Remove the use of `Linq` and unneeded enumerators from Mvc `TagHelper`s.

- Prior to this change we were using `Linq` throughout our `TagHelper`s which resulted excess allocations.
- Also went through our data structures and changed some from `Enumerable<T>` to `IList<T>` where we were just not exposing the more accessible types (list).
- Changed several locations of `foreach` to `for` to remove the enumerator allocation.

#3599

src/Microsoft.AspNet.Mvc.TagHelpers/AnchorTagHelper.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Mvc.ViewFeatures;
 using Microsoft.AspNet.Razor.TagHelpers;
@@ -32,6 +31,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         private const string RouteValuesDictionaryName = "asp-all-route-data";
         private const string RouteValuesPrefix = "asp-route-";
         private const string Href = "href";
+        private IDictionary<string, string> _routeValues;
 
         /// <summary>
         /// Creates a new <see cref="AnchorTagHelper"/>.
@@ -98,8 +98,22 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         /// Additional parameters for the route.
         /// </summary>
         [HtmlAttributeName(RouteValuesDictionaryName, DictionaryAttributePrefix = RouteValuesPrefix)]
-        public IDictionary<string, string> RouteValues { get; set; } =
+        public IDictionary<string, string> RouteValues
-            new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        {
+            get
+            {
+                if (_routeValues == null)
+                {
+                    _routeValues = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                }
+
+                return _routeValues;
+            }
+            set
+            {
+                _routeValues = value;
+            }
+        }
 
         /// <inheritdoc />
         /// <remarks>Does nothing if user provides an <c>href</c> attribute.</remarks>
@@ -148,11 +162,16 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
             else
             {
-                // Convert from Dictionary<string, string> to Dictionary<string, object>.
+                IDictionary<string, object> routeValues = null;
-                var routeValues = RouteValues.ToDictionary(
+                if (_routeValues != null && _routeValues.Count > 0)
-                    kvp => kvp.Key,
+                {
-                    kvp => (object)kvp.Value,
+                    // Convert from Dictionary<string, string> to Dictionary<string, object>.
-                    StringComparer.OrdinalIgnoreCase);
+                    routeValues = new Dictionary<string, object>(_routeValues.Count, StringComparer.OrdinalIgnoreCase);
+                    foreach (var routeValue in _routeValues)
+                    {
+                        routeValues.Add(routeValue.Key, routeValue.Value);
+                    }
+                }
 
                 TagBuilder tagBuilder;
                 if (Route == null)
@@ -180,13 +199,14 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 }
                 else
                 {
-                    tagBuilder = Generator.GenerateRouteLink(linkText: string.Empty,
+                    tagBuilder = Generator.GenerateRouteLink(
-                                                             routeName: Route,
+                        linkText: string.Empty,
-                                                             protocol: Protocol,
+                        routeName: Route,
-                                                             hostName: Host,
+                        protocol: Protocol,
-                                                             fragment: Fragment,
+                        hostName: Host,
-                                                             routeValues: routeValues,
+                        fragment: Fragment,
-                                                             htmlAttributes: null);
+                        routeValues: routeValues,
+                        htmlAttributes: null);
                 }
 
                 if (tagBuilder != null)

src/Microsoft.AspNet.Mvc.TagHelpers/CacheTagHelper.cs

@@ -3,11 +3,9 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Security.Cryptography;
 using System.Text;
 using System.Threading.Tasks;
-using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Mvc.ViewFeatures;
 using Microsoft.AspNet.Razor.TagHelpers;
@@ -101,7 +99,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
         /// <summary>
         /// Gets or sets a value that determines if the cached result is to be varied by the Identity for the logged in
-        /// <see cref="HttpContext.User"/>.
+        /// <see cref="Http.HttpContext.User"/>.
         /// </summary>
         [HtmlAttributeName(VaryByUserAttributeName)]
         public bool VaryByUser { get; set; }
@@ -261,18 +259,19 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                        .Append(keyName)
                        .Append("(");
 
-                var tokenFound = false;
+                var values = Tokenize(value);
-                foreach (var item in Tokenize(value))
-                {
-                    tokenFound = true;
 
+                // Perf: Avoid allocating enumerator
+                for (var i = 0; i < values.Count; i++)
+                {
+                    var item = values[i];
                     builder.Append(item)
                            .Append(CacheKeyTokenSeparator)
                            .Append(sourceCollection[item])
                            .Append(CacheKeyTokenSeparator);
                 }
 
-                if (tokenFound)
+                if (values.Count > 0)
                 {
                     // Remove the trailing separator
                     builder.Length -= CacheKeyTokenSeparator.Length;
@@ -282,12 +281,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
         }
 
