CultureName "c" and "C" get mapped to Invariant Culture

src/libraries/Fuzzing/DotnetFuzzing/Fuzzers/AssemblyNameInfoFuzzer.cs

@@ -68,13 +68,23 @@ private static void Test(Span<char> span)
                     // When converting to AssemblyName, the culture name is lower-cased
                     // by the CultureInfo ctor that calls CultureData.GetCultureData
                     // which lowers the name for caching and normalization purposes.
-                    Assert.Equal(fromTryParse.CultureName.ToLower(), fromParse.ToAssemblyName().CultureName);
+
+                    string lowerCase = fromTryParse.CultureName.ToLower();
+                    if (lowerCase != "c")
+                    {
+                        Assert.Equal(lowerCase, fromParse.ToAssemblyName().CultureName);
+                    }
+                    else
+                    {
+                        // Cultures "c" and "C" get mapped to Invariant Culture.
+                        Assert.Equal("", fromParse.ToAssemblyName().CultureName);
+                        Assert.Equal(CultureInfo.InvariantCulture, fromParse.ToAssemblyName().CultureInfo);
+                    }
                 }
                 else
                 {
                     Assert.True(fromParse.ToAssemblyName().CultureName is null);
                 }
-
 
                 // AssemblyNameInfo.FullName can be different than AssemblyName.FullName:
                 // AssemblyNameInfo includes public key, AssemblyName only its Token.

src/libraries/System.Reflection.Metadata/tests/Metadata/AssemblyNameInfoTests.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Globalization;
 using Xunit;
 
 namespace System.Reflection.Metadata.Tests.Metadata
@@ -116,6 +117,19 @@ public void CultureNameGetLoweredByToAssemblyName()
             Assert.Equal("aa", assemblyNameInfo.ToAssemblyName().CultureName);
         }
 
+        [Theory]
+        // "c" is an invalid culture identifier in Full Framework
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework)]
+        [InlineData('c')]
+        [InlineData('C')]
+        public void Culture_C_IsMappedToInvariantCulture(char culture)
+        {
+            AssemblyNameInfo assemblyNameInfo = AssemblyNameInfo.Parse($"name,culture={culture}".AsSpan());
+            Assert.Equal(culture.ToString(), assemblyNameInfo.CultureName);
+            Assert.Equal("", assemblyNameInfo.ToAssemblyName().CultureName);
+            Assert.Equal(CultureInfo.InvariantCulture, assemblyNameInfo.ToAssemblyName().CultureInfo);
+        }
+
         static void Roundtrip(AssemblyName source)
         {
             AssemblyNameInfo parsed = AssemblyNameInfo.Parse(source.FullName.AsSpan());