Read interface IList.Count once rather than per iteration (#9962)

src/Http/Routing/src/Patterns/RoutePattern.cs

@@ -139,9 +139,12 @@ namespace Microsoft.AspNetCore.Routing.Patterns
                 throw new ArgumentNullException(nameof(name));
             }
 
-            for (var i = 0; i < Parameters.Count; i++)
+            var parameters = Parameters;
+            // Read interface .Count once rather than per iteration
+            var parametersCount = parameters.Count;
+            for (var i = 0; i < parametersCount; i++)
             {
-                var parameter = Parameters[i];
+                var parameter = parameters[i];
                 if (string.Equals(parameter.Name, name, StringComparison.OrdinalIgnoreCase))
                 {
                     return parameter;

src/Http/Routing/src/Template/TemplateBinder.cs

@@ -521,16 +521,20 @@ namespace Microsoft.AspNetCore.Routing.Template
                 }
             }
 
-            for (var i = 0; i < _pattern.PathSegments.Count; i++)
+            var segments = _pattern.PathSegments;
+            // Read interface .Count once rather than per iteration
+            var segmentsCount = segments.Count;
+            for (var i = 0; i < segmentsCount; i++)
             {
                 Debug.Assert(context.BufferState == SegmentState.Beginning);
                 Debug.Assert(context.UriState == SegmentState.Beginning);
 
-                var segment = _pattern.PathSegments[i];
+                var parts = segments[i].Parts;
-
+                // Read interface .Count once rather than per iteration
-                for (var j = 0; j < segment.Parts.Count; j++)
+                var partsCount = parts.Count;
+                for (var j = 0; j < partsCount; j++)
                 {
-                    var part = segment.Parts[j];
+                    var part = parts[j];
 
                     if (part is RoutePatternLiteralPart literalPart)
                     {
@@ -581,7 +585,7 @@ namespace Microsoft.AspNetCore.Routing.Template
                             // for format, so we remove '.' and generate 5.
                             if (!context.Accept(converted, parameterPart.EncodeSlashes))
                             {
-                                if (j != 0 && parameterPart.IsOptional && (separatorPart = segment.Parts[j - 1] as RoutePatternSeparatorPart) != null)
+                                if (j != 0 && parameterPart.IsOptional && (separatorPart = parts[j - 1] as RoutePatternSeparatorPart) != null)
                                 {
                                     context.Remove(separatorPart.Content);
                                 }

src/Http/Routing/src/Tree/LinkGenerationDecisionTree.cs

@@ -113,14 +113,20 @@ namespace Microsoft.AspNetCore.Routing.Tree
         {
             // Any entries in node.Matches have had all their required values satisfied, so add them
             // to the results.
-            for (var i = 0; i < node.Matches.Count; i++)
+            var matches = node.Matches;
+            // Read interface .Count once rather than per iteration
+            var matchesCount = matches.Count;
+            for (var i = 0; i < matchesCount; i++)
             {
-                results.Add(new OutboundMatchResult(node.Matches[i], isFallbackPath));
+                results.Add(new OutboundMatchResult(matches[i], isFallbackPath));
             }
 
-            for (var i = 0; i < node.Criteria.Count; i++)
+            var criteria = node.Criteria;
+            // Read interface .Count once rather than per iteration
+            var criteriaCount = criteria.Count;
+            for (var i = 0; i < criteriaCount; i++)
             {
-                var criterion = node.Criteria[i];
+                var criterion = criteria[i];
                 var key = criterion.Key;
 
                 if (values.TryGetValue(key, out var value))

src/Mvc/Mvc.Core/src/Filters/DefaultFilterProvider.cs

@@ -21,10 +21,12 @@ namespace Microsoft.AspNetCore.Mvc.Filters
 
             if (context.ActionContext.ActionDescriptor.FilterDescriptors != null)
             {
-                // Perf: Avoid allocations
+                var results = context.Results;
-                for (var i = 0; i < context.Results.Count; i++)
+                // Perf: Avoid allocating enumerator and read interface .Count once rather than per iteration
+                var resultsCount = results.Count;
+                for (var i = 0; i < resultsCount; i++)
                 {
-                    ProvideFilter(context, context.Results[i]);
+                    ProvideFilter(context, results[i]);
                 }
             }
         }

src/Mvc/Mvc.Razor/src/RazorViewEngine.cs

@@ -249,15 +249,18 @@ namespace Microsoft.AspNetCore.Mvc.Razor
                 isMainPage);
             Dictionary<string, string> expanderValues = null;
 