-        private static void AddStringCollectionKey<T>(
+        private static void AddStringCollectionKey<TSourceCollection>(
             StringBuilder builder,
             string keyName,
             string value,
-            T sourceCollection,
+            TSourceCollection sourceCollection,
-            Func<T, string, StringValues> accessor)
+            Func<TSourceCollection, string, StringValues> accessor)
         {
             if (!string.IsNullOrEmpty(value))
             {
@@ -296,10 +295,12 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                        .Append(keyName)
                        .Append("(");
 
-                var tokenFound = false;
+                var values = Tokenize(value);
-                foreach (var item in Tokenize(value))
+
+                // Perf: Avoid allocating enumerator
+                for (var i = 0; i < values.Count; i++)
                 {
-                    tokenFound = true;
+                    var item = values[i];
 
                     builder.Append(item)
                            .Append(CacheKeyTokenSeparator)
@@ -307,7 +308,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                            .Append(CacheKeyTokenSeparator);
                 }
 
-                if (tokenFound)
+                if (values.Count > 0)
                 {
                     // Remove the trailing separator
                     builder.Length -= CacheKeyTokenSeparator.Length;
@@ -327,8 +328,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                        .Append(nameof(VaryByRoute))
                        .Append("(");
 
-                foreach (var route in Tokenize(VaryByRoute))
+                var varyByRoutes = Tokenize(VaryByRoute);
+                for (var i = 0; i < varyByRoutes.Count; i++)
                 {
+                    var route = varyByRoutes[i];
                     tokenFound = true;
 
                     builder.Append(route)
@@ -346,11 +349,27 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
         }
 
-        private static IEnumerable<string> Tokenize(string value)
+        private static IList<string> Tokenize(string value)
         {
-            return value.Split(AttributeSeparator, StringSplitOptions.RemoveEmptyEntries)
+            var values = value.Split(AttributeSeparator, StringSplitOptions.RemoveEmptyEntries);
-                        .Select(token => token.Trim())
+            if (values.Length == 0)
-                        .Where(token => token.Length > 0);
+            {
+                return values;
+            }
+
+            var trimmedValues = new List<string>();
+
+            for (var i = 0; i < values.Length; i++)
+            {
+                var trimmedValue = values[i].Trim();
+
+                if (trimmedValue.Length > 0)
+                {
+                    trimmedValues.Add(trimmedValue);
+                }
+            }
+
+            return trimmedValues;
         }
     }
 }

src/Microsoft.AspNet.Mvc.TagHelpers/EnvironmentTagHelper.cs

@@ -2,7 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Linq;
+using System.Collections.Generic;
 using Microsoft.AspNet.Hosting;
 using Microsoft.AspNet.Razor.TagHelpers;
 
@@ -67,30 +67,41 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 return;
             }
 
-            var environments = Names.Split(NameSeparator, StringSplitOptions.RemoveEmptyEntries)
-                                    .Where(name => !string.IsNullOrWhiteSpace(name));
-
-            if (!environments.Any())
-            {
-                // Names contains only commas or empty entries, do nothing
-                return;
-            }
-
             var currentEnvironmentName = HostingEnvironment.EnvironmentName?.Trim();
-
+            if (string.IsNullOrEmpty(currentEnvironmentName))
-            if (string.IsNullOrWhiteSpace(currentEnvironmentName))
             {
                 // No current environment name, do nothing
                 return;
             }
 
-            if (environments.Any(name =>
+            var values = Names.Split(NameSeparator, StringSplitOptions.RemoveEmptyEntries);
-                string.Equals(name.Trim(), currentEnvironmentName, StringComparison.OrdinalIgnoreCase)))
+            var environments = new List<string>();
+
+            for (var i = 0; i < values.Length; i++)
             {
-                // Matching environment name found, do nothing
+                var trimmedValue = values[i].Trim();
+
+                if (trimmedValue.Length > 0)
+                {
+                    environments.Add(trimmedValue);
+                }
+            }
+
+            if (environments.Count == 0)
+            {
+                // Names contains only commas or empty entries, do nothing
                 return;
             }
 
+            for (var i = 0; i < environments.Count; i++)
+            {
+                if (string.Equals(environments[i], currentEnvironmentName, StringComparison.OrdinalIgnoreCase))
+                {
+                    // Matching environment name found, do nothing
+                    return;
+                }
+            }
+
             // No matching environment name found, suppress all output
             output.SuppressOutput();
         }

src/Microsoft.AspNet.Mvc.TagHelpers/FormTagHelper.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Mvc.ViewFeatures;
 using Microsoft.AspNet.Razor.TagHelpers;
@@ -28,6 +27,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         private const string RouteValuesDictionaryName = "asp-all-route-data";
         private const string RouteValuesPrefix = "asp-route-";
         private const string HtmlActionAttributeName = "action";
+        private IDictionary<string, string> _routeValues;
 
