Add `[HtmlAttributeName(..., DictionaryAttributePrefix="prefix")]` part III

- #89 remainder
- support adding attributes (that aren't otherwise bound) to a tag helper dictionary
- use two `TagHelperAttributeDescriptor`s for dictionary and indexer
 - most exising descriptor properties have two meanings depending on new `IsIndexer`
 - add `TagHelperAttributeDescriptor.IsNameMatch()`
- create no `TagHelperAttributeDescriptor`s if property name or `HtmlAttributeNameAttribute`
  is invalid
 - avoid corner case misfeatures where invalidity removes just one descriptor
- extend handling of invalid attribute names to include `[HtmlAttributeName]`
- handle prefix matches in `TagHelperBlockRewriter`
 - add parse error when resolved dictionary key is `string.Empty`
- generate code for indexer property assignments
 - add code generation for runtime error if using indexer when property is `null`
 - use new `GeneratedTagHelperContext.FormatInvalidIndexerAssignmentMethodName` for message
- code generation now handles attributes in source order; thus above errors occur only when
  expected if dictionary is also initialized in the Razor source
 - surprisingly generation order change did not break existing tests!

nits:
- improve `TagHelperDescriptorFactory_InvalidBoundAttributeName` wording
 - rename resource to `TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixStart`
- correct order of arguments to `FormatTagHelperDescriptorFactory_InvalidBoundAttributeName`
- correct `TagHelperBlockRewriter_TagHelperAttributeListMustBeWellFormed` resource
- correct `TagHelperDescriptorFactoryTest` test names
 - remove a few unnecessary `ToArray()` calls
- update `TagHelperAttributeDescriptorComparer` comments

src/Microsoft.AspNet.Razor.Runtime/Properties/Resources.Designer.cs

@@ -186,6 +186,38 @@ namespace Microsoft.AspNet.Razor.Runtime
             return GetString("TagHelperDescriptorFactory_Attribute");
         }
 
+        /// <summary>
+        /// name
+        /// </summary>
+        internal static string TagHelperDescriptorFactory_Name
+        {
+            get { return GetString("TagHelperDescriptorFactory_Name"); }
+        }
+
+        /// <summary>
+        /// name
+        /// </summary>
+        internal static string FormatTagHelperDescriptorFactory_Name()
+        {
+            return GetString("TagHelperDescriptorFactory_Name");
+        }
+
+        /// <summary>
+        /// prefix
+        /// </summary>
+        internal static string TagHelperDescriptorFactory_Prefix
+        {
+            get { return GetString("TagHelperDescriptorFactory_Prefix"); }
+        }
+
+        /// <summary>
+        /// prefix
+        /// </summary>
+        internal static string FormatTagHelperDescriptorFactory_Prefix()
+        {
+            return GetString("TagHelperDescriptorFactory_Prefix");
+        }
+
         /// <summary>
         /// Tag
         /// </summary>
@@ -203,19 +235,67 @@ namespace Microsoft.AspNet.Razor.Runtime
         }
 
         /// <summary>
-        /// Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes beginning with '{2}'.
+        /// Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes with {2} '{3}' because {2} contains a '{4}' character.
         /// </summary>
-        internal static string TagHelperDescriptorFactory_InvalidBoundAttributeName
+        internal static string TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixCharacter
         {
-            get { return GetString("TagHelperDescriptorFactory_InvalidBoundAttributeName"); }
+            get { return GetString("TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixCharacter"); }
         }
 
         /// <summary>
-        /// Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes beginning with '{2}'.
+        /// Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes with {2} '{3}' because {2} contains a '{4}' character.
         /// </summary>
-        internal static string FormatTagHelperDescriptorFactory_InvalidBoundAttributeName(object p0, object p1, object p2)
+        internal static string FormatTagHelperDescriptorFactory_InvalidAttributeNameOrPrefixCharacter(object p0, object p1, object p2, object p3, object p4)
         {
-            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperDescriptorFactory_InvalidBoundAttributeName"), p0, p1, p2);
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixCharacter"), p0, p1, p2, p3, p4);
+        }
+
+        /// <summary>
+        /// Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes with {2} '{3}' because {2} starts with '{4}'.
+        /// </summary>
+        internal static string TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixStart
+        {
+            get { return GetString("TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixStart"); }
+        }
+
+        /// <summary>
+        /// Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes with {2} '{3}' because {2} starts with '{4}'.
+        /// </summary>
+        internal static string FormatTagHelperDescriptorFactory_InvalidAttributeNameOrPrefixStart(object p0, object p1, object p2, object p3, object p4)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixStart"), p0, p1, p2, p3, p4);
+        }
+
+        /// <summary>
+        /// Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes with a whitespace {2}.
+        /// </summary>
+        internal static string TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixWhitespace
+        {
+            get { return GetString("TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixWhitespace"); }
+        }
+
+        /// <summary>
+        /// Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes with a whitespace {2}.
+        /// </summary>
+        internal static string FormatTagHelperDescriptorFactory_InvalidAttributeNameOrPrefixWhitespace(object p0, object p1, object p2)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixWhitespace"), p0, p1, p2);
+        }
+
+        /// <summary>
+        /// Invalid tag helper bound property '{0}.{1}'. '{2}.{3}' must be null unless property type implements '{4}'.
+        /// </summary>
+        internal static string TagHelperDescriptorFactory_InvalidAttributePrefix
+        {
+            get { return GetString("TagHelperDescriptorFactory_InvalidAttributePrefix"); }
+        }
+
+        /// <summary>
+        /// Invalid tag helper bound property '{0}.{1}'. '{2}.{3}' must be null unless property type implements '{4}'.
+        /// </summary>
+        internal static string FormatTagHelperDescriptorFactory_InvalidAttributePrefix(object p0, object p1, object p2, object p3, object p4)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperDescriptorFactory_InvalidAttributePrefix"), p0, p1, p2, p3, p4);
         }
 
         /// <summary>

src/Microsoft.AspNet.Razor.Runtime/Resources.resx

@@ -1,17 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
 <root>
-  <!-- 
+  <!--
-    Microsoft ResX Schema 
+    Microsoft ResX Schema
-    
+
     Version 2.0
-    
+
-    The primary goals of this format is to allow a simple XML format 
+    The primary goals of this format is to allow a simple XML format
-    that is mostly human readable. The generation and parsing of the 
+    that is mostly human readable. The generation and parsing of the
-    various data types are done through the TypeConverter classes 
+    various data types are done through the TypeConverter classes
     associated with the data types.
-    
+
     Example:
-    
+
     ... ado.net/XML headers & schema ...
     <resheader name="resmimetype">text/microsoft-resx</resheader>
     <resheader name="version">2.0</resheader>
@@ -26,36 +26,36 @@
         <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
         <comment>This is a comment</comment>
     </data>
-                
+
-    There are any number of "resheader" rows that contain simple 
+    There are any number of "resheader" rows that contain simple
     name/value pairs.
-    
+
-    Each data row contains a name, and value. The row also contains a 
+    Each data row contains a name, and value. The row also contains a
-    type or mimetype. Type corresponds to a .NET class that support 
+    type or mimetype. Type corresponds to a .NET class that support
-    text/value conversion through the TypeConverter architecture. 
+    text/value conversion through the TypeConverter architecture.
-    Classes that don't support this are serialized and stored with the 
+    Classes that don't support this are serialized and stored with the
     mimetype set.
-    
+
-    The mimetype is used for serialized objects, and tells the 
+    The mimetype is used for serialized objects, and tells the
-    ResXResourceReader how to depersist the object. This is currently not 
+    ResXResourceReader how to depersist the object. This is currently not
     extensible. For a given mimetype the value must be set accordingly:
-    
+
-    Note - application/x-microsoft.net.object.binary.base64 is the format 
+    Note - application/x-microsoft.net.object.binary.base64 is the format
-    that the ResXResourceWriter will generate, however the reader can 
+    that the ResXResourceWriter will generate, however the reader can
     read any of the formats listed below.
-    
+
     mimetype: application/x-microsoft.net.object.binary.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
             : and then encoded with base64 encoding.
-    
+
     mimetype: application/x-microsoft.net.object.soap.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
             : and then encoded with base64 encoding.
 
     mimetype: application/x-microsoft.net.object.bytearray.base64
-    value   : The object must be serialized into a byte array 
+    value   : The object must be serialized into a byte array
             : using a System.ComponentModel.TypeConverter
             : and then encoded with base64 encoding.
     -->
@@ -150,11 +150,26 @@
   <data name="TagHelperDescriptorFactory_Attribute" xml:space="preserve">
     <value>Attribute</value>
   </data>
+  <data name="TagHelperDescriptorFactory_Name" xml:space="preserve">
+    <value>name</value>
+  </data>
+  <data name="TagHelperDescriptorFactory_Prefix" xml:space="preserve">
+    <value>prefix</value>
+  </data>
   <data name="TagHelperDescriptorFactory_Tag" xml:space="preserve">
     <value>Tag</value>
   </data>
-  <data name="TagHelperDescriptorFactory_InvalidBoundAttributeName" xml:space="preserve">
+  <data name="TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixCharacter" xml:space="preserve">
-    <value>Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes beginning with '{2}'.</value>
+    <value>Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes with {2} '{3}' because {2} contains a '{4}' character.</value>
+  </data>
+  <data name="TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixStart" xml:space="preserve">
+    <value>Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes with {2} '{3}' because {2} starts with '{4}'.</value>
+  </data>
+  <data name="TagHelperDescriptorFactory_InvalidAttributeNameOrPrefixWhitespace" xml:space="preserve">
+    <value>Invalid tag helper bound property '{0}.{1}'. Tag helpers cannot bind to HTML attributes with a whitespace {2}.</value>
+  </data>
+  <data name="TagHelperDescriptorFactory_InvalidAttributePrefix" xml:space="preserve">
+    <value>Invalid tag helper bound property '{0}.{1}'. '{2}.{3}' must be null unless property type implements '{4}'.</value>
   </data>
   <data name="TagHelperAttributeList_CannotAddWithNullName" xml:space="preserve">
     <value>Cannot add a '{0}' with a null '{1}'.</value>

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/HtmlAttributeNameAttribute.cs

@@ -11,6 +11,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
     [AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
     public sealed class HtmlAttributeNameAttribute : Attribute
     {
+        private string _dictionaryAttributePrefix;
+
         /// <summary>
         /// Instantiates a new instance of the <see cref="HtmlAttributeNameAttribute"/> class.
         /// </summary>
@@ -29,5 +31,39 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         /// HTML attribute name of the associated property.
         /// </summary>
         public string Name { get; }
+
+        /// <summary>
+        /// Gets or sets the prefix used to match HTML attribute names. Matching attributes are added to the
+        /// associated property (an <see cref="System.Collections.Generic.IDictionary{TKey, TValue}"/>).
+        /// </summary>
+        /// <remarks>
+        /// If non-<c>null</c> associated property must be compatible with
+        /// <see cref="System.Collections.Generic.IDictionary{TKey, TValue}"/> where <c>TKey</c> is
+        /// <see cref="string"/>.
+        /// </remarks>
+        /// <value>
+        /// If associated property is compatible with
+        /// <see cref="System.Collections.Generic.IDictionary{TKey, TValue}"/>, default value is <c>Name + "-"</c>.
+        /// Otherwise default value is <c>null</c>.
+        /// </value>
+        public string DictionaryAttributePrefix
+        {
+            get
+            {
+                return _dictionaryAttributePrefix;
+            }
+            set
+            {
+                _dictionaryAttributePrefix = value;
+                DictionaryAttributePrefixSet = true;
+            }
+        }
+
+        /// <summary>
+        /// Gets an indication whether <see cref="DictionaryAttributePrefix"/> has been set. Used to distinguish an
+        /// uninitialized <see cref="DictionaryAttributePrefix"/> value from an explicit <c>null</c> setting.
+        /// </summary>
+        /// <value><c>true</c> if <see cref="DictionaryAttributePrefix"/> was set. <c>false</c> otherwise.</value>
+        public bool DictionaryAttributePrefixSet { get; private set; }
     }
 }

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperDescriptorFactory.cs

@@ -38,7 +38,9 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         /// </summary>
         /// <param name="assemblyName">The assembly name that contains <paramref name="type"/>.</param>
         /// <param name="type">The type to create a <see cref="TagHelperDescriptor"/> from.</param>
-        /// <returns>A <see cref="TagHelperDescriptor"/> that describes the given <paramref name="type"/>.</returns>
+        /// <returns>
+        /// A collection of <see cref="TagHelperDescriptor"/>s that describe the given <paramref name="type"/>.
+        /// </returns>
         public static IEnumerable<TagHelperDescriptor> CreateDescriptors(
             string assemblyName,
             [NotNull] Type type,
@@ -209,48 +211,195 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             var accessibleProperties = type.GetRuntimeProperties().Where(IsAccessibleProperty);
             var attributeDescriptors = new List<TagHelperAttributeDescriptor>();
 
+            // Keep indexer descriptors separate to avoid sorting the combined list later.
+            var indexerDescriptors = new List<TagHelperAttributeDescriptor>();
+
             foreach (var property in accessibleProperties)
             {
-                var descriptor = ToAttributeDescriptor(property);
+                var attributeNameAttribute = property.GetCustomAttribute<HtmlAttributeNameAttribute>(inherit: false);
+                var descriptor = ToAttributeDescriptor(property, attributeNameAttribute);
                 if (ValidateTagHelperAttributeDescriptor(descriptor, type, errorSink))
                 {
+                    bool isInvalid;
+                    var indexerDescriptor = ToIndexerAttributeDescriptor(
+                        property,
+                        attributeNameAttribute,
+                        parentType: type,
+                        errorSink: errorSink,
+                        defaultPrefix: descriptor.Name + "-",
+                        isInvalid: out isInvalid);
+
+                    if (indexerDescriptor != null &&
+                        !ValidateTagHelperAttributeDescriptor(indexerDescriptor, type, errorSink))
+                    {
+                        isInvalid = true;
+                    }
+
+                    if (isInvalid)
+                    {
+                        // HtmlAttributeNameAttribute was not valid. Ignore this property completely.
+                        continue;
+                    }
+
                     attributeDescriptors.Add(descriptor);
+                    if (indexerDescriptor != null)
+                    {
+                        indexerDescriptors.Add(indexerDescriptor);
+                    }
                 }
             }
 
+            attributeDescriptors.AddRange(indexerDescriptors);
+
             return attributeDescriptors;
         }
 
