert.Single(tagHelperOutput.Attributes, attr => attr.Name.Equals("class"));
             Assert.Equal(expectedAttribute1.Value, attribute.Value);
-            attribute = Assert.Single(tagHelperOutput.Attributes, kvp => kvp.Key.Equals("class2"));
+            attribute = Assert.Single(tagHelperOutput.Attributes, attr => attr.Name.Equals("class2"));
             Assert.Equal(expectedAttribute2.Value, attribute.Value);
         }
 
@@ -273,8 +530,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>());
-            var expectedAttribute = new KeyValuePair<string, object>("class", "btn");
+                attributes: new TagHelperAttributeList());
+            var expectedAttribute = new TagHelperAttribute("class", "btn");
             tagHelperOutput.Attributes.Add(expectedAttribute);
 
             var tagBuilder = new TagBuilder("p", new CommonTestEncoder());
@@ -293,12 +550,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Arrange
             var tagHelperOutput = new TagHelperOutput(
                 "p",
-                attributes: new Dictionary<string, object>());
-            var expectedOutputAttribute = new KeyValuePair<string, object>("class", "btn");
+                attributes: new TagHelperAttributeList());
+            var expectedOutputAttribute = new TagHelperAttribute("class", "btn");
             tagHelperOutput.Attributes.Add(expectedOutputAttribute);
 
             var tagBuilder = new TagBuilder("p", new CommonTestEncoder());
-            var expectedBuilderAttribute = new KeyValuePair<string, object>("for", "hello");
+            var expectedBuilderAttribute = new TagHelperAttribute("for", "hello");
             tagBuilder.Attributes.Add("for", "hello");
 
             // Act
@@ -306,10 +563,40 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             Assert.Equal(tagHelperOutput.Attributes.Count, 2);
-            var attribute = Assert.Single(tagHelperOutput.Attributes, kvp => kvp.Key.Equals("class"));
+            var attribute = Assert.Single(tagHelperOutput.Attributes, attr => attr.Name.Equals("class"));
             Assert.Equal(expectedOutputAttribute.Value, attribute.Value);
-            attribute = Assert.Single(tagHelperOutput.Attributes, kvp => kvp.Key.Equals("for"));
+            attribute = Assert.Single(tagHelperOutput.Attributes, attr => attr.Name.Equals("for"));
             Assert.Equal(expectedBuilderAttribute.Value, attribute.Value);
         }
+
+        private class CaseSensitiveTagHelperAttributeComparer : IEqualityComparer<IReadOnlyTagHelperAttribute>
+        {
+            public readonly static CaseSensitiveTagHelperAttributeComparer Default =
+                new CaseSensitiveTagHelperAttributeComparer();
+
+            private CaseSensitiveTagHelperAttributeComparer()
+            {
+            }
+
+            public bool Equals(
+                IReadOnlyTagHelperAttribute attributeX,
+                IReadOnlyTagHelperAttribute attributeY)
+            {
+                return
+                    attributeX == attributeY ||
+                    // Normal comparer doesn't care about the Name case, in tests we do.
+                    string.Equals(attributeX.Name, attributeY.Name, StringComparison.Ordinal) &&
+                    Equals(attributeX.Value, attributeY.Value);
+            }
+
+            public int GetHashCode(IReadOnlyTagHelperAttribute attribute)
+            {
+                return HashCodeCombiner
+                    .Start()
+                    .Add(attribute.Name, StringComparer.Ordinal)
+                    .Add(attribute.Value)
+                    .CombinedHash;
+            }
+        }
     }
 }
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/TextAreaTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/TextAreaTagHelperTest.cs
index 19f384094a..0cfa27f757 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/TextAreaTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/TextAreaTagHelperTest.cs
@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Mvc.ModelBinding;
 using Microsoft.AspNet.Mvc.Rendering;
@@ -88,7 +89,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             string expectedContent)
         {
             // Arrange
-            var expectedAttributes = new Dictionary<string, object>
+            var expectedAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
                 { "id", nameAndId.Id },
@@ -113,7 +114,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             };
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -122,7 +124,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     tagHelperContent.SetContent("Something");
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
-            var htmlAttributes = new Dictionary<string, object>
+            var htmlAttributes = new TagHelperAttributeList
             {
                 { "class", "form-control" },
             };
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ValidationMessageTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ValidationMessageTagHelperTest.cs
index da52bc0bdc..f249c59273 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ValidationMessageTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ValidationMessageTagHelperTest.cs
@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.ModelBinding;
@@ -34,7 +35,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedPostContent = "original post-content";
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>
+                allAttributes: new TagHelperAttributeList
                 {
                     { "id", "myvalidationmessage" },
                     { "for", modelExpression },
@@ -49,7 +50,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 expectedTagName,
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     { "id", "myvalidationmessage" }
                 });
@@ -68,13 +69,13 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             Assert.Equal(4, output.Attributes.Count);
-            var attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("id"));
+            var attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("id"));
             Assert.Equal("myvalidationmessage", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("class"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("class"));
             Assert.Equal("field-validation-valid", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-valmsg-for"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("data-valmsg-for"));
             Assert.Equal("Name", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-valmsg-replace"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("data-valmsg-replace"));
             Assert.Equal("true", attribute.Value);
             Assert.Equal(expectedPreContent, output.PreContent.GetContent());
             Assert.Equal(expectedContent, output.Content.GetContent());
@@ -94,7 +95,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedContent = "original content";
             var expectedPostContent = "original post-content";
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -105,7 +107,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 "span",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             output.PreContent.SetContent(expectedPreContent);
             output.Content.SetContent(expectedContent);
             output.PostContent.SetContent(expectedPostContent);
@@ -145,11 +147,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             };
             var output = new TagHelperOutput(
                 "span",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             output.Content.SetContent(outputContent);
 