         /// <summary>
         /// Creates a new <see cref="FormTagHelper"/>.
@@ -85,8 +85,22 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         /// Additional parameters for the route.
         /// </summary>
         [HtmlAttributeName(RouteValuesDictionaryName, DictionaryAttributePrefix = RouteValuesPrefix)]
-        public IDictionary<string, string> RouteValues { get; set; } =
+        public IDictionary<string, string> RouteValues
-            new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        {
+            get
+            {
+                if (_routeValues == null)
+                {
+                    _routeValues = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                }
+
+                return _routeValues;
+            }
+            set
+            {
+                _routeValues = value;
+            }
+        }
 
         /// <inheritdoc />
         /// <remarks>
@@ -133,11 +147,16 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             }
             else
             {
-                // Convert from Dictionary<string, string> to Dictionary<string, object>.
+                IDictionary<string, object> routeValues = null;
-                var routeValues = RouteValues.ToDictionary(
+                if (_routeValues != null && _routeValues.Count > 0)
-                    kvp => kvp.Key,
+                {
-                    kvp => (object)kvp.Value,
+                    // Convert from Dictionary<string, string> to Dictionary<string, object>.
-                    StringComparer.OrdinalIgnoreCase);
+                    routeValues = new Dictionary<string, object>(_routeValues.Count, StringComparer.OrdinalIgnoreCase);
+                    foreach (var routeValue in _routeValues)
+                    {
+                        routeValues.Add(routeValue.Key, routeValue.Value);
+                    }
+                }
 
                 TagBuilder tagBuilder;
                 if (Route == null)

src/Microsoft.AspNet.Mvc.TagHelpers/InputTagHelper.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.ModelBinding;
 using Microsoft.AspNet.Mvc.Rendering;
@@ -247,9 +246,11 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             // Prepare to move attributes from current element to <input type="checkbox"/> generated just below.
             var htmlAttributes = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
 
+            // Perf: Avoid allocating enumerator
             // Construct attributes correctly (first attribute wins).
-            foreach (var attribute in output.Attributes)
+            for (var i = 0; i < output.Attributes.Count; i++)
             {
+                var attribute = output.Attributes[i];
                 if (!htmlAttributes.ContainsKey(attribute.Name))
                 {
                     htmlAttributes.Add(attribute.Name, attribute.Value);
@@ -389,15 +390,14 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         // A variant of TemplateRenderer.GetViewNames(). Main change relates to bool? handling.
         private static IEnumerable<string> GetInputTypeHints(ModelExplorer modelExplorer)
         {
-            var inputTypeHints = new string[]
+            if (!string.IsNullOrEmpty(modelExplorer.Metadata.TemplateHint))
             {
-                modelExplorer.Metadata.TemplateHint,
+                yield return modelExplorer.Metadata.TemplateHint;
-                modelExplorer.Metadata.DataTypeName,
+            }
-            };
 
-            foreach (string inputTypeHint in inputTypeHints.Where(s => !string.IsNullOrEmpty(s)))
+            if (!string.IsNullOrEmpty(modelExplorer.Metadata.DataTypeName))
             {
-                yield return inputTypeHint;
+                yield return modelExplorer.Metadata.DataTypeName;
             }
 
             // In most cases, we don't want to search for Nullable<T>. We want to search for T, which should handle

src/Microsoft.AspNet.Mvc.TagHelpers/Internal/AttributeMatcher.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using Microsoft.AspNet.Razor.TagHelpers;
 
 namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
@@ -23,7 +22,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
         /// <returns>The <see cref="ModeMatchResult{TMode}"/>.</returns>
         public static ModeMatchResult<TMode> DetermineMode<TMode>(
             TagHelperContext context,
-            IEnumerable<ModeAttributes<TMode>> modeInfos)
+            IReadOnlyList<ModeAttributes<TMode>> modeInfos)
         {
             if (context == null)
             {
@@ -39,27 +38,32 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
             var matchedAttributes = new Dictionary<string, bool>(StringComparer.OrdinalIgnoreCase);
             var result = new ModeMatchResult<TMode>();
 
-            foreach (var modeInfo in modeInfos)
+            // Perf: Avoid allocating enumerator
+            for (var i = 0; i < modeInfos.Count; i++)
             {
+                var modeInfo = modeInfos[i];
                 var modeAttributes = GetPresentMissingAttributes(context, modeInfo.Attributes);
 
