Fix KeyValuePair nullability detection on Mono

[medhatiwari]

Fix KeyValuePair nullability detection on Mono

Summary

Fixes incorrect nullability detection for KeyValuePair<TKey, TValue> generic parameters on Mono, which was causing ASP.NET Core DataAnnotations tests to fail.

Problem

On Mono, KeyValuePair<TKey, TValue> generic parameters (TKey and TValue) were being detected as NotNull instead of Nullable, while CoreCLR correctly detects them as Nullable.

Root Cause: Mono's KeyValuePair<TKey, TValue> generic parameters are missing the NullableAttribute(2) that CoreCLR has. This causes TryUpdateGenericParameterNullability to return false and fall back to NullableContextAttribute(1) = NotNull.

Failing Tests

• Microsoft.AspNetCore.Mvc.DataAnnotations.DataAnnotationsMetadataProviderTest.IsNullableReferenceType_ReturnsFalse_ForKeyValuePairWithoutNullableConstraints
• Microsoft.AspNetCore.Mvc.DataAnnotations.DataAnnotationsMetadataProviderTest.IsNullableReferenceType_ReturnsTrue_ForKeyValuePairWithNullableConstraints

Solution

Added special handling in TryUpdateGenericParameterNullability to treat KeyValuePair<TKey, TValue> generic parameters as nullable by default when nullable attributes are missing.

The fix:

1. Checks if the generic parameter belongs to System.Collections.Generic.KeyValuePair2`
2. If so, sets ReadState and WriteState to Nullable
3. Only activates when existing attribute parsing fails (regression-safe for CoreCLR)

Testing

Reproduction Case

using System;
using System.Collections.Generic;
using System.Reflection;

var kvpType = typeof(KeyValuePair<string, object>);
var keyProperty = kvpType.GetProperty("Key");
var nullabilityContext = new NullabilityInfoContext();
var result = nullabilityContext.Create(keyProperty);

Console.WriteLine($"ReadState: {result.ReadState}");
// Expected: Nullable
// Mono (before fix): NotNull  
// Mono (after fix): Nullable 

Test Results

• Before fix: 361 pass, 3 fail (including 2 KeyValuePair tests)
• After fix: 363 pass, 1 fail (KeyValuePair tests now pass)
• Net improvement: +2 tests fixed, 0 regressions

Files Changed

• src/libraries/System.Private.CoreLib/src/System/Reflection/NullabilityInfoContext.cs

Verification

The fix has been tested with the full ASP.NET Core DataAnnotations test suite and successfully resolves the failing tests without introducing regressions.

cc: @giritrivedi

[jkotas]

Mono's KeyValuePair<TKey, TValue> generic parameters are missing the NullableAttribute(2) that CoreCLR has

This sounds like a bug in Mono-specific reflection implementation. Instead of adding workaround for this to shared NullabilityInfoContext.cs, could you please fix the Mono reflection implementation instead?

[medhatiwari]

This sounds like a bug in Mono-specific reflection implementation. Instead of adding workaround for this to shared NullabilityInfoContext.cs, could you please fix the Mono reflection implementation instead?

You're absolutely right - I've fixed this at the Mono native level instead. The reflection APIs are actually working correctly; the issue was metadata token corruption in mono_class_layout_fields().
When debugging, I found that mono_custom_attrs_from_class_checked() was receiving generic_param_token = 0x00000000, even though the PE file contains the correct NullableAttribute metadata with valid tokens. The problem is that MonoClassSizes is a union where class_size and generic_param_token share memory - assigning class_size = 0 was overwriting the token that was correctly set during class creation.
The fix (in src/mono/mono/metadata/class-init.c) skips the class_size assignment for generic parameter types (MONO_TYPE_VAR/MONO_TYPE_MVAR), preserving the token. I've removed the managed workaround and pushed the native fix.