-            if (_options.ViewLocationExpanders.Count > 0)
+            var expanders = _options.ViewLocationExpanders;
+            // Read interface .Count once rather than per iteration
+            var expandersCount = expanders.Count;
+            if (expandersCount > 0)
             {
                 expanderValues = new Dictionary<string, string>(StringComparer.Ordinal);
                 expanderContext.Values = expanderValues;
 
                 // Perf: Avoid allocations
-                for (var i = 0; i < _options.ViewLocationExpanders.Count; i++)
+                for (var i = 0; i < expandersCount; i++)
                 {
-                    _options.ViewLocationExpanders[i].PopulateValues(expanderContext);
+                    expanders[i].PopulateValues(expanderContext);
                 }
             }
 
@@ -350,9 +353,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         {
             var viewLocations = GetViewLocationFormats(expanderContext);
 
-            for (var i = 0; i < _options.ViewLocationExpanders.Count; i++)
+            var expanders = _options.ViewLocationExpanders;
+            // Read interface .Count once rather than per iteration
+            var expandersCount = expanders.Count;
+            for (var i = 0; i < expandersCount; i++)
             {
-                viewLocations = _options.ViewLocationExpanders[i].ExpandViewLocations(expanderContext, viewLocations);
+                viewLocations = expanders[i].ExpandViewLocations(expanderContext, viewLocations);
             }
 
             ViewLocationCacheResult cacheResult = null;
@@ -404,9 +410,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             var viewDescriptor = factoryResult.ViewDescriptor;
             if (viewDescriptor?.ExpirationTokens != null)
             {
-                for (var i = 0; i < viewDescriptor.ExpirationTokens.Count; i++)
+                var viewExpirationTokens = viewDescriptor.ExpirationTokens;
+                // Read interface .Count once rather than per iteration
+                var viewExpirationTokensCount = viewExpirationTokens.Count;
+                for (var i = 0; i < viewExpirationTokensCount; i++)
                 {
-                    expirationTokens.Add(viewDescriptor.ExpirationTokens[i]);
+                    expirationTokens.Add(viewExpirationTokens[i]);
                 }
             }
 

src/Mvc/Mvc.Razor/src/TagHelpers/UrlResolutionTagHelper.cs

@@ -156,9 +156,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
                 throw new ArgumentNullException(nameof(output));
             }
 
-            for (var i = 0; i < output.Attributes.Count; i++)
+            var attributes = output.Attributes;
+            // Read interface .Count once rather than per iteration
+            var attributesCount = attributes.Count;
+            for (var i = 0; i < attributesCount; i++)
             {
-                var attribute = output.Attributes[i];
+                var attribute = attributes[i];
                 if (!string.Equals(attribute.Name, attributeName, StringComparison.OrdinalIgnoreCase))
                 {
                     continue;
@@ -170,7 +173,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
                     string resolvedUrl;
                     if (TryResolveUrl(stringValue, resolvedUrl: out resolvedUrl))
                     {
-                        output.Attributes[i] = new TagHelperAttribute(
+                        attributes[i] = new TagHelperAttribute(
                             attribute.Name,
                             resolvedUrl,
                             attribute.ValueStyle);
@@ -199,7 +202,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
                         IHtmlContent resolvedUrl;
                         if (TryResolveUrl(stringValue, resolvedUrl: out resolvedUrl))
                         {
-                            output.Attributes[i] = new TagHelperAttribute(
+                            attributes[i] = new TagHelperAttribute(
                                 attribute.Name,
                                 resolvedUrl,
                                 attribute.ValueStyle);
@@ -207,7 +210,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
                         else if (htmlString == null)
                         {
                             // Not a ~/ URL. Just avoid re-encoding the attribute value later.
-                            output.Attributes[i] = new TagHelperAttribute(
+                            attributes[i] = new TagHelperAttribute(
                                 attribute.Name,
                                 new HtmlString(stringValue),
                                 attribute.ValueStyle);

src/Mvc/Mvc.TagHelpers/src/AttributeMatcher.cs

@@ -44,13 +44,20 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             }
 
             var foundResult = false;
-            result = default(TMode);
+            result = default;
 