-                if (modeAttributes.Present.Any())
+                if (modeAttributes.Present.Count > 0)
                 {
-                    if (!modeAttributes.Missing.Any())
+                    if (modeAttributes.Missing.Count == 0)
                     {
+                        // Perf: Avoid allocating enumerator
                         // A complete match, mark the attribute as fully matched
-                        foreach (var attribute in modeAttributes.Present)
+                        for (var j = 0; j < modeAttributes.Present.Count; j++)
                         {
-                            matchedAttributes[attribute] = true;
+                            matchedAttributes[modeAttributes.Present[j]] = true;
                         }
 
                         result.FullMatches.Add(ModeMatchAttributes.Create(modeInfo.Mode, modeInfo.Attributes));
                     }
                     else
                     {
+                        // Perf: Avoid allocating enumerator
                         // A partial match, mark the attribute as partially matched if not already fully matched
-                        foreach (var attribute in modeAttributes.Present)
+                        for (var j = 0; j < modeAttributes.Present.Count; j++)
                         {
+                            var attribute = modeAttributes.Present[j];
                             bool attributeMatch;
                             if (!matchedAttributes.TryGetValue(attribute, out attributeMatch))
                             {
@@ -87,14 +91,16 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
 
         private static PresentMissingAttributes GetPresentMissingAttributes(
             TagHelperContext context,
-            IEnumerable<string> requiredAttributes)
+            string[] requiredAttributes)
         {
             // Check for all attribute values
             var presentAttributes = new List<string>();
             var missingAttributes = new List<string>();
 
-            foreach (var attribute in requiredAttributes)
+            // Perf: Avoid allocating enumerator
+            for (var i = 0; i < requiredAttributes.Length; i++)
             {
+                var attribute = requiredAttributes[i];
                 if (!context.AllAttributes.ContainsName(attribute) ||
                     context.AllAttributes[attribute] == null ||
                     (context.AllAttributes[attribute].Value is string &&
@@ -114,9 +120,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
 
         private class PresentMissingAttributes
         {
-            public IEnumerable<string> Present { get; set; }
+            public List<string> Present { get; set; }
 
-            public IEnumerable<string> Missing { get; set; }
+            public List<string> Missing { get; set; }
         }
     }
 }

src/Microsoft.AspNet.Mvc.TagHelpers/Internal/GlobbingUrlBuilder.cs

@@ -73,7 +73,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
         /// <param name="includePattern">The file globbing include pattern.</param>
         /// <param name="excludePattern">The file globbing exclude pattern.</param>
         /// <returns>The list of URLs</returns>
-        public virtual IEnumerable<string> BuildUrlList(string staticUrl, string includePattern, string excludePattern)
+        public virtual ICollection<string> BuildUrlList(string staticUrl, string includePattern, string excludePattern)
         {
             var urls = new HashSet<string>(StringComparer.Ordinal);
 
@@ -112,9 +112,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
                 if (!Cache.TryGetValue(cacheKey, out files))
                 {
                     var options = new MemoryCacheEntryOptions();
-                    foreach (var pattern in includePatterns)
+
+                    for (var i = 0; i < includePatterns.Length; i++)
                     {
-                        var changeToken = FileProvider.Watch(pattern);
+                        var changeToken = FileProvider.Watch(includePatterns[i]);
                         options.AddExpirationToken(changeToken);
                     }
 
@@ -128,15 +129,24 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
             return FindFiles(includePatterns, excludePatterns);
         }
 
-        private IEnumerable<string> FindFiles(IEnumerable<string> includePatterns, IEnumerable<string> excludePatterns)
+        private IEnumerable<string> FindFiles(string[] includePatterns, string[] excludePatterns)
         {
             var matcher = MatcherBuilder != null ? MatcherBuilder() : new Matcher();
-
+            var trimmedIncludePatterns = new List<string>();
-            matcher.AddIncludePatterns(includePatterns.Select(pattern => TrimLeadingTildeSlash(pattern)));
+            for (var i = 0; i < includePatterns.Length; i++)
+            {
+                trimmedIncludePatterns.Add(TrimLeadingTildeSlash(includePatterns[i]));
+            }
+            matcher.AddIncludePatterns(trimmedIncludePatterns);
 
             if (excludePatterns != null)
             {
-                matcher.AddExcludePatterns(excludePatterns.Select(pattern => TrimLeadingTildeSlash(pattern)));
+                var trimmedExcludePatterns = new List<string>();
+                for (var i = 0; i < excludePatterns.Length; i++)
+                {
+                    trimmedExcludePatterns.Add(TrimLeadingTildeSlash(excludePatterns[i]));
+                }
+                matcher.AddExcludePatterns(trimmedExcludePatterns);
             }
 
             var matches = matcher.Execute(_baseGlobbingDirectory);

src/Microsoft.AspNet.Mvc.TagHelpers/Internal/JavaScriptStringArrayEncoder.cs

@@ -15,7 +15,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
         /// <summary>
         /// Encodes a .NET string array for safe use as a JavaScript array literal, including inline in an HTML file.
         /// </summary>
-        public static string Encode(JavaScriptEncoder encoder, IEnumerable<string> values)
+        public static string Encode(JavaScriptEncoder encoder, string[] values)
         {
             var writer = new StringWriter();
 