-        private static bool ValidateTagHelperAttributeDescriptor(
+        // Internal for testing.
+        internal static bool ValidateTagHelperAttributeDescriptor(
             TagHelperAttributeDescriptor attributeDescriptor,
             Type parentType,
             ErrorSink errorSink)
         {
+            var nameOrPrefix = attributeDescriptor.IsIndexer ?
+                Resources.TagHelperDescriptorFactory_Prefix :
+                Resources.TagHelperDescriptorFactory_Name;
+            return ValidateTagHelperAttributeNameOrPrefix(
+                attributeDescriptor.Name,
+                parentType,
+                attributeDescriptor.PropertyName,
+                errorSink,
+                nameOrPrefix);
+        }
+
+        private static bool ValidateTagHelperAttributeNameOrPrefix(
+            string attributeNameOrPrefix,
+            Type parentType,
+            string propertyName,
+            ErrorSink errorSink,
+            string nameOrPrefix)
+        {
+            if (string.IsNullOrEmpty(attributeNameOrPrefix))
+            {
+                // HtmlAttributeNameAttribute validates Name is non-null and non-empty. Both are valid for
+                // DictionaryAttributePrefix. (Empty DictionaryAttributePrefix is a corner case which would bind every
+                // attribute of a target element. Likely not particularly useful but unclear what minimum length
+                // should be required and what scenarios a minimum length would break.)
+                return true;
+            }
+
+            if (string.IsNullOrWhiteSpace(attributeNameOrPrefix))
+            {
+                // Provide a single error if the entire name is whitespace, not an error per character.
+                errorSink.OnError(
+                    SourceLocation.Zero,
+                    Resources.FormatTagHelperDescriptorFactory_InvalidAttributeNameOrPrefixWhitespace(
+                        parentType.FullName,
+                        propertyName,
+                        nameOrPrefix));
+
+                return false;
+            }
+
             // data-* attributes are explicitly not implemented by user agents and are not intended for use on
             // the server; therefore it's invalid for TagHelpers to bind to them.
-            if (attributeDescriptor.Name.StartsWith(DataDashPrefix, StringComparison.OrdinalIgnoreCase))
+            if (attributeNameOrPrefix.StartsWith(DataDashPrefix, StringComparison.OrdinalIgnoreCase))
             {
                 errorSink.OnError(
                     SourceLocation.Zero,
-                    Resources.FormatTagHelperDescriptorFactory_InvalidBoundAttributeName(
+                    Resources.FormatTagHelperDescriptorFactory_InvalidAttributeNameOrPrefixStart(
-                        attributeDescriptor.PropertyName,
                         parentType.FullName,
+                        propertyName,
+                        nameOrPrefix,
+                        attributeNameOrPrefix,
                         DataDashPrefix));
 
                 return false;
             }
 
-            return true;
+            var isValid = true;
+            foreach (var character in attributeNameOrPrefix)
+            {
+                if (char.IsWhiteSpace(character) || InvalidNonWhitespaceNameCharacters.Contains(character))
+                {
+                    errorSink.OnError(
+                        SourceLocation.Zero,
+                        Resources.FormatTagHelperDescriptorFactory_InvalidAttributeNameOrPrefixCharacter(
+                            parentType.FullName,
+                            propertyName,
+                            nameOrPrefix,
+                            attributeNameOrPrefix,
+                            character));
+
+                    isValid = false;
+                }
+            }
+
+            return isValid;
         }
 
-        private static TagHelperAttributeDescriptor ToAttributeDescriptor(PropertyInfo property)
+        private static TagHelperAttributeDescriptor ToAttributeDescriptor(
+            PropertyInfo property,
+            HtmlAttributeNameAttribute attributeNameAttribute)
         {
-            var attributeNameAttribute = property.GetCustomAttribute<HtmlAttributeNameAttribute>(inherit: false);
             var attributeName = attributeNameAttribute != null ?
                                 attributeNameAttribute.Name :
                                 ToHtmlCase(property.Name);
 
-            return new TagHelperAttributeDescriptor(attributeName, property.Name, property.PropertyType.FullName);
+            return new TagHelperAttributeDescriptor(
+                attributeName,
+                property.Name,
+                property.PropertyType.FullName,
+                isIndexer: false);
+        }
+
+        private static TagHelperAttributeDescriptor ToIndexerAttributeDescriptor(
+            PropertyInfo property,
+            HtmlAttributeNameAttribute attributeNameAttribute,
+            Type parentType,
+            ErrorSink errorSink,
+            string defaultPrefix,
+            out bool isInvalid)
+        {
+            isInvalid = false;
+            var dictionaryTypeArguments = ClosedGenericMatcher.ExtractGenericInterface(
+                    property.PropertyType,
+                    typeof(IDictionary<,>))
+                ?.GenericTypeArguments;
+            if (dictionaryTypeArguments?[0] != typeof(string))
+            {
+                if (attributeNameAttribute?.DictionaryAttributePrefix != null)
+                {
+                    // DictionaryAttributePrefix is not supported unless associated with an
+                    // IDictionary<string, TValue> property.
+                    isInvalid = true;
+                    errorSink.OnError(
+                        SourceLocation.Zero,
+                        Resources.FormatTagHelperDescriptorFactory_InvalidAttributePrefix(
+                            parentType.FullName,
+                            property.Name,
+                            nameof(HtmlAttributeNameAttribute),
+                            nameof(HtmlAttributeNameAttribute.DictionaryAttributePrefix),
+                            "IDictionary<string, TValue>"));
+                }
+
+                return null;
+            }
+
+            // Potential prefix case. Use default prefix (based on name)?
+            var useDefault = attributeNameAttribute == null || !attributeNameAttribute.DictionaryAttributePrefixSet;
+
+            var prefix = useDefault ? defaultPrefix : attributeNameAttribute.DictionaryAttributePrefix;
+            if (prefix == null)
+            {
+                // DictionaryAttributePrefix explicitly set to null. Ignore.
+                return null;
+            }
+
+            return new TagHelperAttributeDescriptor(
+                name: prefix,
+                propertyName: property.Name,
+                typeName: dictionaryTypeArguments[1].FullName,
+                isIndexer: true);
         }
 
         private static bool IsAccessibleProperty(PropertyInfo property)

src/Microsoft.AspNet.Razor.Runtime/project.json

@@ -4,6 +4,7 @@
     "dependencies": {
         "Microsoft.AspNet.Razor": "4.0.0-*",
         "Microsoft.Framework.BufferEntryCollection.Sources": { "type": "build", "version": "1.0.0-*" },
+        "Microsoft.Framework.ClosedGenericMatcher.Sources": { "type": "build", "version": "1.0.0-*" },
         "Microsoft.Framework.CopyOnWriteDictionary.Sources": { "type": "build", "version": "1.0.0-*" },
         "Microsoft.Framework.NotNullAttribute.Sources": { "type": "build", "version": "1.0.0-*" }
     },

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/CSharpTagHelperCodeRenderer.cs

@@ -69,10 +69,9 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
 
             // Determine what attributes exist in the element and divide them up.
             var htmlAttributes = chunk.Attributes;
-            var attributeDescriptors = tagHelperDescriptors.SelectMany(descriptor => descriptor.Attributes);
+            var attributeDescriptors = tagHelperDescriptors.SelectMany(descriptor => descriptor.Attributes).ToArray();
-            var unboundHtmlAttributes = htmlAttributes.Where(
+            var unboundHtmlAttributes = htmlAttributes.Where(attribute => !attributeDescriptors.Any(
-                attribute => !attributeDescriptors.Any(
+                attributeDescriptor => attributeDescriptor.IsNameMatch(attribute.Key)));
-                    descriptor => string.Equals(attribute.Key, descriptor.Name, StringComparison.OrdinalIgnoreCase)));
 
             RenderUnboundHTMLAttributes(unboundHtmlAttributes);
 
@@ -182,6 +181,7 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
                 // Render all of the bound attribute values for the tag helper.
                 RenderBoundHTMLAttributes(
                     chunk.Attributes,
+                    tagHelperDescriptor.TypeName,
                     tagHelperVariableName,
                     tagHelperDescriptor.Attributes,
                     htmlAttributeValues);
@@ -190,81 +190,126 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
 
         private void RenderBoundHTMLAttributes(
             IList<KeyValuePair<string, Chunk>> chunkAttributes,
+            string tagHelperTypeName,
             string tagHelperVariableName,
             IEnumerable<TagHelperAttributeDescriptor> attributeDescriptors,
             Dictionary<string, string> htmlAttributeValues)
         {
-            foreach (var attributeDescriptor in attributeDescriptors)
+            // Track dictionary properties we have confirmed are non-null.
+            var confirmedDictionaries = new HashSet<string>(StringComparer.Ordinal);
+
+            // First attribute wins, even if there are duplicates.
+            var distinctAttributes = chunkAttributes.Distinct(KeyValuePairKeyComparer.Default);
+
+            // Go through the HTML attributes in source order, assigning to properties or indexers as we go.
+            foreach (var attributeKeyValuePair in distinctAttributes)
             {
-                var matchingAttributes = chunkAttributes.Where(
+                var attributeName = attributeKeyValuePair.Key;
-                    kvp => string.Equals(kvp.Key, attributeDescriptor.Name, StringComparison.OrdinalIgnoreCase));
+                var attributeValueChunk = attributeKeyValuePair.Value;
-
+                if (attributeValueChunk == null)
-                if (matchingAttributes.Any())
                 {
-                    // First attribute wins, even if there's duplicates.
+                    // Minimized attributes are not valid for bound attributes. TagHelperBlockRewriter has already
-                    var firstAttribute = matchingAttributes.First();
+                    // logged an error if it was a bound attribute; so we can skip.
-                    var attributeValueChunk = firstAttribute.Value;
+                    continue;
+                }
 
-                    // Minimized attributes are not valid for bound attributes. There will be an error for the bound
+                // Find the matching TagHelperAttributeDescriptor.
-                    // attribute logged by TagHelperBlockRewriter already so we can skip.
+                var attributeDescriptor = attributeDescriptors.FirstOrDefault(
-                    if (attributeValueChunk == null)
+                    descriptor => descriptor.IsNameMatch(attributeName));
+                if (attributeDescriptor == null)
+                {
+                    // Attribute is not bound to a property or indexer in this tag helper.
+                    continue;
+                }
+
+                // We capture the tag helper's property value accessor so we can retrieve it later (if we need to).
+                var valueAccessor = string.Format(
+                    CultureInfo.InvariantCulture,
+                    "{0}.{1}",
+                    tagHelperVariableName,
+                    attributeDescriptor.PropertyName);
+
+                if (attributeDescriptor.IsIndexer)
+                {
+                    // Need a different valueAccessor in this case. But first need to throw a reasonable Exception at
+                    // runtime if the property is null. The check is not required at design time.
+                    if (!_designTimeMode && confirmedDictionaries.Add(attributeDescriptor.PropertyName))
+                    {
+                        _writer
+                            .Write("if (")
+                            .Write(valueAccessor)
+                            .WriteLine(" == null)");
+                        using (_writer.BuildScope())
+                        {
+                            // System is in Host.NamespaceImports for all MVC scenarios. No need to generate FullName
+                            // of InvalidOperationException type.
+                            _writer
+                                .Write("throw ")
+                                .WriteStartNewObject(nameof(InvalidOperationException))
+                                .WriteStartMethodInvocation(_tagHelperContext.FormatInvalidIndexerAssignmentMethodName)
+                                .WriteStringLiteral(attributeName)
+                                .WriteParameterSeparator()
+                                .WriteStringLiteral(tagHelperTypeName)
+                                .WriteParameterSeparator()
+                                .WriteStringLiteral(attributeDescriptor.PropertyName)
+                                .WriteEndMethodInvocation(endLine: false)   // End of method call
+                                .WriteEndMethodInvocation(endLine: true);   // End of new expression / throw statement
+                        }
+                    }
+
+                    var dictionaryKey = attributeName.Substring(attributeDescriptor.Name.Length);
+                    valueAccessor = string.Format(
+                        CultureInfo.InvariantCulture,
+                        "{0}.{1}[\"{2}\"]",
+                        tagHelperVariableName,
+                        attributeDescriptor.PropertyName,
+                        dictionaryKey);
+                }
+
+                // If we haven't recorded this attribute value before then we need to record its value.
+                var attributeValueRecorded = htmlAttributeValues.ContainsKey(attributeName);
+                if (!attributeValueRecorded)
+                {
+                    // We only need to create attribute values once per HTML element (not once per tag helper).
+                    // We're saving the value accessor so we can retrieve it later if there are more tag
+                    // helpers that need the value.
+                    htmlAttributeValues.Add(attributeName, valueAccessor);
+
+                    // Bufferable attributes are attributes that can have Razor code inside of them. Such
+                    // attributes have string values and may be calculated using a temporary TextWriter or other
+                    // buffer.
+                    var bufferableAttribute = attributeDescriptor.IsStringProperty;
+
+                    RenderNewAttributeValueAssignment(
+                        attributeDescriptor,
+                        bufferableAttribute,
+                        attributeValueChunk,
+                        valueAccessor);
+
+                    // Execution contexts are a runtime feature.
+                    if (_designTimeMode)
                     {
                         continue;
                     }
 
