Allow retaining the original casing for known header values (#80754)

Author: MihaZupan

src/libraries/System.Net.Http/src/System/Net/Http/GlobalHttpSettings.cs

@@ -8,6 +8,12 @@ namespace System.Net.Http
     /// </summary>
     internal static class GlobalHttpSettings
     {
+        // Switch to disable ignore-case matching for known header values. Enabled by default.
+        public static bool IgnoreCaseForKnownHeaderValues { get; } = RuntimeSettingParser.QueryRuntimeSettingSwitch(
+            "System.Net.Http.IgnoreCaseForKnownHeaderValues",
+            "DOTNET_SYSTEM_NET_HTTP_IGNORECASEFORKNOWNHEADERVALUES",
+            true);
+
         internal static class DiagnosticsHandler
         {
             public static bool EnableActivityPropagation { get; } = RuntimeSettingParser.QueryRuntimeSettingSwitch(

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HeaderDescriptor.cs

@@ -132,7 +132,9 @@ public string GetHeaderValue(ReadOnlySpan<byte> headerValue, Encoding? valueEnco
                 {
                     for (int i = 0; i < knownValues.Length; i++)
                     {
-                        if (ByteArrayHelpers.EqualsOrdinalAsciiIgnoreCase(knownValues[i], headerValue))
+                        if (GlobalHttpSettings.IgnoreCaseForKnownHeaderValues
+                            ? ByteArrayHelpers.EqualsOrdinalAsciiIgnoreCase(knownValues[i], headerValue)
+                            : ByteArrayHelpers.EqualsOrdinalAscii(knownValues[i], headerValue))
                         {
                             return knownValues[i];
                         }
@@ -239,9 +241,17 @@ public string GetHeaderValue(ReadOnlySpan<byte> headerValue, Encoding? valueEnco
 
             Debug.Assert(candidate is null || candidate.Length == contentTypeValue.Length);
 
-            return candidate != null && ByteArrayHelpers.EqualsOrdinalAsciiIgnoreCase(candidate, contentTypeValue) ?
-                candidate :
-                null;
+            if (candidate is not null)
+            {
+                if (GlobalHttpSettings.IgnoreCaseForKnownHeaderValues
+                    ? ByteArrayHelpers.EqualsOrdinalAsciiIgnoreCase(candidate, contentTypeValue)
+                    : ByteArrayHelpers.EqualsOrdinalAscii(candidate, contentTypeValue))
+                {
+                    return candidate;
+                }
+            }
+
+            return null;
         }
 
         private static bool TryDecodeUtf8(ReadOnlySpan<byte> input, [NotNullWhen(true)] out string? decoded)

src/libraries/System.Net.Http/tests/UnitTests/Headers/KnownHeadersTest.cs

@@ -3,6 +3,8 @@
 
 using System.Linq;
 using System.Net.Http.Headers;
+using System.Text;
+using Microsoft.DotNet.RemoteExecutor;
 using Xunit;
 
 namespace System.Net.Http.Tests
@@ -245,5 +247,28 @@ public void GetKnownHeaderValue_Unknown_NotFound(string name, string value)
             Assert.Equal(value, v2);
             Assert.NotSame(v1, v2);
         }
+
+        [ConditionalTheory(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        [InlineData("Access-Control-Allow-Credentials", "true")]
+        [InlineData("Cache-Control", "no-cache")]
+        [InlineData("Content-Type", "text/plain")]
+        public void GetKnownHeaderValue_RespectsAppContextIgnoreCaseSwitch(string name, string value)
+        {
+            RemoteExecutor.Invoke(static (name, value) =>
+            {
+                AppContext.SetSwitch("System.Net.Http.IgnoreCaseForKnownHeaderValues", false);
+
+                KnownHeader knownHeader = KnownHeaders.TryGetKnownHeader(name);
+                Assert.NotNull(knownHeader);
+
+                string sameCase = knownHeader.Descriptor.GetHeaderValue(Encoding.ASCII.GetBytes(value), valueEncoding: null);
+                Assert.NotNull(sameCase);
+                Assert.Equal(value, sameCase);
+
+                string differentCase = knownHeader.Descriptor.GetHeaderValue(Encoding.ASCII.GetBytes(value.ToUpperInvariant()), valueEncoding: null);
+                Assert.NotNull(differentCase);
+                Assert.NotEqual(value, differentCase);
+            }, name, value).Dispose();
+        }
     }
 }

src/libraries/System.Net.Http/tests/UnitTests/System.Net.Http.Unit.Tests.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <StringResourcesPath>../../src/Resources/Strings.resx</StringResourcesPath>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
@@ -78,6 +78,8 @@
          Link="ProductionCode\System\Net\Http\EmptyReadStream.cs" />
     <Compile Include="..\..\src\System\Net\Http\FormUrlEncodedContent.cs"
              Link="ProductionCode\System\Net\Http\FormUrlEncodedContent.cs" />
+    <Compile Include="..\..\src\System\Net\Http\GlobalHttpSettings.cs"
+             Link="ProductionCode\System\Net\Http\GlobalHttpSettings.cs" />
     <Compile Include="..\..\src\System\Net\Http\HeaderEncodingSelector.cs"
              Link="ProductionCode\System\Net\Http\HeaderEncodingSelector.cs" />
     <Compile Include="..\..\src\System\Net\Http\Headers\AltSvcHeaderParser.cs"