@@ -23,14 +23,15 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
 
             writer.Write('[');
 
-            foreach (var value in values)
+            // Perf: Avoid allocating enumerator
+            for (var i = 0; i < values.Length; i++)
             {
                 if (firstAdded)
                 {
                     writer.Write(',');
                 }
                 writer.Write('"');
-                encoder.Encode(writer, value);
+                encoder.Encode(writer, values[i]);
                 writer.Write('"');
                 firstAdded = true;
             }

src/Microsoft.AspNet.Mvc.TagHelpers/Internal/ModeAttributes.cs

@@ -13,7 +13,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
         /// <summary>
         /// Creates an <see cref="ModeAttributes{TMode}"/>/
         /// </summary>
-        public static ModeAttributes<TMode> Create<TMode>(TMode mode, IEnumerable<string> attributes)
+        public static ModeAttributes<TMode> Create<TMode>(TMode mode, string[] attributes)
         {
             return new ModeAttributes<TMode>
             {

src/Microsoft.AspNet.Mvc.TagHelpers/Internal/ModeAttributesOfT.cs

@@ -1,25 +1,22 @@
 // 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.Collections.Generic;
-using Microsoft.AspNet.Razor.TagHelpers;
-
 namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
 {
     /// <summary>
-    /// A mapping of a <see cref="ITagHelper"/> mode to its required attributes.
+    /// A mapping of a <see cref="AspNet.Razor.TagHelpers.ITagHelper"/> mode to its required attributes.
     /// </summary>
-    /// <typeparam name="TMode">The type representing the <see cref="ITagHelper"/>'s mode.</typeparam>
+    /// <typeparam name="TMode">The type representing the <see cref="AspNet.Razor.TagHelpers.ITagHelper"/>'s mode.</typeparam>
     public class ModeAttributes<TMode>
     {
         /// <summary>
-        /// The <see cref="ITagHelper"/>'s mode.
+        /// The <see cref="AspNet.Razor.TagHelpers.ITagHelper"/>'s mode.
         /// </summary>
         public TMode Mode { get; set; }
 
         /// <summary>
         /// The names of attributes required for this mode.
         /// </summary>
-        public IEnumerable<string> Attributes { get; set; }
+        public string[] Attributes { get; set; }
     }
 }

src/Microsoft.AspNet.Mvc.TagHelpers/Internal/ModeMatchAttributes.cs

@@ -15,7 +15,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
         /// </summary>
         public static ModeMatchAttributes<TMode> Create<TMode>(
            TMode mode,
-           IEnumerable<string> presentAttributes)
+           IList<string> presentAttributes)
         {
             return Create(mode, presentAttributes, missingAttributes: null);
         }
@@ -25,8 +25,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
         /// </summary>
         public static ModeMatchAttributes<TMode> Create<TMode>(
             TMode mode,
-            IEnumerable<string> presentAttributes,
+            IList<string> presentAttributes,
-            IEnumerable<string> missingAttributes)
+            IList<string> missingAttributes)
         {
             return new ModeMatchAttributes<TMode>
             {

src/Microsoft.AspNet.Mvc.TagHelpers/Internal/ModeMatchAttributesOfT.cs

@@ -2,29 +2,28 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Collections.Generic;
-using Microsoft.AspNet.Razor.TagHelpers;
 
 namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
 {
     /// <summary>
-    /// A mapping of a <see cref="ITagHelper"/> mode to its missing and present attributes.
+    /// A mapping of a <see cref="AspNet.Razor.TagHelpers.ITagHelper"/> mode to its missing and present attributes.
     /// </summary>
-    /// <typeparam name="TMode">The type representing the <see cref="ITagHelper"/>'s mode.</typeparam>
+    /// <typeparam name="TMode">The type representing the <see cref="AspNet.Razor.TagHelpers.ITagHelper"/>'s mode.</typeparam>
     public class ModeMatchAttributes<TMode>
     {
         /// <summary>
-        /// The <see cref="ITagHelper"/>'s mode.
+        /// The <see cref="AspNet.Razor.TagHelpers.ITagHelper"/>'s mode.
         /// </summary>
         public TMode Mode { get; set; }
 
         /// <summary>
         /// The names of attributes that were present in this match.
         /// </summary>
-        public IEnumerable<string> PresentAttributes { get; set; }
+        public IList<string> PresentAttributes { get; set; }
 