-                    // We capture the tag helpers property value accessor so we can retrieve it later (if we need to).
+                    // We need to inform the context of the attribute value.
-                    var valueAccessor = string.Format(
+                    _writer
-                        CultureInfo.InvariantCulture,
+                        .WriteStartInstanceMethodInvocation(
-                        "{0}.{1}",
+                            ExecutionContextVariableName,
-                        tagHelperVariableName,
+                            _tagHelperContext.ExecutionContextAddTagHelperAttributeMethodName)
-                        attributeDescriptor.PropertyName);
+                        .WriteStringLiteral(attributeName)
-
+                        .WriteParameterSeparator()
-                    // If we haven't recorded this attribute value before then we need to record its value.
+                        .Write(valueAccessor)
-                    var attributeValueRecorded = htmlAttributeValues.ContainsKey(attributeDescriptor.Name);
+                        .WriteEndMethodInvocation();
-                    if (!attributeValueRecorded)
+                }
-                    {
+                else
-                        // We only need to create attribute values once per HTML element (not once per tag helper).
+                {
-                        // We're saving the value accessor so we can retrieve it later if there are more tag
+                    // The attribute value has already been recorded, lets retrieve it from the stored value
-                        // helpers that need the value.
+                    // accessors.
-                        htmlAttributeValues.Add(attributeDescriptor.Name, valueAccessor);
+                    _writer
-
+                        .WriteStartAssignment(valueAccessor)
-                        // Bufferable attributes are attributes that can have Razor code inside of them. Such
+                        .Write(htmlAttributeValues[attributeName])
-                        // attributes have string values and may be calculated using a temporary TextWriter or other
+                        .WriteLine(";");
-                        // buffer.
-                        var bufferableAttribute = attributeDescriptor.IsStringProperty;
-
-                        RenderNewAttributeValueAssignment(
-                            attributeDescriptor,
-                            bufferableAttribute,
-                            attributeValueChunk,
-                            valueAccessor);
-
-                        // Execution contexts are a runtime feature.
-                        if (_designTimeMode)
-                        {
-                            continue;
-                        }
-
-                        // We need to inform the context of the attribute value.
-                        var attributeName = firstAttribute.Key;
-                        _writer
-                            .WriteStartInstanceMethodInvocation(
-                                ExecutionContextVariableName,
-                                _tagHelperContext.ExecutionContextAddTagHelperAttributeMethodName)
-                            .WriteStringLiteral(attributeName)
-                            .WriteParameterSeparator()
-                            .Write(valueAccessor)
-                            .WriteEndMethodInvocation();
-                    }
-                    else
-                    {
-                        // The attribute value has already been recorded, lets retrieve it from the stored value
-                        // accessors.
-                        _writer
-                            .WriteStartAssignment(valueAccessor)
-                            .Write(htmlAttributeValues[attributeDescriptor.Name])
-                            .WriteLine(";");
-                    }
                 }
             }
         }
@@ -580,6 +625,26 @@ namespace Microsoft.AspNet.Razor.Generator.Compiler.CSharp
             return true;
         }
 
+        // An IEqualityComparer for string -> Chunk mappings which compares only the Key.
+        private class KeyValuePairKeyComparer : IEqualityComparer<KeyValuePair<string, Chunk>>
+        {
+            public static KeyValuePairKeyComparer Default = new KeyValuePairKeyComparer();
+
+            private KeyValuePairKeyComparer()
+            {
+            }
+
+            public bool Equals(KeyValuePair<string, Chunk> keyValuePairX, KeyValuePair<string, Chunk> keyValuePairY)
+            {
+                return string.Equals(keyValuePairX.Key, keyValuePairY.Key, StringComparison.OrdinalIgnoreCase);
+            }
+
+            public int GetHashCode(KeyValuePair<string, Chunk> keyValuePair)
+            {
+                return keyValuePair.Key.GetHashCode();
+            }
+        }
+
         // A CSharpCodeVisitor which does not HTML encode values. Used when rendering bound string attribute values.
         private class CSharpLiteralCodeVisitor : CSharpCodeVisitor
         {

src/Microsoft.AspNet.Razor/Generator/GeneratedTagHelperContext.cs

@@ -22,6 +22,7 @@ namespace Microsoft.AspNet.Razor.Generator
             ExecutionContextAddMinimizedHtmlAttributeMethodName = "AddMinimizedHtmlAttribute";
             ExecutionContextAddHtmlAttributeMethodName = "AddHtmlAttribute";
             ExecutionContextOutputPropertyName = "Output";
+            FormatInvalidIndexerAssignmentMethodName = "FormatInvalidIndexerAssignment";
             MarkAsHtmlEncodedMethodName = "Html.Raw";
             StartTagHelperWritingScopeMethodName = "StartTagHelperWritingScope";
             EndTagHelperWritingScopeMethodName = "EndTagHelperWritingScope";
@@ -78,6 +79,21 @@ namespace Microsoft.AspNet.Razor.Generator
         /// </summary>
         public string ExecutionContextOutputPropertyName { get; set; }
 
+        /// <summary>
+        /// The name of the method used to format an error message about using an indexer when the tag helper property
+        /// is <c>null</c>.
+        /// </summary>
+        /// <remarks>
+        /// Method signature should be
+        /// <code>
+        /// public string FormatInvalidIndexerAssignment(
+        ///     string attributeName,       // Name of the HTML attribute associated with the indexer.
+        ///     string tagHelperTypeName,   // Full name of the tag helper type.
+        ///     string propertyName)        // Dictionary property in the tag helper.
+        /// </code>
+        /// </remarks>
+        public string FormatInvalidIndexerAssignmentMethodName { get; set; }
+
         /// <summary>
         /// The name of the method used to wrap a <see cref="string"/> value and mark it as HTML-encoded.
         /// </summary>

src/Microsoft.AspNet.Razor/Parser/TagHelpers/TagHelperBlockRewriter.cs

@@ -1,7 +1,6 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
-using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -65,16 +64,18 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                 // Only want to track the attribute if we succeeded in parsing its corresponding Block/Span.
                 if (result != null)
                 {
+                    SourceLocation? errorLocation = null;
+
                     // Check if it's a bound attribute that is minimized or if it's a bound non-string attribute that
                     // is null or whitespace.
                     if ((result.IsBoundAttribute && result.AttributeValueNode == null) ||
                         (result.IsBoundNonStringAttribute &&
                          IsNullOrWhitespaceAttributeValue(result.AttributeValueNode)))
                     {
-                        var errorLocation = GetAttributeNameStartLocation(child);
+                        errorLocation = GetAttributeNameStartLocation(child);
 
                         errorSink.OnError(
-                            errorLocation,
+                            errorLocation.Value,
                             RazorResources.FormatRewriterError_EmptyTagHelperBoundAttribute(
                                 result.AttributeName,
                                 tagName,
@@ -82,6 +83,23 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                             result.AttributeName.Length);
                     }
 
+                    // Check if the attribute was a prefix match for a tag helper dictionary property but the
+                    // dictionary key would be the empty string.
+                    if (result.IsMissingDictionaryKey)
+                    {
+                        if (!errorLocation.HasValue)
+                        {
+                            errorLocation = GetAttributeNameStartLocation(child);
+                        }
+
+                        errorSink.OnError(
+                            errorLocation.Value,
+                            RazorResources.FormatTagHelperBlockRewriter_IndexerAttributeNameMustIncludeKey(
+                                result.AttributeName,
+                                tagName),
+                            result.AttributeName.Length);
+                    }
+
                     attributes.Add(
                         new KeyValuePair<string, SyntaxTreeNode>(result.AttributeName, result.AttributeValueNode));
                 }
@@ -224,24 +242,19 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                 {
                     errorSink.OnError(
                         span.Start,
-                        RazorResources.TagHelperBlockRewriter_TagHelperAttributeListMustBeWelformed,
+                        RazorResources.TagHelperBlockRewriter_TagHelperAttributeListMustBeWellFormed,
                         span.Content.Length);
                 }
 
                 return null;
             }
 
-            bool isBoundNonStringAttribute;
+            var result = CreateTryParseResult(name, descriptors);
-            var result = new TryParseResult
-            {
-                IsBoundAttribute = IsBoundAttribute(name, descriptors, out isBoundNonStringAttribute),
-                AttributeName = name,
-            };
-            result.IsBoundNonStringAttribute = isBoundNonStringAttribute;
 
             // If we're not after an equal then we should treat the value as if it were a minimized attribute.
             var attributeValueBuilder = afterEquals ? builder : null;
-            result.AttributeValueNode = CreateMarkupAttribute(name, attributeValueBuilder, isBoundNonStringAttribute);
+            result.AttributeValueNode =
+                CreateMarkupAttribute(name, attributeValueBuilder, result.IsBoundNonStringAttribute);
 
             return result;
         }
@@ -286,13 +299,7 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             }
 
             // Have a name now. Able to determine correct isBoundNonStringAttribute value.
-            bool isBoundNonStringAttribute;
+            var result = CreateTryParseResult(name, descriptors);
-            var result = new TryParseResult
-            {
-                IsBoundAttribute = IsBoundAttribute(name, descriptors, out isBoundNonStringAttribute),
-                AttributeName = name,
-            };
-            result.IsBoundNonStringAttribute = isBoundNonStringAttribute;
 
             // Remove first child i.e. foo="
             builder.Children.RemoveAt(0);
@@ -324,13 +331,13 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             if (block.Children.Count() == 1)
             {
                 var child = block.Children.First() as Span;
-
                 if (child != null)
                 {
                     // After pulling apart the block we just have a value span.
                     var spanBuilder = new SpanBuilder(child);
 
-                    result.AttributeValueNode = CreateMarkupAttribute(name, spanBuilder, isBoundNonStringAttribute);
+                    result.AttributeValueNode =
+                        CreateMarkupAttribute(name, spanBuilder, result.IsBoundNonStringAttribute);
 
                     return result;
                 }
@@ -485,17 +492,23 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             return firstBoundAttribute?.TypeName;
         }
 
-        // Determines whether an attribute with the given name is bound to a non-string tag helper property.
+        // Create a TryParseResult for given name, filling in binding details.
-        private static bool IsBoundAttribute(
+        private static TryParseResult CreateTryParseResult(string name, IEnumerable<TagHelperDescriptor> descriptors)
-            string name,
-            IEnumerable<TagHelperDescriptor> descriptors,
-            out bool isBoundNonStringAttribute)
         {
             var firstBoundAttribute = FindFirstBoundAttribute(name, descriptors);
             var isBoundAttribute = firstBoundAttribute != null;
-            isBoundNonStringAttribute = isBoundAttribute && !firstBoundAttribute.IsStringProperty;
+            var isBoundNonStringAttribute = isBoundAttribute && !firstBoundAttribute.IsStringProperty;
+            var isMissingDictionaryKey = isBoundAttribute &&
+                firstBoundAttribute.IsIndexer &&
+                name.Length == firstBoundAttribute.Name.Length;
 
-            return isBoundAttribute;
+            return new TryParseResult
+            {
+                AttributeName = name,
+                IsBoundAttribute = isBoundAttribute,
+                IsBoundNonStringAttribute = isBoundNonStringAttribute,
+                IsMissingDictionaryKey = isMissingDictionaryKey,
+            };
         }
 
         // Finds first TagHelperAttributeDescriptor matching given name.
@@ -503,9 +516,13 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             string name,
             IEnumerable<TagHelperDescriptor> descriptors)
         {
-            return descriptors
+            // Non-indexers (exact HTML attribute name matches) have higher precedence than indexers (prefix matches).
+            // Attributes already sorted to ensure this precedence.
+            var firstBoundAttribute = descriptors
                 .SelectMany(descriptor => descriptor.Attributes)
-                .FirstOrDefault(attribute => string.Equals(attribute.Name, name, StringComparison.OrdinalIgnoreCase));
+                .FirstOrDefault(attributeDescriptor => attributeDescriptor.IsNameMatch(name));
+
+            return firstBoundAttribute;
         }
 
         private static bool IsQuote(HtmlSymbol htmlSymbol)
@@ -523,6 +540,8 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             public bool IsBoundAttribute { get; set; }
 
             public bool IsBoundNonStringAttribute { get; set; }
+
+            public bool IsMissingDictionaryKey { get; set; }
         }
     }
 }

src/Microsoft.AspNet.Razor/Properties/RazorResources.Designer.cs

@@ -1367,19 +1367,35 @@ namespace Microsoft.AspNet.Razor
         }
 
         /// <summary>
-        /// TagHelper attributes must be welformed.
+        /// TagHelper attributes must be well-formed.
         /// </summary>
-        internal static string TagHelperBlockRewriter_TagHelperAttributeListMustBeWelformed
+        internal static string TagHelperBlockRewriter_TagHelperAttributeListMustBeWellFormed
         {
-            get { return GetString("TagHelperBlockRewriter_TagHelperAttributeListMustBeWelformed"); }
+            get { return GetString("TagHelperBlockRewriter_TagHelperAttributeListMustBeWellFormed"); }
         }
 
         /// <summary>
-        /// TagHelper attributes must be welformed.
+        /// TagHelper attributes must be well-formed.
         /// </summary>
-        internal static string FormatTagHelperBlockRewriter_TagHelperAttributeListMustBeWelformed()
+        internal static string FormatTagHelperBlockRewriter_TagHelperAttributeListMustBeWellFormed()
         {
-            return GetString("TagHelperBlockRewriter_TagHelperAttributeListMustBeWelformed");
+            return GetString("TagHelperBlockRewriter_TagHelperAttributeListMustBeWellFormed");
+        }
+
+        /// <summary>
+        /// The tag helper attribute '{0}' in element '{1}' is missing a key. The syntax is '&lt;{1} {0}{{ key }}="value"&gt;'.
+        /// </summary>
+        internal static string TagHelperBlockRewriter_IndexerAttributeNameMustIncludeKey
+        {
+            get { return GetString("TagHelperBlockRewriter_IndexerAttributeNameMustIncludeKey"); }
+        }
+
+        /// <summary>
+        /// The tag helper attribute '{0}' in element '{1}' is missing a key. The syntax is '&lt;{1} {0}{{ key }}="value"&gt;'.
+        /// </summary>
+        internal static string FormatTagHelperBlockRewriter_IndexerAttributeNameMustIncludeKey(object p0, object p1)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("TagHelperBlockRewriter_IndexerAttributeNameMustIncludeKey"), p0, p1);
         }
 
         /// <summary>

src/Microsoft.AspNet.Razor/RazorResources.resx

@@ -1,17 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
 <root>
-  <!-- 
+  <!--
-    Microsoft ResX Schema 
+    Microsoft ResX Schema
-    
+
     Version 2.0
-    
+
-    The primary goals of this format is to allow a simple XML format 
+    The primary goals of this format is to allow a simple XML format
-    that is mostly human readable. The generation and parsing of the 
+    that is mostly human readable. The generation and parsing of the
-    various data types are done through the TypeConverter classes 
+    various data types are done through the TypeConverter classes
     associated with the data types.
-    
+
     Example:
-    
+
     ... ado.net/XML headers & schema ...
     <resheader name="resmimetype">text/microsoft-resx</resheader>
     <resheader name="version">2.0</resheader>
@@ -26,36 +26,36 @@
         <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
         <comment>This is a comment</comment>
     </data>
-                
+
-    There are any number of "resheader" rows that contain simple 
+    There are any number of "resheader" rows that contain simple
     name/value pairs.
-    
+
-    Each data row contains a name, and value. The row also contains a 
+    Each data row contains a name, and value. The row also contains a
-    type or mimetype. Type corresponds to a .NET class that support 
+    type or mimetype. Type corresponds to a .NET class that support
-    text/value conversion through the TypeConverter architecture. 
+    text/value conversion through the TypeConverter architecture.
-    Classes that don't support this are serialized and stored with the 
+    Classes that don't support this are serialized and stored with the
     mimetype set.
-    
+
-    The mimetype is used for serialized objects, and tells the 
+    The mimetype is used for serialized objects, and tells the
-    ResXResourceReader how to depersist the object. This is currently not 
+    ResXResourceReader how to depersist the object. This is currently not
     extensible. For a given mimetype the value must be set accordingly:
-    
+
-    Note - application/x-microsoft.net.object.binary.base64 is the format 
+    Note - application/x-microsoft.net.object.binary.base64 is the format
-    that the ResXResourceWriter will generate, however the reader can 
+    that the ResXResourceWriter will generate, however the reader can
     read any of the formats listed below.
-    
+
     mimetype: application/x-microsoft.net.object.binary.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
             : and then encoded with base64 encoding.
-    
+
     mimetype: application/x-microsoft.net.object.soap.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
             : and then encoded with base64 encoding.
 
