Optimize allocations of List<ISymbol> and related

src/Microsoft.AspNet.Razor/Parser/SyntaxTree/Span.cs

@@ -16,6 +16,7 @@ namespace Microsoft.AspNet.Razor.Parser.SyntaxTree
     public class Span : SyntaxTreeNode
     {
         private static readonly int TypeHashCode = typeof(Span).GetHashCode();
+        private string _content;
         private SourceLocation _start;
 
         public Span(SpanBuilder builder)
@@ -24,7 +25,7 @@ namespace Microsoft.AspNet.Razor.Parser.SyntaxTree
         }
 
         public SpanKind Kind { get; protected set; }
-        public IEnumerable<ISymbol> Symbols { get; protected set; }
+        public IReadOnlyList<ISymbol> Symbols { get; protected set; }
 
         // Allow test code to re-link spans
         public Span Previous { get; protected internal set; }
@@ -48,7 +49,25 @@ namespace Microsoft.AspNet.Razor.Parser.SyntaxTree
             get { return _start; }
         }
 
-        public string Content { get; private set; }
+        public string Content
+        {
+            get
+            {
+                if (_content == null)
+                {
+                    var builder = new StringBuilder();
+                    for (var i = 0; i < Symbols.Count; i++)
+                    {
+                        var symbol = Symbols[i];
+                        builder.Append(symbol.Content);
+                    }
+
+                    _content = builder.ToString();
+                }
+
+                return _content;
+            }
+        }
 
         public void Change(Action<SpanBuilder> changes)
         {
@@ -59,19 +78,15 @@ namespace Microsoft.AspNet.Razor.Parser.SyntaxTree
 
         public void ReplaceWith(SpanBuilder builder)
         {
-            Debug.Assert(!builder.Symbols.Any() || builder.Symbols.All(s => s != null));
-
             Kind = builder.Kind;
             Symbols = builder.Symbols;
             EditHandler = builder.EditHandler;
             ChunkGenerator = builder.ChunkGenerator ?? SpanChunkGenerator.Null;
             _start = builder.Start;
+            _content = null;
 
             // Since we took references to the values in SpanBuilder, clear its references out
             builder.Reset();
-
-            // Calculate other properties
-            Content = Symbols.Aggregate(new StringBuilder(), (sb, sym) => sb.Append(sym.Content), sb => sb.ToString());
         }
 
         /// <summary>

src/Microsoft.AspNet.Razor/Parser/SyntaxTree/SpanBuilder.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
 using System.Linq;
 using Microsoft.AspNet.Razor.Editor;
 using Microsoft.AspNet.Razor.Chunks.Generators;
@@ -13,7 +12,7 @@ namespace Microsoft.AspNet.Razor.Parser.SyntaxTree
 {
     public class SpanBuilder
     {
-        private IList<ISymbol> _symbols = new List<ISymbol>();
+        private List<ISymbol> _symbols;
         private SourceLocationTracker _tracker = new SourceLocationTracker();
 
         public SpanBuilder(Span original)
@@ -31,11 +30,20 @@ namespace Microsoft.AspNet.Razor.Parser.SyntaxTree
         }
 
         public SourceLocation Start { get; set; }
+
         public SpanKind Kind { get; set; }
 
-        public ReadOnlyCollection<ISymbol> Symbols
+        public IReadOnlyList<ISymbol> Symbols
         {
-            get { return new ReadOnlyCollection<ISymbol>(_symbols); }
+            get
+            {
+                if (_symbols == null)
+                {
+                    _symbols = new List<ISymbol>();
+                }
+
+                return _symbols;
+            }
         }
 
         public SpanEditHandler EditHandler { get; set; }
@@ -43,7 +51,10 @@ namespace Microsoft.AspNet.Razor.Parser.SyntaxTree
 
         public void Reset()
         {
-            _symbols = new List<ISymbol>();
+            // Need to potentially allocate a new list because Span.ReplaceWith takes ownership
+            // of the original list.
+            _symbols = null;
+
             EditHandler = SpanEditHandler.CreateDefault(s => Enumerable.Empty<ISymbol>());
             ChunkGenerator = SpanChunkGenerator.Null;
             Start = SourceLocation.Zero;
@@ -67,6 +78,11 @@ namespace Microsoft.AspNet.Razor.Parser.SyntaxTree
                 return;
             }
 
+            if (_symbols == null)
+            {
+                _symbols = new List<ISymbol>();
+            }
+
             if (_symbols.Count == 0)
             {
                 Start = symbol.Start;

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

@@ -647,8 +647,9 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             EnsureTagBlock(tagBlock);
 
             var childSpan = (Span)tagBlock.Children.First();
+
             // We grab the symbol that could be forward slash
-            var relevantSymbol = (HtmlSymbol)childSpan.Symbols.Take(2).Last();
+            var relevantSymbol = (HtmlSymbol)childSpan.Symbols[childSpan.Symbols.Count == 1 ? 0 : 1];
 
             return relevantSymbol.Type == HtmlSymbolType.ForwardSlash;
         }