         /// <summary>
         /// The names of attributes that were missing in this match.
         /// </summary>
-        public IEnumerable<string> MissingAttributes { get; set; }
+        public IList<string> MissingAttributes { get; set; }
     }
 }

src/Microsoft.AspNet.Mvc.TagHelpers/Internal/ModeMatchResult.cs

@@ -1,20 +1,14 @@
 // 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.Linq;
-using Microsoft.AspNet.Mvc.Logging;
-using Microsoft.AspNet.Mvc.TagHelpers.Logging;
-using Microsoft.AspNet.Razor.TagHelpers;
-using Microsoft.Extensions.Logging;
 
 namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
 {
     /// <summary>
-    /// Result of determining the mode an <see cref="ITagHelper"/> will run in.
+    /// Result of determining the mode an <see cref="AspNet.Razor.TagHelpers.ITagHelper"/> will run in.
     /// </summary>
-    /// <typeparam name="TMode">The type representing the <see cref="ITagHelper"/>'s mode.</typeparam>
+    /// <typeparam name="TMode">The type representing the <see cref="AspNet.Razor.TagHelpers.ITagHelper"/>'s mode.</typeparam>
     public class ModeMatchResult<TMode>
     {
         /// <summary>
@@ -32,47 +26,5 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
         /// <see cref="FullMatches"/>.
         /// </summary>
         public IList<string> PartiallyMatchedAttributes { get; } = new List<string>();
-
-        /// <summary>
-        /// Logs the details of the <see cref="ModeMatchResult{TMode}"/>.
-        /// </summary>
-        /// <param name="logger">The <see cref="ILogger"/>.</param>
-        /// <param name="tagHelper">The <see cref="ITagHelper"/>.</param>
-        /// <param name="uniqueId">The value of <see cref="TagHelperContext.UniqueId"/>.</param>
-        /// <param name="viewPath">The path to the view the <see cref="ITagHelper"/> is on.</param>
-        public void LogDetails<TTagHelper>(
-            ILogger logger,
-            TTagHelper tagHelper,
-            string uniqueId,
-            string viewPath)
-            where TTagHelper : ITagHelper
-        {
-            if (logger == null)
-            {
-                throw new ArgumentNullException(nameof(logger));
-            }
-
-            if (tagHelper == null)
-            {
-                throw new ArgumentNullException(nameof(tagHelper));
-            }
-
-            if (logger.IsEnabled(LogLevel.Warning) && PartiallyMatchedAttributes.Any())
-            {
-                // Build the list of partial matches that contain attributes not appearing in at least one full match
-                var partialOnlyMatches = PartialMatches.Where(
-                    match => match.PresentAttributes.Any(
-                        attribute => PartiallyMatchedAttributes.Contains(
-                            attribute, StringComparer.OrdinalIgnoreCase)));
-                logger.TagHelperPartialMatches<TMode>(uniqueId, viewPath, partialOnlyMatches);
-            }
-
-            if (logger.IsEnabled(LogLevel.Verbose) && !FullMatches.Any())
-            {
-                logger.TagHelperSkippingProcessing(
-                    tagHelper,
-                    uniqueId);
-            }
-        }
     }
 }

src/Microsoft.AspNet.Mvc.TagHelpers/LinkTagHelper.cs

@@ -10,6 +10,7 @@ using Microsoft.AspNet.Hosting;
 using Microsoft.AspNet.Mvc.Razor.TagHelpers;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Mvc.TagHelpers.Internal;
+using Microsoft.AspNet.Mvc.TagHelpers.Logging;
 using Microsoft.AspNet.Mvc.ViewFeatures;
 using Microsoft.AspNet.Razor.TagHelpers;
 using Microsoft.Extensions.Caching.Memory;
@@ -241,10 +242,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Href = output.Attributes[HrefAttributeName]?.Value as string;
 
             var modeResult = AttributeMatcher.DetermineMode(context, ModeDetails);
+            Logger.TagHelperModeMatchResult(modeResult, context.UniqueId, ViewContext.View.Path, this);
 
-            modeResult.LogDetails(Logger, this, context.UniqueId, ViewContext.View.Path);
+            if (modeResult.FullMatches.Count == 0)
-
-            if (!modeResult.FullMatches.Any())
             {
                 // No attributes matched so we have nothing to do
                 return;
@@ -266,7 +266,15 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var builder = new DefaultTagHelperContent();
 
             // Get the highest matched mode
-            var mode = modeResult.FullMatches.Select(match => match.Mode).Max();
+            var mode = modeResult.FullMatches[0].Mode;
+            for (var i = 1; i < modeResult.FullMatches.Count; i++)
+            {
+                var currentMode = modeResult.FullMatches[i].Mode;
+                if (mode < currentMode)
+                {
+                    mode = currentMode;
+                }
+            }
 
             if (mode == Mode.GlobbedHref || mode == Mode.Fallback && !string.IsNullOrEmpty(HrefInclude))
             {
@@ -384,8 +392,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             builder.AppendHtml("<link ");
 