     mimetype: application/x-microsoft.net.object.bytearray.base64
-    value   : The object must be serialized into a byte array 
+    value   : The object must be serialized into a byte array
             : using a System.ComponentModel.TypeConverter
             : and then encoded with base64 encoding.
     -->
@@ -390,8 +390,11 @@ Instead, wrap the contents of the block in "{{}}":
   <data name="TagHelpersParseTreeRewriter_MissingCloseAngle" xml:space="preserve">
     <value>Missing close angle for tag helper '{0}'.</value>
   </data>
-  <data name="TagHelperBlockRewriter_TagHelperAttributeListMustBeWelformed" xml:space="preserve">
+  <data name="TagHelperBlockRewriter_TagHelperAttributeListMustBeWellFormed" xml:space="preserve">
-    <value>TagHelper attributes must be welformed.</value>
+    <value>TagHelper attributes must be well-formed.</value>
+  </data>
+  <data name="TagHelperBlockRewriter_IndexerAttributeNameMustIncludeKey" xml:space="preserve">
+    <value>The tag helper attribute '{0}' in element '{1}' is missing a key. The syntax is '&lt;{1} {0}{{ key }}="value"&gt;'.</value>
   </data>
   <data name="TagHelpers_AttributeExpressionRequired" xml:space="preserve">
     <value>Non-string tag helper attribute values must not be empty. Add an expression to this attribute value.</value>

src/Microsoft.AspNet.Razor/TagHelpers/TagHelperAttributeDescriptor.cs

@@ -18,6 +18,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                   name,
                   propertyInfo.Name,
                   propertyInfo.PropertyType.FullName,
+                  isIndexer: false,
                   isStringProperty: propertyInfo.PropertyType == typeof(string))
         {
         }
@@ -25,48 +26,77 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <summary>
         /// Instantiates a new instance of the <see cref="TagHelperAttributeDescriptor"/> class.
         /// </summary>
-        /// <param name="name">The HTML attribute name.</param>
+        /// <param name="name">
+        /// The HTML attribute name or, if <paramref name="isIndexer"/> is <c>true</c>, the prefix for matching
+        /// attribute names.
+        /// </param>
         /// <param name="propertyName">The name of the CLR property that corresponds to the HTML attribute.</param>
         /// <param name="typeName">
-        /// The full name of the named (see <paramref name="propertyName"/>) property's <see cref="System.Type"/>.
+        /// The full name of the named (see <paramref name="propertyName"/>) property's <see cref="Type"/> or,
+        /// if <paramref name="isIndexer"/> is <c>true</c>, the full name of the indexer's value <see cref="Type"/>.
         /// </param>
+        /// <param name="isIndexer">
+        /// If <c>true</c> this <see cref="TagHelperAttributeDescriptor"/> is used for dictionary indexer assignments.
+        /// Otherwise this <see cref="TagHelperAttributeDescriptor"/> is used for property assignment.
+        /// </param>
+        /// <remarks>
+        /// HTML attribute names are matched case-insensitively, regardless of <paramref name="isIndexer"/>.
+        /// </remarks>
         public TagHelperAttributeDescriptor(
             [NotNull] string name,
             [NotNull] string propertyName,
-            [NotNull] string typeName)
+            [NotNull] string typeName,
+            bool isIndexer)
             : this(
                   name,
                   propertyName,
                   typeName,
+                  isIndexer,
                   isStringProperty: string.Equals(typeName, typeof(string).FullName, StringComparison.Ordinal))
         {
         }
 
-        // Internal for testing i.e. for confirming above constructor sets `IsStringProperty` as expected.
+        // Internal for testing i.e. for confirming above constructor sets IsStringProperty as expected.
         internal TagHelperAttributeDescriptor(
             [NotNull] string name,
             [NotNull] string propertyName,
             [NotNull] string typeName,
+            bool isIndexer,
             bool isStringProperty)
         {
             Name = name;
             PropertyName = propertyName;
             TypeName = typeName;
+            IsIndexer = isIndexer;
             IsStringProperty = isStringProperty;
         }
 
         /// <summary>
-        /// Gets an indication whether this property is of type <see cref="string"/>.
+        /// Gets an indication whether this <see cref="TagHelperAttributeDescriptor"/> is used for dictionary indexer
+        /// assignments.
+        /// </summary>
+        /// <value>
+        /// If <c>true</c> this <see cref="TagHelperAttributeDescriptor"/> should be associated with all HTML
+        /// attributes that have names starting with <see cref="Name"/>. Otherwise this
+        /// <see cref="TagHelperAttributeDescriptor"/> is used for property assignment and is only associated with an
+        /// HTML attribute that has the exact <see cref="Name"/>.
+        /// </value>
+        public bool IsIndexer { get; }
+
+        /// <summary>
+        /// Gets an indication whether this property is of type <see cref="string"/> or, if <see cref="IsIndexer"/> is
+        /// <c>true</c>, whether the indexer's value is of type <see cref="string"/>.
         /// </summary>
         /// <value>
         /// If <c>true</c> the <see cref="TypeName"/> is for <see cref="string"/>. This causes the Razor parser
-        /// to allow empty values for attributes that have names matching <see cref="Name"/>. If <c>false</c>
+        /// to allow empty values for HTML attributes matching this <see cref="TagHelperAttributeDescriptor"/>. If
-        /// empty values for such matching attributes lead to errors.
+        /// <c>false</c> empty values for such matching attributes lead to errors.
         /// </value>
         public bool IsStringProperty { get; }
 
         /// <summary>
-        /// The HTML attribute name.
+        /// The HTML attribute name or, if <see cref="IsIndexer"/> is <c>true</c>, the prefix for matching attribute
+        /// names.
         /// </summary>
         public string Name { get; }
 
@@ -76,8 +106,30 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         public string PropertyName { get; }
 
         /// <summary>
-        /// The full name of the named (see <see name="PropertyName"/>) property's <see cref="System.Type"/>.
+        /// The full name of the named (see <see name="PropertyName"/>) property's <see cref="Type"/> or, if
+        /// <see cref="IsIndexer"/> is <c>true</c>, the full name of the indexer's value <see cref="Type"/>.
         /// </summary>
         public string TypeName { get; }
+
+        /// <summary>
+        /// Determines whether HTML attribute <paramref name="name"/> matches this
+        /// <see cref="TagHelperAttributeDescriptor"/>.
+        /// </summary>
+        /// <param name="name">Name of the HTML attribute to check.</param>
+        /// <returns>
+        /// <c>true</c> if this <see cref="TagHelperAttributeDescriptor"/> matches <paramref name="name"/>.
+        /// <c>false</c> otherwise.
+        /// </returns>
+        public bool IsNameMatch(string name)
+        {
+            if (IsIndexer)
+            {
+                return name.StartsWith(Name, StringComparison.OrdinalIgnoreCase);
+            }
+            else
+            {
+                return string.Equals(name, Name, StringComparison.OrdinalIgnoreCase);
+            }
+        }
     }
 }

src/Microsoft.AspNet.Razor/TagHelpers/TagHelperAttributeDescriptorComparer.cs

@@ -29,9 +29,11 @@ namespace Microsoft.AspNet.Razor.TagHelpers
 
         /// <inheritdoc />
         /// <remarks>
-        /// Determines equality based on <see cref="TagHelperAttributeDescriptor.Name"/>,
+        /// Determines equality based on <see cref="TagHelperAttributeDescriptor.IsIndexer"/>,
-        /// <see cref="TagHelperAttributeDescriptor.PropertyName"/>,
+        /// <see cref="TagHelperAttributeDescriptor.Name"/>, <see cref="TagHelperAttributeDescriptor.PropertyName"/>,
-        /// and <see cref="TagHelperAttributeDescriptor.TypeName"/>.
+        /// and <see cref="TagHelperAttributeDescriptor.TypeName"/>. Ignores
+        /// <see cref="TagHelperAttributeDescriptor.IsStringProperty"/> because it can be inferred directly from
+        /// <see cref="TagHelperAttributeDescriptor.TypeName"/>.
         /// </remarks>
         public virtual bool Equals(TagHelperAttributeDescriptor descriptorX, TagHelperAttributeDescriptor descriptorY)
         {
@@ -40,7 +42,11 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 return true;
             }
 
+            // Check Name and TypeName though each property in a particular tag helper has at most two
+            // TagHelperAttributeDescriptors (one for the indexer and one not). May be comparing attributes between
+            // tag helpers and should be as specific as we can.
             return descriptorX != null &&
+                descriptorX.IsIndexer == descriptorY.IsIndexer &&
                 string.Equals(descriptorX.Name, descriptorY.Name, StringComparison.OrdinalIgnoreCase) &&
                 string.Equals(descriptorX.PropertyName, descriptorY.PropertyName, StringComparison.Ordinal) &&
                 string.Equals(descriptorX.TypeName, descriptorY.TypeName, StringComparison.Ordinal);
@@ -49,7 +55,10 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <inheritdoc />
         public virtual int GetHashCode([NotNull] TagHelperAttributeDescriptor descriptor)
         {
+            // Rarely if ever hash TagHelperAttributeDescriptor. If we do, include the Name and TypeName since context
+            // information is not available in the hash.
             return HashCodeCombiner.Start()
+                .Add(descriptor.IsIndexer)
                 .Add(descriptor.Name, StringComparer.OrdinalIgnoreCase)
                 .Add(descriptor.PropertyName, StringComparer.Ordinal)
                 .Add(descriptor.TypeName, StringComparer.Ordinal)

test/Microsoft.AspNet.Razor.Runtime.Test/TagHelpers/CaseSensitiveTagHelperAttributeDescriptorComparer.cs

@@ -24,8 +24,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 return true;
             }
 
-            // Normal comparer doesn't care about case, in tests we do. Also double-check IsStringProperty though
+            // Base comparer does not care about Name case but in tests we do. Also double-check IsStringProperty
-            // it is inferred from TypeName.
+            // though it is inferred from TypeName.
             return base.Equals(descriptorX, descriptorY) &&
                 descriptorX.IsStringProperty == descriptorY.IsStringProperty &&
                 string.Equals(descriptorX.Name, descriptorY.Name, StringComparison.Ordinal);
@@ -33,8 +33,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
 
         public override int GetHashCode(TagHelperAttributeDescriptor descriptor)
         {
-            // Rarely if ever hash TagHelperAttributeDescriptor. If we do, ignore IsStringProperty since it should
+            // Ignore IsStringProperty because it is directly inferred from TypeName and thus won't vary the hash
-            // not vary for a given TypeName i.e. will not change the bucket.
+            // bucket. Base comparer does not care about Name case in its hash code but in tests we do.
             return HashCodeCombiner.Start()
                 .Add(base.GetHashCode(descriptor))
                 .Add(descriptor.Name, StringComparer.Ordinal)

test/Microsoft.AspNet.Razor.Test/Generator/CSharpTagHelperRenderingTest.cs

@@ -34,7 +34,8 @@ namespace Microsoft.AspNet.Razor.Test.Generator
                             new TagHelperAttributeDescriptor(
                                 "catchall-bound-string",
                                 "BoundRequiredString",
-                                typeof(string).FullName),
+                                typeof(string).FullName,
+                                isIndexer: false),
                         },
                         requiredAttributes: new[] { "catchall-unbound-required" }),
                     new TagHelperDescriptor(
@@ -46,11 +47,13 @@ namespace Microsoft.AspNet.Razor.Test.Generator
                             new TagHelperAttributeDescriptor(
                                 "input-bound-required-string",
                                 "BoundRequiredString",
-                                typeof(string).FullName),
+                                typeof(string).FullName,
+                                isIndexer: false),
                             new TagHelperAttributeDescriptor(
                                 "input-bound-string",
                                 "BoundString",
-                                typeof(string).FullName)
+                                typeof(string).FullName,
+                                isIndexer: false)
                         },
                         requiredAttributes: new[] { "input-bound-required-string", "input-unbound-required" }),
                 };
@@ -152,6 +155,80 @@ namespace Microsoft.AspNet.Razor.Test.Generator
             }
         }
 