             // Perf: Avoid allocating enumerator
-            for (var i = 0; i < modeInfos.Count; i++)
+            var modeInfosCount = modeInfos.Count;
+            var allAttributes = context.AllAttributes;
+            // Read interface .Count once rather than per iteration
+            var allAttributesCount = allAttributes.Count;
+            for (var i = 0; i < modeInfosCount; i++)
             {
                 var modeInfo = modeInfos[i];
-                if (!HasMissingAttributes(context, modeInfo.Attributes) &&
+                var requiredAttributes = modeInfo.Attributes;
+                // If there are fewer attributes present than required, one or more of them must be missing.
+                if (allAttributesCount >= requiredAttributes.Length && 
+                    !HasMissingAttributes(allAttributes, requiredAttributes) &&
                     compare(result, modeInfo.Mode) <= 0)
                 {
                     foundResult = true;
@@ -61,19 +68,13 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             return foundResult;
         }
 
-        private static bool HasMissingAttributes(TagHelperContext context, string[] requiredAttributes)
+        private static bool HasMissingAttributes(ReadOnlyTagHelperAttributeList allAttributes, string[] requiredAttributes)
         {
-            if (context.AllAttributes.Count < requiredAttributes.Length)
-            {
-                // If there are fewer attributes present than required, one or more of them must be missing.
-                return true;
-            }
-
             // Check for all attribute values
             // Perf: Avoid allocating enumerator
             for (var i = 0; i < requiredAttributes.Length; i++)
             {
-                if (!context.AllAttributes.TryGetAttribute(requiredAttributes[i], out var attribute))
+                if (!allAttributes.TryGetAttribute(requiredAttributes[i], out var attribute))
                 {
                     // Missing attribute.
                     return true;

src/Mvc/Mvc.TagHelpers/src/LinkTagHelper.cs

@@ -381,8 +381,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
             builder.AppendHtml(", \"");
 
-            // Perf: Avoid allocating enumerator
+            // Perf: Avoid allocating enumerator and read interface .Count once rather than per iteration
-            for (var i = 0; i < attributes.Count; i++)
+            var attributesCount = attributes.Count;
+            for (var i = 0; i < attributesCount; i++)
             {
                 var attribute = attributes[i];
                 if (string.Equals(attribute.Name, HrefAttributeName, StringComparison.OrdinalIgnoreCase))
@@ -440,8 +441,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
         {
             builder.AppendHtml("[");
             var firstAdded = false;
-            // Perf: Avoid allocating enumerator
+
-            for (var i = 0; i < fallbackHrefs.Count; i++)
+            // Perf: Avoid allocating enumerator and read interface .Count once rather than per iteration
+            var fallbackHrefsCount = fallbackHrefs.Count;
+            for (var i = 0; i < fallbackHrefsCount; i++)
             {
                 if (firstAdded)
                 {
@@ -498,8 +501,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
             var addHref = true;
 
-            // Perf: Avoid allocating enumerator
+            // Perf: Avoid allocating enumerator and read interface .Count once rather than per iteration
-            for (var i = 0; i < attributes.Count; i++)
+            var attributesCount = attributes.Count;
+            for (var i = 0; i < attributesCount; i++)
             {
                 var attribute = attributes[i];
 

src/Mvc/Mvc.TagHelpers/src/ScriptTagHelper.cs

@@ -331,8 +331,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
                     var addSrc = true;
 
-                    // Perf: Avoid allocating enumerator
+                    // Perf: Avoid allocating enumerator and read interface .Count once rather than per iteration
-                    for (var i = 0; i < attributes.Count; i++)
+                    var attributesCount = attributes.Count;
+                    for (var i = 0; i < attributesCount; i++)
                     {
                         var attribute = attributes[i];
                         if (!attribute.Name.Equals(SrcAttributeName, StringComparison.OrdinalIgnoreCase))
@@ -434,8 +435,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
             var addSrc = true;
 
-            // Perf: Avoid allocating enumerator
+            // Perf: Avoid allocating enumerator and read interface .Count once rather than per iteration
-            for (var i = 0; i < attributes.Count; i++)
+            var attributesCount = attributes.Count;
+            for (var i = 0; i < attributesCount; i++)
             {
                 var attribute = attributes[i];
                 if (!attribute.Name.Equals(SrcAttributeName, StringComparison.OrdinalIgnoreCase))

src/Mvc/Mvc.TagHelpers/src/TagHelperOutputExtensions.cs

@@ -321,13 +321,14 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             TagHelperOutput tagHelperOutput,
             TagHelperContext context)
         {
-            var existingAttribute = context.AllAttributes[allAttributeIndex];
+            var allAttributes = context.AllAttributes;
+            var existingAttribute = allAttributes[allAttributeIndex];
 
