Improve HeaderSplit perf

src/Http/Http.Abstractions/src/Internal/ParsingHelpers.cs

@@ -19,22 +19,22 @@ public static StringValues GetHeader(IHeaderDictionary headers, string key)
         public static StringValues GetHeaderSplit(IHeaderDictionary headers, string key)
         {
             var values = GetHeaderUnmodified(headers, key);
-            return new StringValues(GetHeaderSplitImplementation(values).ToArray());
-        }
 
-        private static IEnumerable<string> GetHeaderSplitImplementation(StringValues values)
-        {
+            StringValues result = default;
+
             foreach (var segment in new HeaderSegmentCollection(values))
             {
                 if (!StringSegment.IsNullOrEmpty(segment.Data))
                 {
                     var value = DeQuote(segment.Data.Value);
                     if (!string.IsNullOrEmpty(value))
                     {
-                        yield return value;
+                        result = StringValues.Concat(in result, value);
                     }
                 }
             }
+
+            return result;
         }
 
         public static StringValues GetHeaderUnmodified(IHeaderDictionary headers, string key)

src/Http/perf/Microbenchmarks/AssemblyInfo.cs

@@ -0,0 +1 @@
+[assembly: BenchmarkDotNet.Attributes.AspNetCoreBenchmark]

src/Http/perf/Microbenchmarks/GetHeaderSplitBenchmark.cs

@@ -0,0 +1,63 @@
+// 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 BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Configs;
+using Microsoft.AspNetCore.Http.Internal;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Primitives;
+
+namespace Microsoft.AspNetCore.Http.Abstractions.Microbenchmarks
+{
+    public class GetHeaderSplitBenchmark
+    {
+        HeaderDictionary _dictionary;
+
+        [GlobalSetup]
+        public void GlobalSetup()
+        {
+            var dict = new Dictionary<string, StringValues>()
+            {
+                { "singleValue", new StringValues("single") },
+                { "singleValueQuoted", new StringValues("\"single\"") },
+                { "doubleValue", new StringValues(new [] { "first", "second" }) },
+                { "manyValue", new StringValues(new [] { "first", "second", "third", "fourth", "fifth", "sixth" }) }
+            };
+            _dictionary = new HeaderDictionary(dict);
+        }
+
+        [Benchmark]
+        public void SplitSingleHeader()
+        {
+            var values = ParsingHelpers.GetHeaderSplit(_dictionary, "singleValue");
+            if (values.Count != 1)
+                throw new Exception();
+        }
+
+        [Benchmark]
+        public void SplitSingleQuotedHeader()
+        {
+            var values = ParsingHelpers.GetHeaderSplit(_dictionary, "singleValueQuoted");
+            if (values.Count != 1)
+                throw new Exception();
+        }
+
+        [Benchmark]
+        public void SplitDoubleHeader()
+        {
+            var values = ParsingHelpers.GetHeaderSplit(_dictionary, "doubleValue");
+            if (values.Count != 2)
+                throw new Exception();
+        }
+
+        [Benchmark]
+        public void SplitManyHeaders()
+        {
+            var values = ParsingHelpers.GetHeaderSplit(_dictionary, "manyValue");
+            if (values.Count != 6)
+                throw new Exception();
+        }
+    }
+}

src/Http/perf/Microbenchmarks/Microsoft.AspNetCore.Http.Microbenchmarks.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Reference Include="BenchmarkDotNet" />
+    <Reference Include="Microsoft.AspNetCore.BenchmarkRunner.Sources" />
+    <Reference Include="Microsoft.AspNetCore.Http.Abstractions" />
+    <Reference Include="Microsoft.AspNetCore.Http" />
+  </ItemGroup>
+</Project>