+        private static IEnumerable<TagHelperDescriptor> PrefixedAttributeTagHelperDescriptors
+        {
+            get
+            {
+                return new[]
+                {
+                    new TagHelperDescriptor(
+                        tagName: "input",
+                        typeName: "InputTagHelper1",
+                        assemblyName: "SomeAssembly",
+                        attributes: new[]
+                        {
+                            new TagHelperAttributeDescriptor(
+                                name: "int-prefix-grabber",
+                                propertyName: "IntProperty",
+                                typeName: typeof(int).FullName,
+                                isIndexer: false),
+                            new TagHelperAttributeDescriptor(
+                                name: "int-dictionary",
+                                propertyName: "IntDictionaryProperty",
+                                typeName: typeof(IDictionary<string, int>).FullName,
+                                isIndexer: false),
+                            new TagHelperAttributeDescriptor(
+                                name: "string-dictionary",
+                                propertyName: "StringDictionaryProperty",
+                                typeName: "Namespace.DictionaryWithoutParameterlessConstructor<string, string>",
+                                isIndexer: false),
+                            new TagHelperAttributeDescriptor(
+                                name: "string-prefix-grabber",
+                                propertyName: "StringProperty",
+                                typeName: typeof(string).FullName,
+                                isIndexer: false),
+                            new TagHelperAttributeDescriptor(
+                                name: "int-prefix-",
+                                propertyName: "IntDictionaryProperty",
+                                typeName: typeof(int).FullName,
+                                isIndexer: true),
+                            new TagHelperAttributeDescriptor(
+                                name: "string-prefix-",
+                                propertyName: "StringDictionaryProperty",
+                                typeName: typeof(string).FullName,
+                                isIndexer: true),
+                        }),
+                    new TagHelperDescriptor(
+                        tagName: "input",
+                        typeName: "InputTagHelper2",
+                        assemblyName: "SomeAssembly",
+                        attributes: new[]
+                        {
+                            new TagHelperAttributeDescriptor(
+                                name: "int-dictionary",
+                                propertyName: "IntDictionaryProperty",
+                                typeName: typeof(IDictionary<string, int>).FullName,
+                                isIndexer: false),
+                            new TagHelperAttributeDescriptor(
+                                name: "string-dictionary",
+                                propertyName: "StringDictionaryProperty",
+                                typeName: "Namespace.DictionaryWithoutParameterlessConstructor<string, string>",
+                                isIndexer: false),
+                            new TagHelperAttributeDescriptor(
+                                name: "int-prefix-",
+                                propertyName: "IntDictionaryProperty",
+                                typeName: typeof(int).FullName,
+                                isIndexer: true),
+                            new TagHelperAttributeDescriptor(
+                                name: "string-prefix-",
+                                propertyName: "StringDictionaryProperty",
+                                typeName: typeof(string).FullName,
+                                isIndexer: true),
+                        }),
+                };
+            }
+        }
+
         public static TheoryData TagHelperDescriptorFlowTestData
         {
             get
@@ -498,6 +575,101 @@ namespace Microsoft.AspNet.Razor.Test.Generator
                                              contentLength: 4)
                         }
                     },
+                    {
+                        "PrefixedAttributeTagHelpers",
+                        "PrefixedAttributeTagHelpers.DesignTime",
+                        PrefixedAttributeTagHelperDescriptors,
+                        new List<LineMapping>
+                        {
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 14,
+                                documentLineIndex: 0,
+                                generatedAbsoluteIndex: 499,
+                                generatedLineIndex: 15,
+                                characterOffsetIndex: 14,
+                                contentLength: 17),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 37,
+                                documentLineIndex: 2,
+                                generatedAbsoluteIndex: 996,
+                                generatedLineIndex: 34,
+                                characterOffsetIndex: 2,
+                                contentLength: 242),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 370,
+                                documentLineIndex: 15,
+                                generatedAbsoluteIndex: 1430,
+                                generatedLineIndex: 50,
+                                characterOffsetIndex: 43,
+                                contentLength: 13),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 404,
+                                documentLineIndex: 15,
+                                generatedAbsoluteIndex: 1601,
+                                generatedLineIndex: 55,
+                                characterOffsetIndex: 77,
+                                contentLength: 16),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 468,
+                                documentLineIndex: 16,
+                                generatedAbsoluteIndex: 2077,
+                                generatedLineIndex: 64,
+                                characterOffsetIndex: 43,
+                                contentLength: 13),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 502,
+                                documentLineIndex: 16,
+                                generatedAbsoluteIndex: 2248,
+                                generatedLineIndex: 69,
+                                characterOffsetIndex: 77,
+                                contentLength: 2),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 526,
+                                documentLineIndex: 16,
+                                generatedAbsoluteIndex: 2432,
+                                generatedLineIndex: 74,
+                                characterOffsetIndex: 101,
+                                contentLength: 2),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 590,
+                                documentLineIndex: 18,
+                                documentCharacterOffsetIndex: 31,
+                                generatedAbsoluteIndex: 2994,
+                                generatedLineIndex: 84,
+                                generatedCharacterOffsetIndex: 32,
+                                contentLength: 2),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 611,
+                                documentLineIndex: 18,
+                                generatedAbsoluteIndex: 3129,
+                                generatedLineIndex: 89,
+                                characterOffsetIndex: 52,
+                                contentLength: 2),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 634,
+                                documentLineIndex: 18,
+                                generatedAbsoluteIndex: 3287,
+                                generatedLineIndex: 94,
+                                characterOffsetIndex: 75,
+                                contentLength: 2),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 783,
+                                documentLineIndex: 20,
+                                documentCharacterOffsetIndex: 42,
+                                generatedAbsoluteIndex: 3521,
+                                generatedLineIndex: 101,
+                                generatedCharacterOffsetIndex: 6,
+                                contentLength: 8),
+                            BuildLineMapping(
+                                documentAbsoluteIndex: 826,
+                                documentLineIndex: 21,
+                                documentCharacterOffsetIndex: 29,
+                                generatedAbsoluteIndex: 4552,
+                                generatedLineIndex: 115,
+                                generatedCharacterOffsetIndex: 51,
+                                contentLength: 2),
+                        }
+                    },
                 };
             }
         }
@@ -523,28 +695,36 @@ namespace Microsoft.AspNet.Razor.Test.Generator
             {
                 // Test resource name, expected TagHelperDescriptors
                 // Note: The baseline resource name is equivalent to the test resource name.
-                return new TheoryData<string, IEnumerable<TagHelperDescriptor>>
+                return new TheoryData<string, string, IEnumerable<TagHelperDescriptor>>
                 {
-                    { "SingleTagHelper", DefaultPAndInputTagHelperDescriptors },
+                    { "SingleTagHelper", null, DefaultPAndInputTagHelperDescriptors },
-                    { "BasicTagHelpers", DefaultPAndInputTagHelperDescriptors },
+                    { "BasicTagHelpers", null, DefaultPAndInputTagHelperDescriptors },
-                    { "BasicTagHelpers.RemoveTagHelper", DefaultPAndInputTagHelperDescriptors },
+                    { "BasicTagHelpers.RemoveTagHelper", null, DefaultPAndInputTagHelperDescriptors },
-                    { "BasicTagHelpers.Prefixed", PrefixedPAndInputTagHelperDescriptors },
+                    { "BasicTagHelpers.Prefixed", null, PrefixedPAndInputTagHelperDescriptors },
-                    { "ComplexTagHelpers", DefaultPAndInputTagHelperDescriptors },
+                    { "ComplexTagHelpers", null, DefaultPAndInputTagHelperDescriptors },
-                    { "DuplicateTargetTagHelper", DuplicateTargetTagHelperDescriptors },
+                    { "DuplicateTargetTagHelper", null, DuplicateTargetTagHelperDescriptors },
-                    { "EmptyAttributeTagHelpers", DefaultPAndInputTagHelperDescriptors },
+                    { "EmptyAttributeTagHelpers", null, DefaultPAndInputTagHelperDescriptors },
-                    { "EscapedTagHelpers", DefaultPAndInputTagHelperDescriptors },
+                    { "EscapedTagHelpers", null, DefaultPAndInputTagHelperDescriptors },
-                    { "AttributeTargetingTagHelpers", AttributeTargetingTagHelperDescriptors },
+                    { "AttributeTargetingTagHelpers", null, AttributeTargetingTagHelperDescriptors },
+                    { "PrefixedAttributeTagHelpers", null, PrefixedAttributeTagHelperDescriptors },
+                    {
+                        "PrefixedAttributeTagHelpers",
+                        "PrefixedAttributeTagHelpers.Reversed",
+                        PrefixedAttributeTagHelperDescriptors.Reverse()
+                    },
                 };
             }
         }
 
         [Theory]
         [MemberData(nameof(RuntimeTimeTagHelperTestData))]
-        public void TagHelpers_GenerateExpectedRuntimeOutput(string testName,
+        public void TagHelpers_GenerateExpectedRuntimeOutput(
-                                                             IEnumerable<TagHelperDescriptor> tagHelperDescriptors)
+            string testName,
+            string baseLineName,
+            IEnumerable<TagHelperDescriptor> tagHelperDescriptors)
         {
-            // Act & Assert
+            // Arrange & Act & Assert
-            RunTagHelperTest(testName, tagHelperDescriptors: tagHelperDescriptors);
+            RunTagHelperTest(testName, baseLineName, tagHelperDescriptors: tagHelperDescriptors);
         }
 
         [Fact]

test/Microsoft.AspNet.Razor.Test/TagHelpers/TagHelperBlockRewriterTest.cs

@@ -181,6 +181,8 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 var noErrors = new RazorError[0];
                 var errorFormat = "Attribute '{0}' on tag helper element '{1}' requires a value. Tag helper bound " +
                     "attributes of type '{2}' cannot be empty or contain only whitespace.";
+                var emptyKeyFormat = "The tag helper attribute '{0}' in element '{1}' is missing a key. The " +
+                    "syntax is '<{1} {0}{{ key }}=\"value\">'.";
                 var stringType = typeof(string).FullName;
                 var intType = typeof(int).FullName;
                 var expressionString = "@DateTime.Now + 1";
@@ -269,6 +271,196 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                                 })),
                         new[] { new RazorError(string.Format(errorFormat, "bound-int", "p", intType), 3, 0, 3, 9) }
                     },
+                    {
+                        "<input int-dictionary/>",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("int-dictionary", null),
+                                })),
+                        new[]
+                        {
+                            new RazorError(
+                                string.Format(errorFormat, "int-dictionary", "input", typeof(IDictionary<string, int>).FullName),
+                                absoluteIndex: 7,
+                                lineIndex: 0,
+                                columnIndex: 7,
+                                length: 14),
+                        }
+                    },
+                    {
+                        "<input string-dictionary />",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("string-dictionary", null),
+                                })),
+                        new[]
+                        {
+                            new RazorError(
+                                string.Format(errorFormat, "string-dictionary", "input", typeof(IDictionary<string, string>).FullName),
+                                absoluteIndex: 7,
+                                lineIndex: 0,
+                                columnIndex: 7,
+                                length: 17),
+                        }
+                    },
+                    {
+                        "<input int-prefix- />",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("int-prefix-", null),
+                                })),
+                        new[]
+                        {
+                            new RazorError(
+                                string.Format(errorFormat, "int-prefix-", "input", typeof(int).FullName),
+                                absoluteIndex: 7,
+                                lineIndex: 0,
+                                columnIndex: 7,
+                                length: 11),
+                            new RazorError(
+                                string.Format(emptyKeyFormat, "int-prefix-", "input"),
+                                absoluteIndex: 7,
+                                lineIndex: 0,
+                                columnIndex: 7,
+                                length: 11),
+                        }
+                    },
+                    {
+                        "<input string-prefix-/>",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("string-prefix-", null),
+                                })),
+                        new[]
+                        {
+                            new RazorError(
+                                string.Format(errorFormat, "string-prefix-", "input", typeof(string).FullName),
+                                absoluteIndex: 7,
+                                lineIndex: 0,
+                                columnIndex: 7,
+                                length: 14),
+                            new RazorError(
+                                string.Format(emptyKeyFormat, "string-prefix-", "input"),
+                                absoluteIndex: 7,
+                                lineIndex: 0,
+                                columnIndex: 7,
+                                length: 14),
+                        }
+                    },
+                    {
+                        "<input int-prefix-value/>",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("int-prefix-value", null),
+                                })),
+                        new[]
+                        {
+                            new RazorError(
+                                string.Format(errorFormat, "int-prefix-value", "input", typeof(int).FullName),
+                                absoluteIndex: 7,
+                                lineIndex: 0,
+                                columnIndex: 7,
+                                length: 16),
+                        }
+                    },
+                    {
+                        "<input string-prefix-value />",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("string-prefix-value", null),
+                                })),
+                        new[]
+                        {
+                            new RazorError(
+                                string.Format(errorFormat, "string-prefix-value", "input", typeof(string).FullName),
+                                absoluteIndex: 7,
+                                lineIndex: 0,
+                                columnIndex: 7,
+                                length: 19),
+                        }
+                    },
+                    {
+                        "<input int-prefix-value='' />",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("int-prefix-value", new MarkupBlock()),
+                                })),
+                        new[]
+                        {
+                            new RazorError(
+                                string.Format(errorFormat, "int-prefix-value", "input", typeof(int).FullName),
+                                absoluteIndex: 7,
+                                lineIndex: 0,
+                                columnIndex: 7,
+                                length: 16),
+                        }
+                    },
+                    {
+                        "<input string-prefix-value=''/>",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("string-prefix-value", new MarkupBlock()),
+                                })),
+                        new RazorError[0]
+                    },
+                    {
+                        "<input int-prefix-value='3'/>",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("int-prefix-value", factory.CodeMarkup("3")),
+                                })),
+                        new RazorError[0]
+                    },
+                    {
+                        "<input string-prefix-value='some string' />",
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock(
+                                "input",
+                                selfClosing: true,
+                                attributes: new List<KeyValuePair<string, SyntaxTreeNode>>
+                                {
+                                    new KeyValuePair<string, SyntaxTreeNode>("string-prefix-value", new MarkupBlock(
+                                        factory.Markup("some"),
+                                        factory.Markup(" string"))),
+                                })),
+                        new RazorError[0]
+                    },
                     {
                         "<input unbound-required bound-required-string />",
                         new MarkupBlock(
@@ -878,7 +1070,8 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                             new TagHelperAttributeDescriptor(
                                 "bound-required-string",
                                 "BoundRequiredString",
-                                typeof(string).FullName)
+                                typeof(string).FullName,
+                                isIndexer: false)
                         },
                         requiredAttributes: new[] { "unbound-required" }),
                     new TagHelperDescriptor(
@@ -890,7 +1083,8 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                             new TagHelperAttributeDescriptor(
                                 "bound-required-string",
                                 "BoundRequiredString",
-                                typeof(string).FullName)
+                                typeof(string).FullName,
+                                isIndexer: false)
                         },
                         requiredAttributes: new[] { "bound-required-string" }),
                     new TagHelperDescriptor(
@@ -902,9 +1096,38 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                             new TagHelperAttributeDescriptor(
                                 "bound-required-int",
                                 "BoundRequiredInt",
-                                typeof(int).FullName)
+                                typeof(int).FullName,
+                                isIndexer: false)
                         },
                         requiredAttributes: new[] { "bound-required-int" }),
+                    new TagHelperDescriptor(
+                        tagName: "input",
+                        typeName: "InputTagHelper3",
+                        assemblyName: "SomeAssembly",
+                        attributes: new[]
+                        {
+                            new TagHelperAttributeDescriptor(
+                                "int-dictionary",
+                                "DictionaryOfIntProperty",
+                                typeof(IDictionary<string, int>).FullName,
+                                isIndexer: false),
+                            new TagHelperAttributeDescriptor(
+                                "string-dictionary",
+                                "DictionaryOfStringProperty",
+                                typeof(IDictionary<string, string>).FullName,
+                                isIndexer: false),
+                            new TagHelperAttributeDescriptor(
+                                "int-prefix-",
+                                "DictionaryOfIntProperty",
+                                typeof(int).FullName,
+                                isIndexer: true),
+                            new TagHelperAttributeDescriptor(
+                                "string-prefix-",
+                                "DictionaryOfStringProperty",
+                                typeof(string).FullName,
+                                isIndexer: true),
+                        },
+                        requiredAttributes: Enumerable.Empty<string>()),
                     new TagHelperDescriptor(
                         tagName: "p",
                         typeName: "PTagHelper",
@@ -914,11 +1137,13 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                             new TagHelperAttributeDescriptor(
                                 "bound-string",
                                 "BoundRequiredString",
-                                typeof(string).FullName),
+                                typeof(string).FullName,
+                                isIndexer: false),
                             new TagHelperAttributeDescriptor(
                                 "bound-int",
                                 "BoundRequiredString",
-                                typeof(int).FullName)
+                                typeof(int).FullName,
+                                isIndexer: false)
                         },
                         requiredAttributes: Enumerable.Empty<string>()),
                 };