             // Move backwards through context.AllAttributes from the provided index until we find a familiar attribute
             // in tagHelperOutput where we can insert the copied value after the familiar one.
             for (var i = allAttributeIndex - 1; i >= 0; i--)
             {
-                var previousName = context.AllAttributes[i].Name;
+                var previousName = allAttributes[i].Name;
                 var index = IndexOfFirstMatch(previousName, tagHelperOutput.Attributes);
                 if (index != -1)
                 {
@@ -336,11 +337,13 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 }
             }
 
+            // Read interface .Count once rather than per iteration
+            var allAttributesCount = allAttributes.Count;
             // Move forward through context.AllAttributes from the provided index until we find a familiar attribute in
             // tagHelperOutput where we can insert the copied value.
-            for (var i = allAttributeIndex + 1; i < context.AllAttributes.Count; i++)
+            for (var i = allAttributeIndex + 1; i < allAttributesCount; i++)
             {
-                var nextName = context.AllAttributes[i].Name;
+                var nextName = allAttributes[i].Name;
                 var index = IndexOfFirstMatch(nextName, tagHelperOutput.Attributes);
                 if (index != -1)
                 {
@@ -355,7 +358,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
         private static int IndexOfFirstMatch(string name, TagHelperAttributeList attributes)
         {
-            for (var i = 0; i < attributes.Count; i++)
+            // Read interface .Count once rather than per iteration
+            var attributesCount = attributes.Count;
+            for (var i = 0; i < attributesCount; i++)
             {
                 if (string.Equals(name, attributes[i].Name, StringComparison.OrdinalIgnoreCase))
                 {

src/Razor/Razor.Runtime/ref/Microsoft.AspNetCore.Razor.Runtime.netcoreapp3.0.cs

@@ -93,7 +93,6 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
     public partial class TagHelperRunner
     {
         public TagHelperRunner() { }
-        [System.Diagnostics.DebuggerStepThroughAttribute]
         public System.Threading.Tasks.Task RunAsync(Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperExecutionContext executionContext) { throw null; }
     }
     public partial class TagHelperScopeManager

src/Razor/Razor.Runtime/src/Runtime/TagHelpers/TagHelperRunner.cs

@@ -20,7 +20,7 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
         /// </param>
         /// <returns>Resulting <see cref="TagHelperOutput"/> from processing all of the
         /// <paramref name="executionContext"/>'s <see cref="ITagHelper"/>s.</returns>
-        public async Task RunAsync(TagHelperExecutionContext executionContext)
+        public Task RunAsync(TagHelperExecutionContext executionContext)
         {
             if (executionContext == null)
             {
@@ -28,19 +28,40 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
             }
 
             var tagHelperContext = executionContext.Context;
+            var tagHelpers = executionContext.TagHelpers;
+            OrderTagHelpers(tagHelpers);
 
-            OrderTagHelpers(executionContext.TagHelpers);
+            // Read interface .Count once rather than per iteration
-
+            var count = tagHelpers.Count;
-            for (var i = 0; i < executionContext.TagHelpers.Count; i++)
+            for (var i = 0; i < count; i++)
             {
-                executionContext.TagHelpers[i].Init(tagHelperContext);
+                tagHelpers[i].Init(tagHelperContext);
             }
 
             var tagHelperOutput = executionContext.Output;
 
-            for (var i = 0; i < executionContext.TagHelpers.Count; i++)
+            for (var i = 0; i < count; i++)
             {
-                await executionContext.TagHelpers[i].ProcessAsync(tagHelperContext, tagHelperOutput);
+                var task = tagHelpers[i].ProcessAsync(tagHelperContext, tagHelperOutput);
+                if (!task.IsCompletedSuccessfully)
+                {
+                    return Awaited(task, executionContext, i + 1, count);
+                }
+            }
+
+            return Task.CompletedTask;
+
+            static async Task Awaited(Task task, TagHelperExecutionContext executionContext, int i, int count)
+            {
+                await task;
+
+                var tagHelpers = executionContext.TagHelpers;
+                var tagHelperOutput = executionContext.Output;
+                var tagHelperContext = executionContext.Context;
+                for (; i < count; i++)
+                {
+                    await tagHelpers[i].ProcessAsync(tagHelperContext, tagHelperOutput);
+                }
             }
         }
 
