Optimize `TagHelperAttributeList` allocations.

- Today `TagHelperAttributeList` and corresponding infrastructure copy themselves too frequently. I've reduced the copying since we own `TagHelperExecutionContext`.
- Removed usage of linq inside of `TagHelperAttributeList` to reduce allocations.

#599

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

@@ -5,7 +5,6 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
-using Microsoft.Extensions.Internal;
 
 namespace Microsoft.AspNet.Razor.TagHelpers
 {
@@ -50,13 +49,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         protected List<TAttribute> Attributes { get; }
 
         /// <inheritdoc />
-        public TAttribute this[int index]
+        public TAttribute this[int index] => Attributes[index];
-        {
-            get
-            {
-                return Attributes[index];
-            }
-        }
 
         /// <summary>
         /// Gets the first <typeparamref name="TAttribute"/> with <see cref="IReadOnlyTagHelperAttribute.Name"/>
@@ -78,18 +71,21 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                     throw new ArgumentNullException(nameof(name));
                 }
 
-                return Attributes.FirstOrDefault(attribute => NameEquals(name, attribute));
+                // Perf: Avoid allocating enumerator
+                for (var i = 0; i < Attributes.Count; i++)
+                {
+                    if (NameEquals(name, Attributes[i]))
+                    {
+                        return Attributes[i];
+                    }
+                }
+
+                return default(TAttribute);
             }
         }
 
         /// <inheritdoc />
-        public int Count
+        public int Count => Attributes.Count;
-        {
-            get
-            {
-                return Attributes.Count;
-            }
-        }
 
         /// <summary>
         /// Determines whether a <typeparamref name="TAttribute"/> matching <paramref name="item"/> exists in the
@@ -131,7 +127,16 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 throw new ArgumentNullException(nameof(name));
             }
 
-            return Attributes.Any(attribute => NameEquals(name, attribute));
+            // Perf: Avoid allocating enumerator
+            for (var i = 0; i < Attributes.Count; i++)
+            {
+                if (NameEquals(name, Attributes[i]))
+                {
+                    return true;
+                }
+            }
+
+            return false;
         }
 
         /// <summary>
@@ -173,7 +178,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 throw new ArgumentNullException(nameof(name));
             }
 
-            attribute = Attributes.FirstOrDefault(attr => NameEquals(name, attr));
+            attribute = this[name];
 
             return attribute != null;
         }
@@ -197,9 +202,23 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 throw new ArgumentNullException(nameof(name));
             }
 
-            attributes = Attributes.Where(attribute => NameEquals(name, attribute));
+            // Perf: Avoid allocating enumerator
+            List<TAttribute> matchedAttributes = null;
+            for (var i = 0; i < Attributes.Count; i++)
+            {
+                if (NameEquals(name, Attributes[i]))
+                {
+                    if (matchedAttributes == null)
+                    {
+                        matchedAttributes = new List<TAttribute>();
+                    }
 
-            return attributes.Any();
+                    matchedAttributes.Add(Attributes[i]);
+                }
+            }
+            attributes = matchedAttributes ?? Enumerable.Empty<TAttribute>();
+
+            return matchedAttributes != null;
         }
 
         /// <inheritdoc />

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

@@ -4,7 +4,6 @@
 using System;
 using System.Collections.Generic;
 using Microsoft.AspNet.Razor.Runtime;
-using Microsoft.Extensions.Internal;
 
 namespace Microsoft.AspNet.Razor.TagHelpers
 {
@@ -134,6 +133,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
 
                 var attributeReplaced = false;
 
+                // Perf: Avoid allocating enumerator
                 for (var i = 0; i < Attributes.Count; i++)
                 {
                     if (NameEquals(name, Attributes[i]))
@@ -280,7 +280,18 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 throw new ArgumentNullException(nameof(name));
             }
 
-            return Attributes.RemoveAll(attribute => NameEquals(name, attribute)) > 0;
+            // Perf: Avoid allocating enumerator
+            var removedAtLeastOne = false;
+            for (var i = Attributes.Count - 1; i >= 0; i--)
+            {
+                if (NameEquals(name, Attributes[i]))
+                {
+                    Attributes.RemoveAt(i);
+                    removedAtLeastOne = true;
+                }
+            }
+
+            return removedAtLeastOne;
         }
 
         /// <inheritdoc />

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

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 
 namespace Microsoft.AspNet.Razor.TagHelpers
 {
@@ -12,6 +11,9 @@ namespace Microsoft.AspNet.Razor.TagHelpers
     /// </summary>
     public class TagHelperContext
     {
+        private ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute> _allAttributes;
+        private IEnumerable<IReadOnlyTagHelperAttribute> _allAttributesData;
+
         /// <summary>
         /// Instantiates a new <see cref="TagHelperContext"/>.
         /// </summary>
@@ -39,8 +41,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 throw new ArgumentNullException(nameof(uniqueId));
             }
 
-            AllAttributes = new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
+            _allAttributesData = allAttributes;
-                allAttributes.Select(attribute => new TagHelperAttribute(attribute.Name, attribute.Value)));
             Items = items;
             UniqueId = uniqueId;
         }
@@ -48,7 +49,18 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         /// <summary>
         /// Every attribute associated with the current HTML element.
         /// </summary>
-        public ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute> AllAttributes { get; }
+        public ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute> AllAttributes
+        {
+            get
+            {
+                if (_allAttributes == null)
+                {
+                    _allAttributes = new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(_allAttributesData);
+                }
+
+                return _allAttributes;
+            }
+        }
 
         /// <summary>
         /// Gets the collection of items used to communicate with other <see cref="ITagHelper"/>s.

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

@@ -54,7 +54,7 @@ namespace Microsoft.AspNet.Razor.TagHelpers
             }
 
             TagName = tagName;
-            Attributes = new TagHelperAttributeList(attributes);
+            Attributes = attributes;
             _getChildContentAsync = getChildContentAsync;
         }
 
@@ -243,8 +243,10 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                 writer.Write('<');
                 writer.Write(TagName);
 
-                foreach (var attribute in Attributes)
+                // Perf: Avoid allocating enumerator
+                for (var i = 0; i < Attributes.Count; i++)
                 {
+                    var attribute = Attributes[i];
                     writer.Write(' ');
                     writer.Write(attribute.Name);