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -184,9 +187,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Assert
             Assert.Equal("span", output.TagName);
             Assert.Equal(2, output.Attributes.Count);
-            var attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-foo"));
+            var attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("data-foo"));
             Assert.Equal("bar", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-hello"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("data-hello"));
             Assert.Equal("world", attribute.Value);
             Assert.Equal(expectedOutputContent, output.Content.GetContent());
         }
@@ -204,10 +207,11 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             };
             var output = new TagHelperOutput(
                 "span",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
 
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -242,9 +246,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Assert
             Assert.Equal("span", output.TagName);
             Assert.Equal(2, output.Attributes.Count);
-            var attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-foo"));
+            var attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("data-foo"));
             Assert.Equal("bar", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-hello"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("data-hello"));
             Assert.Equal("world", attribute.Value);
             Assert.Equal(expectedOutputContent, output.Content.GetContent());
         }
@@ -259,7 +263,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedPostContent = "original post-content";
             var output = new TagHelperOutput(
                 "span",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             output.PreContent.SetContent(expectedPreContent);
             output.Content.SetContent(expectedContent);
             output.PostContent.SetContent(expectedPostContent);
diff --git a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ValidationSummaryTagHelperTest.cs b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ValidationSummaryTagHelperTest.cs
index b02b9feb0f..73e2e89aab 100644
--- a/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ValidationSummaryTagHelperTest.cs
+++ b/test/Microsoft.AspNet.Mvc.TagHelpers.Test/ValidationSummaryTagHelperTest.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.ModelBinding;
@@ -44,7 +45,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedPreContent = "original pre-content";
             var expectedContent = "original content";
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(),
+                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test",
                 getChildContentAsync: () =>
@@ -55,7 +57,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 });
             var output = new TagHelperOutput(
                 expectedTagName,
-                attributes: new Dictionary<string, object>
+                attributes: new TagHelperAttributeList
                 {
                     { "class", "form-control" }
                 });
@@ -74,9 +76,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Assert
             Assert.Equal(2, output.Attributes.Count);
-            var attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("class"));
+            var attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("class"));
             Assert.Equal("form-control validation-summary-valid", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-valmsg-summary"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("data-valmsg-summary"));
             Assert.Equal("true", attribute.Value);
             Assert.Equal(expectedPreContent, output.PreContent.GetContent());
             Assert.Equal(expectedContent, output.Content.GetContent());
@@ -102,7 +104,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedPostContent = "original post-content";
             var output = new TagHelperOutput(
                 "div",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             output.PreContent.SetContent(expectedPreContent);
             output.Content.SetContent(expectedContent);
             output.PostContent.SetContent(expectedPostContent);
@@ -144,7 +146,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedContent = "original content";
             var output = new TagHelperOutput(
                 "div",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             output.PreContent.SetContent(expectedPreContent);
             output.Content.SetContent(expectedContent);
             output.PostContent.SetContent("Content of validation summary");
@@ -177,11 +179,11 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Assert
             Assert.Equal("div", output.TagName);
             Assert.Equal(3, output.Attributes.Count);
-            var attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-foo"));
+            var attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("data-foo"));
             Assert.Equal("bar", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("data-hello"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("data-hello"));
             Assert.Equal("world", attribute.Value);
-            attribute = Assert.Single(output.Attributes, kvp => kvp.Key.Equals("anything"));
+            attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("anything"));
             Assert.Equal("something", attribute.Value);
             Assert.Equal(expectedPreContent, output.PreContent.GetContent());
             Assert.Equal(expectedContent, output.Content.GetContent());
@@ -201,7 +203,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedPostContent = "original post-content";
             var output = new TagHelperOutput(
                 "div",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             output.PreContent.SetContent(expectedPreContent);
             output.Content.SetContent(expectedContent);
             output.PostContent.SetContent(expectedPostContent);
@@ -236,7 +238,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedContent = "original content";
             var output = new TagHelperOutput(
                 "div",
-                attributes: new Dictionary<string, object>());
+                attributes: new TagHelperAttributeList());
             output.PreContent.SetContent(expectedPreContent);
             output.Content.SetContent(expectedContent);
             output.PostContent.SetContent("Content of validation message");
diff --git a/test/WebSites/TagHelpersWebSite/TagHelpers/ATagHelper.cs b/test/WebSites/TagHelpersWebSite/TagHelpers/ATagHelper.cs
index 96404619c5..6f5a91c50d 100644
--- a/test/WebSites/TagHelpersWebSite/TagHelpers/ATagHelper.cs
+++ b/test/WebSites/TagHelpersWebSite/TagHelpers/ATagHelper.cs
@@ -21,8 +21,8 @@ namespace TagHelpersWebSite.TagHelpers
         {
             if (Controller != null && Action != null)
             {
-                var methodParameters = output.Attributes.ToDictionary(attribute => attribute.Key,
-                                                                      attribute => (object)attribute.Value);
+                var methodParameters = output.Attributes.ToDictionary(attribute => attribute.Name,
+                                                                      attribute => attribute.Value);
 
                 // We remove all attributes from the resulting HTML element because they're supposed to
                 // be parameters to our final href value.