@@ -48,14 +69,16 @@ namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
         {
             // Using bubble-sort here due to its simplicity. It'd be an extreme corner case to ever have more than 3 or
             // 4 tag helpers simultaneously.
-            ITagHelper temp = null;
+
-            for (var i = 0; i < tagHelpers.Count; i++)
+            // Read interface .Count once rather than per iteration
+            var count = tagHelpers.Count;
+            for (var i = 0; i < count; i++)
             {
-                for (var j = i + 1; j < tagHelpers.Count; j++)
+                for (var j = i + 1; j < count; j++)
                 {
                     if (tagHelpers[j].Order < tagHelpers[i].Order)
                     {
-                        temp = tagHelpers[i];
+                        var temp = tagHelpers[i];
                         tagHelpers[i] = tagHelpers[j];
                         tagHelpers[j] = temp;
                     }

src/Razor/Razor/src/TagHelpers/ReadOnlyTagHelperAttributeList.cs

@@ -54,11 +54,15 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                 }
 
                 // Perf: Avoid allocating enumerator
-                for (var i = 0; i < Items.Count; i++)
+                var items = Items;
+                // Read interface .Count once rather than per iteration
+                var itemsCount = items.Count;
+                for (var i = 0; i < itemsCount; i++)
                 {
-                    if (NameEquals(name, Items[i]))
+                    var attribute = items[i];
+                    if (NameEquals(name, attribute))
                     {
-                        return Items[i];
+                        return attribute;
                     }
                 }
 
@@ -126,16 +130,20 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
 
             // Perf: Avoid allocating enumerator
             List<TagHelperAttribute> matchedAttributes = null;
-            for (var i = 0; i < Items.Count; i++)
+            var items = Items;
+            // Read interface .Count once rather than per iteration
+            var itemsCount = items.Count;
+            for (var i = 0; i < itemsCount; i++)
             {
-                if (NameEquals(name, Items[i]))
+                var attribute = items[i];
+                if (NameEquals(name, attribute))
                 {
                     if (matchedAttributes == null)
                     {
                         matchedAttributes = new List<TagHelperAttribute>();
                     }
 
-                    matchedAttributes.Add(Items[i]);
+                    matchedAttributes.Add(attribute);
                 }
             }
             attributes = matchedAttributes ?? EmptyList;
@@ -158,9 +166,12 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                 throw new ArgumentNullException(nameof(name));
             }
 
-            for (var i = 0; i < Items.Count; i++)
+            var items = Items;
+            // Read interface .Count once rather than per iteration
+            var itemsCount = items.Count;
+            for (var i = 0; i < itemsCount; i++)
             {
-                if (NameEquals(name, Items[i]))
+                if (NameEquals(name, items[i]))
                 {
                     return i;
                 }

src/Razor/Razor/src/TagHelpers/TagHelperOutput.cs

@@ -305,8 +305,9 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                 destination.AppendHtml("<");
                 destination.AppendHtml(TagName);
 
-                // Perf: Avoid allocating enumerator
+                // Perf: Avoid allocating enumerator, cache .Count as it goes via interface
-                for (var i = 0; i < Attributes.Count; i++)
+                var count = Attributes.Count;
+                for (var i = 0; i < count; i++)
                 {
                     var attribute = Attributes[i];
                     destination.AppendHtml(" ");
@@ -356,8 +357,9 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                 destination.AppendHtml("<");
                 destination.AppendHtml(TagName);
 
-                // Perf: Avoid allocating enumerator
+                // Perf: Avoid allocating enumerator, cache .Count as it goes via interface
-                for (var i = 0; i < Attributes.Count; i++)
+                var count = Attributes.Count;
+                for (var i = 0; i < count; i++)
                 {
                     var attribute = Attributes[i];
                     destination.AppendHtml(" ");
@@ -416,8 +418,9 @@ namespace Microsoft.AspNetCore.Razor.TagHelpers
                 writer.Write("<");
                 writer.Write(TagName);
 
-                // Perf: Avoid allocating enumerator
+                // Perf: Avoid allocating enumerator, cache .Count as it goes via interface
-                for (var i = 0; i < Attributes.Count; i++)
+                var count = Attributes.Count;
+                for (var i = 0; i < count; i++)
                 {
                     var attribute = Attributes[i];
                     writer.Write(" ");