test/Microsoft.AspNet.Razor.Test/TagHelpers/TagHelperDescriptorTest.cs

@@ -52,11 +52,13 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                     new TagHelperAttributeDescriptor(
                         name: "attribute one",
                         propertyName: "property name",
-                        typeName: "property type name"),
+                        typeName: "property type name",
+                        isIndexer: false),
                     new TagHelperAttributeDescriptor(
                         name: "attribute two",
                         propertyName: "property name",
-                        typeName: typeof(string).FullName),
+                        typeName: typeof(string).FullName,
+                        isIndexer: false),
                 },
                 requiredAttributes: Enumerable.Empty<string>());
             var expectedSerializedDescriptor =
@@ -66,11 +68,62 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 $"\"{ nameof(TagHelperDescriptor.TypeName) }\":\"type name\"," +
                 $"\"{ nameof(TagHelperDescriptor.AssemblyName) }\":\"assembly name\"," +
                 $"\"{ nameof(TagHelperDescriptor.Attributes) }\":[" +
-                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":false," +
+                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsIndexer) }\":false," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":false," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.Name) }\":\"attribute one\"," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.PropertyName) }\":\"property name\"," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.TypeName) }\":\"property type name\"}}," +
-                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":true," +
+                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsIndexer) }\":false," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":true," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.Name) }\":\"attribute two\"," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.PropertyName) }\":\"property name\"," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.TypeName) }\":\"{ typeof(string).FullName }\"}}]," +
+                $"\"{ nameof(TagHelperDescriptor.RequiredAttributes) }\":[]}}";
+
+            // Act
+            var serializedDescriptor = JsonConvert.SerializeObject(descriptor);
+
+            // Assert
+            Assert.Equal(expectedSerializedDescriptor, serializedDescriptor, StringComparer.Ordinal);
+        }
+
+        [Fact]
+        public void TagHelperDescriptor_WithIndexerAttributes_CanBeSerialized()
+        {
+            // Arrange
+            var descriptor = new TagHelperDescriptor(
+                prefix: "prefix:",
+                tagName: "tag name",
+                typeName: "type name",
+                assemblyName: "assembly name",
+                attributes: new[]
+                {
+                    new TagHelperAttributeDescriptor(
+                        name: "attribute one",
+                        propertyName: "property name",
+                        typeName: "property type name",
+                        isIndexer: true),
+                    new TagHelperAttributeDescriptor(
+                        name: "attribute two",
+                        propertyName: "property name",
+                        typeName: typeof(string).FullName,
+                        isIndexer: true),
+                },
+                requiredAttributes: Enumerable.Empty<string>());
+            var expectedSerializedDescriptor =
+                $"{{\"{ nameof(TagHelperDescriptor.Prefix) }\":\"prefix:\"," +
+                $"\"{ nameof(TagHelperDescriptor.TagName) }\":\"tag name\"," +
+                $"\"{ nameof(TagHelperDescriptor.FullTagName) }\":\"prefix:tag name\"," +
+                $"\"{ nameof(TagHelperDescriptor.TypeName) }\":\"type name\"," +
+                $"\"{ nameof(TagHelperDescriptor.AssemblyName) }\":\"assembly name\"," +
+                $"\"{ nameof(TagHelperDescriptor.Attributes) }\":[" +
+                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsIndexer) }\":true," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":false," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.Name) }\":\"attribute one\"," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.PropertyName) }\":\"property name\"," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.TypeName) }\":\"property type name\"}}," +
+                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsIndexer) }\":true," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":true," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.Name) }\":\"attribute two\"," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.PropertyName) }\":\"property name\"," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.TypeName) }\":\"{ typeof(string).FullName }\"}}]," +
@@ -129,11 +182,13 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 $"\"{ nameof(TagHelperDescriptor.TypeName) }\":\"type name\"," +
                 $"\"{ nameof(TagHelperDescriptor.AssemblyName) }\":\"assembly name\"," +
                 $"\"{ nameof(TagHelperDescriptor.Attributes) }\":[" +
-                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":false," +
+                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsIndexer) }\":false," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":false," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.Name) }\":\"attribute one\"," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.PropertyName) }\":\"property name\"," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.TypeName) }\":\"property type name\"}}," +
-                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":true," +
+                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsIndexer) }\":false," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":true," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.Name) }\":\"attribute two\"," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.PropertyName) }\":\"property name\"," +
                 $"\"{ nameof(TagHelperAttributeDescriptor.TypeName) }\":\"{ typeof(string).FullName }\"}}]," +
@@ -148,11 +203,13 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                     new TagHelperAttributeDescriptor(
                         name: "attribute one",
                         propertyName: "property name",
-                        typeName: "property type name"),
+                        typeName: "property type name",
+                        isIndexer: false),
                     new TagHelperAttributeDescriptor(
                         name: "attribute two",
                         propertyName: "property name",
-                        typeName: typeof(string).FullName),
+                        typeName: typeof(string).FullName,
+                        isIndexer: false),
                 },
                 requiredAttributes: Enumerable.Empty<string>());
 
@@ -166,7 +223,9 @@ namespace Microsoft.AspNet.Razor.TagHelpers
             Assert.Equal(expectedDescriptor.FullTagName, descriptor.FullTagName, StringComparer.Ordinal);
             Assert.Equal(expectedDescriptor.TypeName, descriptor.TypeName, StringComparer.Ordinal);
             Assert.Equal(expectedDescriptor.AssemblyName, descriptor.AssemblyName, StringComparer.Ordinal);
+
             Assert.Equal(2, descriptor.Attributes.Count);
+            Assert.Equal(expectedDescriptor.Attributes[0].IsIndexer, descriptor.Attributes[0].IsIndexer);
             Assert.Equal(expectedDescriptor.Attributes[0].IsStringProperty, descriptor.Attributes[0].IsStringProperty);
             Assert.Equal(expectedDescriptor.Attributes[0].Name, descriptor.Attributes[0].Name, StringComparer.Ordinal);
             Assert.Equal(
@@ -177,6 +236,8 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 expectedDescriptor.Attributes[0].TypeName,
                 descriptor.Attributes[0].TypeName,
                 StringComparer.Ordinal);
+
+            Assert.Equal(expectedDescriptor.Attributes[1].IsIndexer, descriptor.Attributes[1].IsIndexer);
             Assert.Equal(expectedDescriptor.Attributes[1].IsStringProperty, descriptor.Attributes[1].IsStringProperty);
             Assert.Equal(expectedDescriptor.Attributes[1].Name, descriptor.Attributes[1].Name, StringComparer.Ordinal);
             Assert.Equal(
@@ -187,6 +248,88 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 expectedDescriptor.Attributes[1].TypeName,
                 descriptor.Attributes[1].TypeName,
                 StringComparer.Ordinal);
+
+            Assert.Empty(descriptor.RequiredAttributes);
+        }
+
+        [Fact]
+        public void TagHelperDescriptor_WithIndexerAttributes_CanBeDeserialized()
+        {
+            // Arrange
+            var serializedDescriptor =
+                $"{{\"{ nameof(TagHelperDescriptor.Prefix) }\":\"prefix:\"," +
+                $"\"{ nameof(TagHelperDescriptor.TagName) }\":\"tag name\"," +
+                $"\"{ nameof(TagHelperDescriptor.FullTagName) }\":\"prefix:tag name\"," +
+                $"\"{ nameof(TagHelperDescriptor.TypeName) }\":\"type name\"," +
+                $"\"{ nameof(TagHelperDescriptor.AssemblyName) }\":\"assembly name\"," +
+                $"\"{ nameof(TagHelperDescriptor.Attributes) }\":[" +
+                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsIndexer) }\":true," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":false," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.Name) }\":\"attribute one\"," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.PropertyName) }\":\"property name\"," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.TypeName) }\":\"property type name\"}}," +
+                $"{{\"{ nameof(TagHelperAttributeDescriptor.IsIndexer) }\":true," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.IsStringProperty) }\":true," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.Name) }\":\"attribute two\"," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.PropertyName) }\":\"property name\"," +
+                $"\"{ nameof(TagHelperAttributeDescriptor.TypeName) }\":\"{ typeof(string).FullName }\"}}]," +
+                $"\"{ nameof(TagHelperDescriptor.RequiredAttributes) }\":[]}}";
+            var expectedDescriptor = new TagHelperDescriptor(
+                prefix: "prefix:",
+                tagName: "tag name",
+                typeName: "type name",
+                assemblyName: "assembly name",
+                attributes: new[]
+                {
+                    new TagHelperAttributeDescriptor(
+                        name: "attribute one",
+                        propertyName: "property name",
+                        typeName: "property type name",
+                        isIndexer: true),
+                    new TagHelperAttributeDescriptor(
+                        name: "attribute two",
+                        propertyName: "property name",
+                        typeName: typeof(string).FullName,
+                        isIndexer: true),
+                },
+                requiredAttributes: Enumerable.Empty<string>());
+
+            // Act
+            var descriptor = JsonConvert.DeserializeObject<TagHelperDescriptor>(serializedDescriptor);
+
+            // Assert
+            Assert.NotNull(descriptor);
+            Assert.Equal(expectedDescriptor.Prefix, descriptor.Prefix, StringComparer.Ordinal);
+            Assert.Equal(expectedDescriptor.TagName, descriptor.TagName, StringComparer.Ordinal);
+            Assert.Equal(expectedDescriptor.FullTagName, descriptor.FullTagName, StringComparer.Ordinal);
+            Assert.Equal(expectedDescriptor.TypeName, descriptor.TypeName, StringComparer.Ordinal);
+            Assert.Equal(expectedDescriptor.AssemblyName, descriptor.AssemblyName, StringComparer.Ordinal);
+
+            Assert.Equal(2, descriptor.Attributes.Count);
+            Assert.Equal(expectedDescriptor.Attributes[0].IsIndexer, descriptor.Attributes[0].IsIndexer);
+            Assert.Equal(expectedDescriptor.Attributes[0].IsStringProperty, descriptor.Attributes[0].IsStringProperty);
+            Assert.Equal(expectedDescriptor.Attributes[0].Name, descriptor.Attributes[0].Name, StringComparer.Ordinal);
+            Assert.Equal(
+                expectedDescriptor.Attributes[0].PropertyName,
+                descriptor.Attributes[0].PropertyName,
+                StringComparer.Ordinal);
+            Assert.Equal(
+                expectedDescriptor.Attributes[0].TypeName,
+                descriptor.Attributes[0].TypeName,
+                StringComparer.Ordinal);
+
+            Assert.Equal(expectedDescriptor.Attributes[1].IsIndexer, descriptor.Attributes[1].IsIndexer);
+            Assert.Equal(expectedDescriptor.Attributes[1].IsStringProperty, descriptor.Attributes[1].IsStringProperty);
+            Assert.Equal(expectedDescriptor.Attributes[1].Name, descriptor.Attributes[1].Name, StringComparer.Ordinal);
+            Assert.Equal(
+                expectedDescriptor.Attributes[1].PropertyName,
+                descriptor.Attributes[1].PropertyName,
+                StringComparer.Ordinal);
+            Assert.Equal(
+                expectedDescriptor.Attributes[1].TypeName,
+                descriptor.Attributes[1].TypeName,
+                StringComparer.Ordinal);
+
             Assert.Empty(descriptor.RequiredAttributes);
         }
     }

test/Microsoft.AspNet.Razor.Test/TagHelpers/TagHelperParseTreeRewriterTest.cs

@@ -921,7 +921,8 @@ namespace Microsoft.AspNet.Razor.Test.TagHelpers
                             new TagHelperAttributeDescriptor(
                                 name: "bound",
                                 propertyName: "Bound",
-                                typeName: typeof(bool).FullName),
+                                typeName: typeof(bool).FullName,
+                                isIndexer: false),
                         },
                         requiredAttributes: Enumerable.Empty<string>())
                 };
@@ -944,7 +945,8 @@ namespace Microsoft.AspNet.Razor.Test.TagHelpers
                             new TagHelperAttributeDescriptor(
                                 name: "bound",
                                 propertyName: "Bound",
-                                typeName: typeof(bool).FullName),
+                                typeName: typeof(bool).FullName,
+                                isIndexer: false),
                         },
                         requiredAttributes: Enumerable.Empty<string>())
                 };
@@ -1508,11 +1510,13 @@ namespace Microsoft.AspNet.Razor.Test.TagHelpers
                             new TagHelperAttributeDescriptor(
                                 name: "bound",
                                 propertyName: "Bound",
-                                typeName: typeof(bool).FullName),
+                                typeName: typeof(bool).FullName,
+                                isIndexer: false),
                             new TagHelperAttributeDescriptor(
                                 name: "name",
                                 propertyName: "Name",
-                                typeName: typeof(string).FullName)
+                                typeName: typeof(string).FullName,
+                                isIndexer: false)
                         })
                 };
             var descriptorProvider = new TagHelperDescriptorProvider(descriptors);
@@ -3396,7 +3400,7 @@ namespace Microsoft.AspNet.Razor.Test.TagHelpers
                                 string.Format(CultureInfo.InvariantCulture, errorFormatUnclosed, "p"),
                                 SourceLocation.Zero),
                             new RazorError(
-                                "TagHelper attributes must be welformed.",
+                                "TagHelper attributes must be well-formed.",
                                 absoluteIndex: 12,
                                 lineIndex: 0,
                                 columnIndex: 12)
@@ -3962,9 +3966,13 @@ namespace Microsoft.AspNet.Razor.Test.TagHelpers
                 new TagHelperDescriptor("person", "PersonTagHelper", "personAssembly",
                     attributes: new[]
                     {
-                        new TagHelperAttributeDescriptor("age", "Age", typeof(int).FullName),
+                        new TagHelperAttributeDescriptor("age", "Age", typeof(int).FullName, isIndexer: false),
-                        new TagHelperAttributeDescriptor("birthday", "BirthDay", typeof(DateTime).FullName),
+                        new TagHelperAttributeDescriptor(
-                        new TagHelperAttributeDescriptor("name", "Name", typeof(string).FullName),
+                            "birthday",
+                            "BirthDay",
+                            typeof(DateTime).FullName,
+                            isIndexer: false),
+                        new TagHelperAttributeDescriptor("name", "Name", typeof(string).FullName, isIndexer: false),
                     })
             };
             var providerContext = new TagHelperDescriptorProvider(descriptors);