-            foreach (var attribute in attributes)
+            // Perf: Avoid allocating enumerator
+            for (var i = 0; i < attributes.Count; i++)
             {
+                var attribute = attributes[i];
                 var attributeValue = attribute.Value;
                 if (AppendVersion == true &&
                     string.Equals(attribute.Name, HrefAttributeName, StringComparison.OrdinalIgnoreCase))

src/Microsoft.AspNet.Mvc.TagHelpers/Logging/ModeMatchResultLoggerExtensions.cs

@@ -19,30 +19,54 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Logging
                 "Skipping processing for tag helper '{TagHelper}' with id '{TagHelperId}'.");
         }
 
-        public static void TagHelperPartialMatches<TMode>(
+        public static void TagHelperModeMatchResult<TMode>(
-                    this ILogger logger,
+            this ILogger logger,
-                    string uniqueId,
+            ModeMatchResult<TMode> modeMatchResult,
-                    string viewPath,
+            string uniqueId,
-                    IEnumerable<ModeMatchAttributes<TMode>> partialMatches)
+            string viewPath,
+            ITagHelper tagHelper)
         {
-            var logValues = new PartialModeMatchLogValues<TMode>(
+            if (logger.IsEnabled(LogLevel.Warning) && modeMatchResult.PartiallyMatchedAttributes.Count > 0)
-                uniqueId,
+            {
-                viewPath,
+                // Build the list of partial matches that contain attributes not appearing in at least one full match
-                partialMatches);
+                var partialOnlyMatches = new List<ModeMatchAttributes<TMode>>();
+                for (var i = 0; i < modeMatchResult.PartialMatches.Count; i++)
+                {
+                    var presentAttributes = modeMatchResult.PartialMatches[i].PresentAttributes;
+                    for (var j = 0; j < presentAttributes.Count; j++)
+                    {
+                        var present = presentAttributes[j];
+                        var presentIsPartialOnlyMatch = false;
+                        for (var k = 0; k < modeMatchResult.PartiallyMatchedAttributes.Count; k++)
+                        {
+                            var partiallyMatched = modeMatchResult.PartiallyMatchedAttributes[k];
+                            if (string.Equals(partiallyMatched, present, StringComparison.OrdinalIgnoreCase))
+                            {
+                                presentIsPartialOnlyMatch = true;
+                                break;
+                            }
+                        }
 
-            logger.LogWarning(logValues);
+                        if (presentIsPartialOnlyMatch)
-        }
+                        {
+                            partialOnlyMatches.Add(modeMatchResult.PartialMatches[i]);
+                            break;
+                        }
+                    }
+                }
 
-        public static void TagHelperSkippingProcessing(
+                var logValues = new PartialModeMatchLogValues<TMode>(
-                    this ILogger logger,
+                    uniqueId,
-                    ITagHelper tagHelper,
+                    viewPath,
-                    string uniqueId)
+                    partialOnlyMatches);
-        {
+
-            _skippingProcessing(
+                logger.LogWarning(logValues);
-                logger,
+            }
-                tagHelper,
+
-                uniqueId,
+            if (logger.IsEnabled(LogLevel.Verbose) && modeMatchResult.FullMatches.Count == 0)
-                null);
+            {
+                _skippingProcessing(logger, tagHelper, uniqueId, null);
+            }
         }
 
         /// <summary>

src/Microsoft.AspNet.Mvc.TagHelpers/RenderAtEndOfFormTagHelper.cs

@@ -57,9 +57,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var formContext = ViewContext.FormContext;
             if (formContext.HasEndOfFormContent)
             {
-                foreach (var content in formContext.EndOfFormContent)
+                // Perf: Avoid allocating enumerator
+                for (var i = 0; i < formContext.EndOfFormContent.Count; i++)
                 {
-                    output.PostContent.Append(content);
+                    output.PostContent.Append(formContext.EndOfFormContent[i]);
                 }
             }
 

src/Microsoft.AspNet.Mvc.TagHelpers/ScriptTagHelper.cs

@@ -3,12 +3,12 @@
 
 using System;
 using System.Diagnostics;
-using System.Linq;
 using System.Text.Encodings.Web;
 using Microsoft.AspNet.Hosting;
 using Microsoft.AspNet.Mvc.Razor.TagHelpers;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Mvc.TagHelpers.Internal;
+using Microsoft.AspNet.Mvc.TagHelpers.Logging;
 using Microsoft.AspNet.Mvc.ViewFeatures;
 using Microsoft.AspNet.Razor.TagHelpers;
 using Microsoft.Extensions.Caching.Memory;
@@ -209,10 +209,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Src = output.Attributes[SrcAttributeName]?.Value as string;
 
             var modeResult = AttributeMatcher.DetermineMode(context, ModeDetails);
+            Logger.TagHelperModeMatchResult(modeResult, context.UniqueId, ViewContext.View.Path, this);
 