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/CS/Output/PrefixedAttributeTagHelpers.DesignTime.cs

@@ -0,0 +1,127 @@
+namespace TestOutput
+{
+    using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+    using System;
+    using System.Threading.Tasks;
+
+    public class PrefixedAttributeTagHelpers
+    {
+        private static object @__o;
+        private void @__RazorDesignTimeHelpers__()
+        {
+            #pragma warning disable 219
+            string __tagHelperDirectiveSyntaxHelper = null;
+            __tagHelperDirectiveSyntaxHelper = 
+#line 1 "PrefixedAttributeTagHelpers.cshtml"
+              "something, nice"
+
+#line default
+#line hidden
+            ;
+            #pragma warning restore 219
+        }
+        #line hidden
+        private InputTagHelper1 __InputTagHelper1 = null;
+        private InputTagHelper2 __InputTagHelper2 = null;
+        #line hidden
+        public PrefixedAttributeTagHelpers()
+        {
+        }
+
+        #pragma warning disable 1998
+        public override async Task ExecuteAsync()
+        {
+#line 3 "PrefixedAttributeTagHelpers.cshtml"
+  
+    var literate = "or illiterate";
+    var intDictionary = new Dictionary<string, int>
+    {
+        { "three", 3 },
+    };
+    var stringDictionary = new SortedDictionary<string, string>
+    {
+        { "name", "value" },
+    };
+
+#line default
+#line hidden
+
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+#line 16 "PrefixedAttributeTagHelpers.cshtml"
+ __InputTagHelper1.IntDictionaryProperty = intDictionary;
+
+#line default
+#line hidden
+#line 16 "PrefixedAttributeTagHelpers.cshtml"
+                                __InputTagHelper1.StringDictionaryProperty = stringDictionary;
+
+#line default
+#line hidden
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __InputTagHelper2.IntDictionaryProperty = __InputTagHelper1.IntDictionaryProperty;
+            __InputTagHelper2.StringDictionaryProperty = __InputTagHelper1.StringDictionaryProperty;
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+#line 17 "PrefixedAttributeTagHelpers.cshtml"
+ __InputTagHelper1.IntDictionaryProperty = intDictionary;
+
+#line default
+#line hidden
+#line 17 "PrefixedAttributeTagHelpers.cshtml"
+                         __InputTagHelper1.IntDictionaryProperty["garlic"] = 37;
+
+#line default
+#line hidden
+#line 17 "PrefixedAttributeTagHelpers.cshtml"
+                                                                     __InputTagHelper1.IntProperty = 42;
+
+#line default
+#line hidden
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __InputTagHelper2.IntDictionaryProperty = __InputTagHelper1.IntDictionaryProperty;
+            __InputTagHelper2.IntDictionaryProperty["garlic"] = __InputTagHelper1.IntDictionaryProperty["garlic"];
+            __InputTagHelper2.IntDictionaryProperty["grabber"] = __InputTagHelper1.IntProperty;
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+#line 19 "PrefixedAttributeTagHelpers.cshtml"
+__InputTagHelper1.IntProperty = 42;
+
+#line default
+#line hidden
+#line 19 "PrefixedAttributeTagHelpers.cshtml"
+  __InputTagHelper1.IntDictionaryProperty["salt"] = 37;
+
+#line default
+#line hidden
+#line 19 "PrefixedAttributeTagHelpers.cshtml"
+                       __InputTagHelper1.IntDictionaryProperty["pepper"] = 98;
+
+#line default
+#line hidden
+            __InputTagHelper1.StringProperty = "string";
+            __InputTagHelper1.StringDictionaryProperty["paprika"] = "another string";
+#line 21 "PrefixedAttributeTagHelpers.cshtml"
+__o = literate;
+
+#line default
+#line hidden
+            __InputTagHelper1.StringDictionaryProperty["cumin"] = string.Empty;
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __InputTagHelper2.IntDictionaryProperty["grabber"] = __InputTagHelper1.IntProperty;
+            __InputTagHelper2.IntDictionaryProperty["salt"] = __InputTagHelper1.IntDictionaryProperty["salt"];
+            __InputTagHelper2.IntDictionaryProperty["pepper"] = __InputTagHelper1.IntDictionaryProperty["pepper"];
+            __InputTagHelper2.StringDictionaryProperty["grabber"] = __InputTagHelper1.StringProperty;
+            __InputTagHelper2.StringDictionaryProperty["paprika"] = __InputTagHelper1.StringDictionaryProperty["paprika"];
+            __InputTagHelper2.StringDictionaryProperty["cumin"] = __InputTagHelper1.StringDictionaryProperty["cumin"];
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+#line 22 "PrefixedAttributeTagHelpers.cshtml"
+__InputTagHelper1.IntDictionaryProperty["value"] = 37;
+
+#line default
+#line hidden
+            __InputTagHelper1.StringDictionaryProperty["thyme"] = "string";
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __InputTagHelper2.IntDictionaryProperty["value"] = __InputTagHelper1.IntDictionaryProperty["value"];
+            __InputTagHelper2.StringDictionaryProperty["thyme"] = __InputTagHelper1.StringDictionaryProperty["thyme"];
+        }
+        #pragma warning restore 1998
+    }
+}

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/CS/Output/PrefixedAttributeTagHelpers.Reversed.cs

@@ -0,0 +1,231 @@
+#pragma checksum "PrefixedAttributeTagHelpers.cshtml" "{ff1816ec-aa5e-4d10-87f7-6f4963833460}" "4e7fe9697b745af1a07d41f6a8532fdc288fa046"
+namespace TestOutput
+{
+    using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+    using System;
+    using System.Threading.Tasks;
+
+    public class PrefixedAttributeTagHelpers
+    {
+        #line hidden
+        #pragma warning disable 0414
+        private TagHelperContent __tagHelperStringValueBuffer = null;
+        #pragma warning restore 0414
+        private TagHelperExecutionContext __tagHelperExecutionContext = null;
+        private TagHelperRunner __tagHelperRunner = null;
+        private TagHelperScopeManager __tagHelperScopeManager = new TagHelperScopeManager();
+        private InputTagHelper2 __InputTagHelper2 = null;
+        private InputTagHelper1 __InputTagHelper1 = null;
+        #line hidden
+        public PrefixedAttributeTagHelpers()
+        {
+        }
+
+        #pragma warning disable 1998
+        public override async Task ExecuteAsync()
+        {
+            __tagHelperRunner = __tagHelperRunner ?? new TagHelperRunner();
+            Instrumentation.BeginContext(33, 2, true);
+            WriteLiteral("\r\n");
+            Instrumentation.EndContext();
+#line 3 "PrefixedAttributeTagHelpers.cshtml"
+  
+    var literate = "or illiterate";
+    var intDictionary = new Dictionary<string, int>
+    {
+        { "three", 3 },
+    };
+    var stringDictionary = new SortedDictionary<string, string>
+    {
+        { "name", "value" },
+    };
+
+#line default
+#line hidden
+
+            Instrumentation.BeginContext(280, 51, true);
+            WriteLiteral("\r\n\r\n<div class=\"randomNonTagHelperAttribute\">\r\n    ");
+            Instrumentation.EndContext();
+            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("input", true, "test", async() => {
+            }
+            , StartTagHelperWritingScope, EndTagHelperWritingScope);
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __tagHelperExecutionContext.Add(__InputTagHelper2);
+#line 16 "PrefixedAttributeTagHelpers.cshtml"
+ __InputTagHelper2.IntDictionaryProperty = intDictionary;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-dictionary", __InputTagHelper2.IntDictionaryProperty);
+#line 16 "PrefixedAttributeTagHelpers.cshtml"
+                                __InputTagHelper2.StringDictionaryProperty = stringDictionary;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-dictionary", __InputTagHelper2.StringDictionaryProperty);
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+            __tagHelperExecutionContext.Add(__InputTagHelper1);
+            __InputTagHelper1.IntDictionaryProperty = __InputTagHelper2.IntDictionaryProperty;
+            __InputTagHelper1.StringDictionaryProperty = __InputTagHelper2.StringDictionaryProperty;
+            __tagHelperExecutionContext.AddHtmlAttribute("type", Html.Raw("checkbox"));
+            __tagHelperExecutionContext.Output = await __tagHelperRunner.RunAsync(__tagHelperExecutionContext);
+            await WriteTagHelperAsync(__tagHelperExecutionContext);
+            __tagHelperExecutionContext = __tagHelperScopeManager.End();
+            Instrumentation.BeginContext(423, 6, true);
+            WriteLiteral("\r\n    ");
+            Instrumentation.EndContext();
+            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("input", true, "test", async() => {
+            }
+            , StartTagHelperWritingScope, EndTagHelperWritingScope);
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __tagHelperExecutionContext.Add(__InputTagHelper2);
+#line 17 "PrefixedAttributeTagHelpers.cshtml"
+ __InputTagHelper2.IntDictionaryProperty = intDictionary;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-dictionary", __InputTagHelper2.IntDictionaryProperty);
+            if (__InputTagHelper2.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-garlic", "InputTagHelper2", "IntDictionaryProperty"));
+            }
+#line 17 "PrefixedAttributeTagHelpers.cshtml"
+                         __InputTagHelper2.IntDictionaryProperty["garlic"] = 37;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-garlic", __InputTagHelper2.IntDictionaryProperty["garlic"]);
+#line 17 "PrefixedAttributeTagHelpers.cshtml"
+                                                __InputTagHelper2.IntDictionaryProperty["grabber"] = 42;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-grabber", __InputTagHelper2.IntDictionaryProperty["grabber"]);
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+            __tagHelperExecutionContext.Add(__InputTagHelper1);
+            __InputTagHelper1.IntDictionaryProperty = __InputTagHelper2.IntDictionaryProperty;
+            if (__InputTagHelper1.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-garlic", "InputTagHelper1", "IntDictionaryProperty"));
+            }
+            __InputTagHelper1.IntDictionaryProperty["garlic"] = __InputTagHelper2.IntDictionaryProperty["garlic"];
+            __InputTagHelper1.IntProperty = __InputTagHelper2.IntDictionaryProperty["grabber"];
+            __tagHelperExecutionContext.AddHtmlAttribute("type", Html.Raw("password"));
+            __tagHelperExecutionContext.Output = await __tagHelperRunner.RunAsync(__tagHelperExecutionContext);
+            await WriteTagHelperAsync(__tagHelperExecutionContext);
+            __tagHelperExecutionContext = __tagHelperScopeManager.End();
+            Instrumentation.BeginContext(532, 6, true);
+            WriteLiteral("\r\n    ");
+            Instrumentation.EndContext();
+            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("input", true, "test", async() => {
+            }
+            , StartTagHelperWritingScope, EndTagHelperWritingScope);
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __tagHelperExecutionContext.Add(__InputTagHelper2);
+            if (__InputTagHelper2.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-grabber", "InputTagHelper2", "IntDictionaryProperty"));
+            }
+#line 19 "PrefixedAttributeTagHelpers.cshtml"
+__InputTagHelper2.IntDictionaryProperty["grabber"] = 42;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-grabber", __InputTagHelper2.IntDictionaryProperty["grabber"]);
+#line 19 "PrefixedAttributeTagHelpers.cshtml"
+  __InputTagHelper2.IntDictionaryProperty["salt"] = 37;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-salt", __InputTagHelper2.IntDictionaryProperty["salt"]);
+#line 19 "PrefixedAttributeTagHelpers.cshtml"
+                       __InputTagHelper2.IntDictionaryProperty["pepper"] = 98;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-pepper", __InputTagHelper2.IntDictionaryProperty["pepper"]);
+            if (__InputTagHelper2.StringDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("string-prefix-grabber", "InputTagHelper2", "StringDictionaryProperty"));
+            }
+            __InputTagHelper2.StringDictionaryProperty["grabber"] = "string";
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-grabber", __InputTagHelper2.StringDictionaryProperty["grabber"]);
+            __InputTagHelper2.StringDictionaryProperty["paprika"] = "another string";
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-paprika", __InputTagHelper2.StringDictionaryProperty["paprika"]);
+            StartTagHelperWritingScope();
+            WriteLiteral("literate ");
+#line 21 "PrefixedAttributeTagHelpers.cshtml"
+WriteLiteral(literate);
+
+#line default
+#line hidden
+            WriteLiteral("?");
+            __tagHelperStringValueBuffer = EndTagHelperWritingScope();
+            __InputTagHelper2.StringDictionaryProperty["cumin"] = __tagHelperStringValueBuffer.ToString();
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-cumin", __InputTagHelper2.StringDictionaryProperty["cumin"]);
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+            __tagHelperExecutionContext.Add(__InputTagHelper1);
+            __InputTagHelper1.IntProperty = __InputTagHelper2.IntDictionaryProperty["grabber"];
+            if (__InputTagHelper1.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-salt", "InputTagHelper1", "IntDictionaryProperty"));
+            }
+            __InputTagHelper1.IntDictionaryProperty["salt"] = __InputTagHelper2.IntDictionaryProperty["salt"];
+            __InputTagHelper1.IntDictionaryProperty["pepper"] = __InputTagHelper2.IntDictionaryProperty["pepper"];
+            __InputTagHelper1.StringProperty = __InputTagHelper2.StringDictionaryProperty["grabber"];
+            if (__InputTagHelper1.StringDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("string-prefix-paprika", "InputTagHelper1", "StringDictionaryProperty"));
+            }
+            __InputTagHelper1.StringDictionaryProperty["paprika"] = __InputTagHelper2.StringDictionaryProperty["paprika"];
+            __InputTagHelper1.StringDictionaryProperty["cumin"] = __InputTagHelper2.StringDictionaryProperty["cumin"];
+            __tagHelperExecutionContext.AddHtmlAttribute("type", Html.Raw("radio"));
+            __tagHelperExecutionContext.Output = await __tagHelperRunner.RunAsync(__tagHelperExecutionContext);
+            await WriteTagHelperAsync(__tagHelperExecutionContext);
+            __tagHelperExecutionContext = __tagHelperScopeManager.End();
+            Instrumentation.BeginContext(795, 6, true);
+            WriteLiteral("\r\n    ");
+            Instrumentation.EndContext();
+            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("input", true, "test", async() => {
+            }
+            , StartTagHelperWritingScope, EndTagHelperWritingScope);
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __tagHelperExecutionContext.Add(__InputTagHelper2);
+            if (__InputTagHelper2.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-value", "InputTagHelper2", "IntDictionaryProperty"));
+            }
+#line 22 "PrefixedAttributeTagHelpers.cshtml"
+__InputTagHelper2.IntDictionaryProperty["value"] = 37;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-value", __InputTagHelper2.IntDictionaryProperty["value"]);
+            if (__InputTagHelper2.StringDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("string-prefix-thyme", "InputTagHelper2", "StringDictionaryProperty"));
+            }
+            __InputTagHelper2.StringDictionaryProperty["thyme"] = "string";
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-thyme", __InputTagHelper2.StringDictionaryProperty["thyme"]);
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+            __tagHelperExecutionContext.Add(__InputTagHelper1);
+            if (__InputTagHelper1.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-value", "InputTagHelper1", "IntDictionaryProperty"));
+            }
+            __InputTagHelper1.IntDictionaryProperty["value"] = __InputTagHelper2.IntDictionaryProperty["value"];
+            if (__InputTagHelper1.StringDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("string-prefix-thyme", "InputTagHelper1", "StringDictionaryProperty"));
+            }
+            __InputTagHelper1.StringDictionaryProperty["thyme"] = __InputTagHelper2.StringDictionaryProperty["thyme"];
+            __tagHelperExecutionContext.Output = await __tagHelperRunner.RunAsync(__tagHelperExecutionContext);
+            await WriteTagHelperAsync(__tagHelperExecutionContext);
+            __tagHelperExecutionContext = __tagHelperScopeManager.End();
+            Instrumentation.BeginContext(861, 8, true);
+            WriteLiteral("\r\n</div>");
+            Instrumentation.EndContext();
+        }
+        #pragma warning restore 1998
+    }
+}

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/CS/Output/PrefixedAttributeTagHelpers.cs

@@ -0,0 +1,231 @@
+#pragma checksum "PrefixedAttributeTagHelpers.cshtml" "{ff1816ec-aa5e-4d10-87f7-6f4963833460}" "4e7fe9697b745af1a07d41f6a8532fdc288fa046"
+namespace TestOutput
+{
+    using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+    using System;
+    using System.Threading.Tasks;
+
+    public class PrefixedAttributeTagHelpers
+    {
+        #line hidden
+        #pragma warning disable 0414
+        private TagHelperContent __tagHelperStringValueBuffer = null;
+        #pragma warning restore 0414
+        private TagHelperExecutionContext __tagHelperExecutionContext = null;
+        private TagHelperRunner __tagHelperRunner = null;
+        private TagHelperScopeManager __tagHelperScopeManager = new TagHelperScopeManager();
+        private InputTagHelper1 __InputTagHelper1 = null;
+        private InputTagHelper2 __InputTagHelper2 = null;
+        #line hidden
+        public PrefixedAttributeTagHelpers()
+        {
+        }
+
+        #pragma warning disable 1998
+        public override async Task ExecuteAsync()
+        {
+            __tagHelperRunner = __tagHelperRunner ?? new TagHelperRunner();
+            Instrumentation.BeginContext(33, 2, true);
+            WriteLiteral("\r\n");
+            Instrumentation.EndContext();
+#line 3 "PrefixedAttributeTagHelpers.cshtml"
+  
+    var literate = "or illiterate";
+    var intDictionary = new Dictionary<string, int>
+    {
+        { "three", 3 },
+    };
+    var stringDictionary = new SortedDictionary<string, string>
+    {
+        { "name", "value" },
+    };
+
+#line default
+#line hidden
+
+            Instrumentation.BeginContext(280, 51, true);
+            WriteLiteral("\r\n\r\n<div class=\"randomNonTagHelperAttribute\">\r\n    ");
+            Instrumentation.EndContext();
+            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("input", true, "test", async() => {
+            }
+            , StartTagHelperWritingScope, EndTagHelperWritingScope);
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+            __tagHelperExecutionContext.Add(__InputTagHelper1);
+#line 16 "PrefixedAttributeTagHelpers.cshtml"
+ __InputTagHelper1.IntDictionaryProperty = intDictionary;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-dictionary", __InputTagHelper1.IntDictionaryProperty);
+#line 16 "PrefixedAttributeTagHelpers.cshtml"
+                                __InputTagHelper1.StringDictionaryProperty = stringDictionary;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-dictionary", __InputTagHelper1.StringDictionaryProperty);
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __tagHelperExecutionContext.Add(__InputTagHelper2);
+            __InputTagHelper2.IntDictionaryProperty = __InputTagHelper1.IntDictionaryProperty;
+            __InputTagHelper2.StringDictionaryProperty = __InputTagHelper1.StringDictionaryProperty;
+            __tagHelperExecutionContext.AddHtmlAttribute("type", Html.Raw("checkbox"));
+            __tagHelperExecutionContext.Output = await __tagHelperRunner.RunAsync(__tagHelperExecutionContext);
+            await WriteTagHelperAsync(__tagHelperExecutionContext);
+            __tagHelperExecutionContext = __tagHelperScopeManager.End();
+            Instrumentation.BeginContext(423, 6, true);
+            WriteLiteral("\r\n    ");
+            Instrumentation.EndContext();
+            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("input", true, "test", async() => {
+            }
+            , StartTagHelperWritingScope, EndTagHelperWritingScope);
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+            __tagHelperExecutionContext.Add(__InputTagHelper1);
+#line 17 "PrefixedAttributeTagHelpers.cshtml"
+ __InputTagHelper1.IntDictionaryProperty = intDictionary;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-dictionary", __InputTagHelper1.IntDictionaryProperty);
+            if (__InputTagHelper1.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-garlic", "InputTagHelper1", "IntDictionaryProperty"));
+            }
+#line 17 "PrefixedAttributeTagHelpers.cshtml"
+                         __InputTagHelper1.IntDictionaryProperty["garlic"] = 37;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-garlic", __InputTagHelper1.IntDictionaryProperty["garlic"]);
+#line 17 "PrefixedAttributeTagHelpers.cshtml"
+                                                                     __InputTagHelper1.IntProperty = 42;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-grabber", __InputTagHelper1.IntProperty);
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __tagHelperExecutionContext.Add(__InputTagHelper2);
+            __InputTagHelper2.IntDictionaryProperty = __InputTagHelper1.IntDictionaryProperty;
+            if (__InputTagHelper2.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-garlic", "InputTagHelper2", "IntDictionaryProperty"));
+            }
+            __InputTagHelper2.IntDictionaryProperty["garlic"] = __InputTagHelper1.IntDictionaryProperty["garlic"];
+            __InputTagHelper2.IntDictionaryProperty["grabber"] = __InputTagHelper1.IntProperty;
+            __tagHelperExecutionContext.AddHtmlAttribute("type", Html.Raw("password"));
+            __tagHelperExecutionContext.Output = await __tagHelperRunner.RunAsync(__tagHelperExecutionContext);
+            await WriteTagHelperAsync(__tagHelperExecutionContext);
+            __tagHelperExecutionContext = __tagHelperScopeManager.End();
+            Instrumentation.BeginContext(532, 6, true);
+            WriteLiteral("\r\n    ");
+            Instrumentation.EndContext();
+            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("input", true, "test", async() => {
+            }
+            , StartTagHelperWritingScope, EndTagHelperWritingScope);
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+            __tagHelperExecutionContext.Add(__InputTagHelper1);
+#line 19 "PrefixedAttributeTagHelpers.cshtml"
+__InputTagHelper1.IntProperty = 42;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-grabber", __InputTagHelper1.IntProperty);
+            if (__InputTagHelper1.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-salt", "InputTagHelper1", "IntDictionaryProperty"));
+            }
+#line 19 "PrefixedAttributeTagHelpers.cshtml"
+  __InputTagHelper1.IntDictionaryProperty["salt"] = 37;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-salt", __InputTagHelper1.IntDictionaryProperty["salt"]);
+#line 19 "PrefixedAttributeTagHelpers.cshtml"
+                       __InputTagHelper1.IntDictionaryProperty["pepper"] = 98;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-pepper", __InputTagHelper1.IntDictionaryProperty["pepper"]);
+            __InputTagHelper1.StringProperty = "string";
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-grabber", __InputTagHelper1.StringProperty);
+            if (__InputTagHelper1.StringDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("string-prefix-paprika", "InputTagHelper1", "StringDictionaryProperty"));
+            }
+            __InputTagHelper1.StringDictionaryProperty["paprika"] = "another string";
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-paprika", __InputTagHelper1.StringDictionaryProperty["paprika"]);
+            StartTagHelperWritingScope();
+            WriteLiteral("literate ");
+#line 21 "PrefixedAttributeTagHelpers.cshtml"
+WriteLiteral(literate);
+
+#line default
+#line hidden
+            WriteLiteral("?");
+            __tagHelperStringValueBuffer = EndTagHelperWritingScope();
+            __InputTagHelper1.StringDictionaryProperty["cumin"] = __tagHelperStringValueBuffer.ToString();
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-cumin", __InputTagHelper1.StringDictionaryProperty["cumin"]);
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __tagHelperExecutionContext.Add(__InputTagHelper2);
+            if (__InputTagHelper2.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-grabber", "InputTagHelper2", "IntDictionaryProperty"));
+            }
+            __InputTagHelper2.IntDictionaryProperty["grabber"] = __InputTagHelper1.IntProperty;
+            __InputTagHelper2.IntDictionaryProperty["salt"] = __InputTagHelper1.IntDictionaryProperty["salt"];
+            __InputTagHelper2.IntDictionaryProperty["pepper"] = __InputTagHelper1.IntDictionaryProperty["pepper"];
+            if (__InputTagHelper2.StringDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("string-prefix-grabber", "InputTagHelper2", "StringDictionaryProperty"));
+            }
+            __InputTagHelper2.StringDictionaryProperty["grabber"] = __InputTagHelper1.StringProperty;
+            __InputTagHelper2.StringDictionaryProperty["paprika"] = __InputTagHelper1.StringDictionaryProperty["paprika"];
+            __InputTagHelper2.StringDictionaryProperty["cumin"] = __InputTagHelper1.StringDictionaryProperty["cumin"];
+            __tagHelperExecutionContext.AddHtmlAttribute("type", Html.Raw("radio"));
+            __tagHelperExecutionContext.Output = await __tagHelperRunner.RunAsync(__tagHelperExecutionContext);
+            await WriteTagHelperAsync(__tagHelperExecutionContext);
+            __tagHelperExecutionContext = __tagHelperScopeManager.End();
+            Instrumentation.BeginContext(795, 6, true);
+            WriteLiteral("\r\n    ");
+            Instrumentation.EndContext();
+            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("input", true, "test", async() => {
+            }
+            , StartTagHelperWritingScope, EndTagHelperWritingScope);
+            __InputTagHelper1 = CreateTagHelper<InputTagHelper1>();
+            __tagHelperExecutionContext.Add(__InputTagHelper1);
+            if (__InputTagHelper1.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-value", "InputTagHelper1", "IntDictionaryProperty"));
+            }
+#line 22 "PrefixedAttributeTagHelpers.cshtml"
+__InputTagHelper1.IntDictionaryProperty["value"] = 37;
+
+#line default
+#line hidden
+            __tagHelperExecutionContext.AddTagHelperAttribute("int-prefix-value", __InputTagHelper1.IntDictionaryProperty["value"]);
+            if (__InputTagHelper1.StringDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("string-prefix-thyme", "InputTagHelper1", "StringDictionaryProperty"));
+            }
+            __InputTagHelper1.StringDictionaryProperty["thyme"] = "string";
+            __tagHelperExecutionContext.AddTagHelperAttribute("string-prefix-thyme", __InputTagHelper1.StringDictionaryProperty["thyme"]);
+            __InputTagHelper2 = CreateTagHelper<InputTagHelper2>();
+            __tagHelperExecutionContext.Add(__InputTagHelper2);
+            if (__InputTagHelper2.IntDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("int-prefix-value", "InputTagHelper2", "IntDictionaryProperty"));
+            }
+            __InputTagHelper2.IntDictionaryProperty["value"] = __InputTagHelper1.IntDictionaryProperty["value"];
+            if (__InputTagHelper2.StringDictionaryProperty == null)
+            {
+                throw new InvalidOperationException(FormatInvalidIndexerAssignment("string-prefix-thyme", "InputTagHelper2", "StringDictionaryProperty"));
+            }
+            __InputTagHelper2.StringDictionaryProperty["thyme"] = __InputTagHelper1.StringDictionaryProperty["thyme"];
+            __tagHelperExecutionContext.Output = await __tagHelperRunner.RunAsync(__tagHelperExecutionContext);
+            await WriteTagHelperAsync(__tagHelperExecutionContext);
+            __tagHelperExecutionContext = __tagHelperScopeManager.End();
+            Instrumentation.BeginContext(861, 8, true);
+            WriteLiteral("\r\n</div>");
+            Instrumentation.EndContext();
+        }
+        #pragma warning restore 1998
+    }
+}

test/Microsoft.AspNet.Razor.Test/TestFiles/CodeGenerator/CS/Source/PrefixedAttributeTagHelpers.cshtml

@@ -0,0 +1,23 @@
+@addTagHelper "something, nice"
+
+@{
+    var literate = "or illiterate";
+    var intDictionary = new Dictionary<string, int>
+    {
+        { "three", 3 },
+    };
+    var stringDictionary = new SortedDictionary<string, string>
+    {
+        { "name", "value" },
+    };
+}
+
+<div class="randomNonTagHelperAttribute">
+    <input type="checkbox" int-dictionary="intDictionary" string-dictionary="stringDictionary"/>
+    <input type="password" int-dictionary="intDictionary" int-prefix-garlic="37" int-prefix-grabber="42" />
+    <input type="radio"
+           int-prefix-grabber="42" int-prefix-salt="37" int-prefix-pepper="98" int-prefix-salt="8"
+           string-prefix-grabber="string" string-prefix-paprika="another string"
+           string-prefix-cumin="literate @literate?"/>
+    <input int-prefix-value="37" string-prefix-thyme="string" />
+</div>