-            modeResult.LogDetails(Logger, this, context.UniqueId, ViewContext.View.Path);
+            if (modeResult.FullMatches.Count == 0)
-
-            if (!modeResult.FullMatches.Any())
             {
                 // No attributes matched so we have nothing to do
                 return;
@@ -234,7 +233,15 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var builder = new DefaultTagHelperContent();
 
             // Get the highest matched mode
-            var mode = modeResult.FullMatches.Select(match => match.Mode).Max();
+            var mode = modeResult.FullMatches[0].Mode;
+            for (var i = 1; i < modeResult.FullMatches.Count; i++)
+            {
+                var currentMode = modeResult.FullMatches[i].Mode;
+                if (mode < currentMode)
+                {
+                    mode = currentMode;
+                }
+            }
 
             if (mode == Mode.GlobbedSrc || mode == Mode.Fallback && !string.IsNullOrEmpty(SrcInclude))
             {
@@ -290,7 +297,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             EnsureGlobbingUrlBuilder();
 
             var fallbackSrcs = GlobbingUrlBuilder.BuildUrlList(FallbackSrc, FallbackSrcInclude, FallbackSrcExclude);
-            if (fallbackSrcs.Any())
+            if (fallbackSrcs.Count > 0)
             {
                 // Build the <script> tag that checks the test method and if it fails, renders the extra script.
                 builder.AppendHtml(Environment.NewLine)
@@ -312,8 +319,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
                     builder.AppendHtml("<script");
 
-                    foreach (var attribute in attributes)
+                    // Perf: Avoid allocating enumerator
+                    for (var i = 0; i < attributes.Count; i++)
                     {
+                        var attribute = attributes[i];
                         if (!attribute.Name.Equals(SrcAttributeName, StringComparison.OrdinalIgnoreCase))
                         {
                             var encodedKey = JavaScriptEncoder.Encode(attribute.Name);
@@ -373,8 +382,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         {
             builder.AppendHtml("<script");
 
-            foreach (var attribute in attributes)
+            // Perf: Avoid allocating enumerator
+            for (var i = 0; i < attributes.Count; i++)
             {
+                var attribute = attributes[i];
                 var attributeValue = attribute.Value;
                 if (AppendVersion == true &&
                     string.Equals(attribute.Name, SrcAttributeName, StringComparison.OrdinalIgnoreCase))

test/Microsoft.AspNet.Mvc.TagHelpers.Test/Internal/ModeMatchResultTest.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using Microsoft.AspNet.Mvc.TagHelpers.Logging;
 using Microsoft.AspNet.Razor.TagHelpers;
 using Microsoft.Extensions.Logging;
 using Moq;
@@ -22,7 +23,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
             var viewPath = "Views/Home/Index.cshtml";
 
             // Act
-            modeMatchResult.LogDetails(logger, tagHelper.Object, uniqueId, viewPath);
+            logger.TagHelperModeMatchResult(modeMatchResult, uniqueId, viewPath, tagHelper.Object);
 
             // Assert
             Mock.Get(logger).Verify(l => l.Log(
@@ -48,7 +49,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
             var viewPath = "Views/Home/Index.cshtml";
 
             // Act
-            modeMatchResult.LogDetails(logger, tagHelper.Object, uniqueId, viewPath);
+            logger.TagHelperModeMatchResult(modeMatchResult, uniqueId, viewPath, tagHelper.Object);
 
             // Assert
             Mock.Get(logger).Verify(l => l.Log(
@@ -79,7 +80,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
             var viewPath = "Views/Home/Index.cshtml";
 
             // Act
-            modeMatchResult.LogDetails(logger, tagHelper.Object, uniqueId, viewPath);
+            logger.TagHelperModeMatchResult(modeMatchResult, uniqueId, viewPath, tagHelper.Object);
 
             // Assert
             Mock.Get(logger).Verify(l => l.Log(
@@ -110,7 +111,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers.Internal
             var viewPath = "Views/Home/Index.cshtml";
 
             // Act
-            modeMatchResult.LogDetails(logger, tagHelper.Object, uniqueId, viewPath);
+            logger.TagHelperModeMatchResult(modeMatchResult, uniqueId, viewPath, tagHelper.Object);
 
             // Assert
             Mock.Get(logger).Verify(l => l.Log(