Regression with generics from .NET 8 To .Net 10

[AndreasHeisel]

Description

I have a generic readonly record struct. It has a single instance member of type System.String. I use it as a strongly typed identifier to various types of objects. To make this customizable, the target object must implement a generic interface with a static virtual property of type StringComparer.

Reproduction Steps

Run the single unit test:

public readonly record struct Id<T>(string Value) : IEquatable<Id<T>> where T : ITarget<T>
{
	public static StringComparer Comparer { get; } = T.IdentifierComparer;

	public override int GetHashCode()
	{
		return Comparer.GetHashCode(Value);
	}

	public bool Equals(Id<T> other)
	{
		return Comparer.Equals(Value, other.Value);
	}

	public override string ToString()
	{
		return Value;
	}
}


public interface ITarget<T> where T : ITarget<T>
{
	static virtual StringComparer IdentifierComparer => StringComparer.Ordinal;

	public Id<T> Id { get; }
}


public record Target(Id<Target> Id) : ITarget<Target>
{
}


[TestClass]
public class Test
{
	[TestMethod]
	public void AssertDictionary()
	{
		var actual = new Dictionary<Id<Target>, Target>();
		var expected = new Dictionary<Id<Target>, Target>();

		var target = new Target(new("1"));
		actual.Add(target.Id, target);
		expected.Add(target.Id, target);

		CollectionAssert.AreEquivalent(expected, actual);
	}
}

Expected behavior

No exception

Actual behavior

Message: 
Test method IdTest.Test.AssertDictionary threw exception:
System.NullReferenceException: Object reference not set to an instance of an object.

  Stack Trace: 
Id`1.get_Comparer() line 7
Id`1.GetHashCode() line 12
ValueType.GetHashCode()
Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
Dictionary`2.set_Item(TKey key, TValue value)
CollectionAssert.GetElementCounts[T](IEnumerable`1 collection, IEqualityComparer`1 comparer, Int32& nullCount)
CollectionAssert.FindMismatchedElement[T](IEnumerable`1 expected, IEnumerable`1 actual, IEqualityComparer`1 comparer, Int32& expectedCount, Int32& actualCount, Object& mismatchedElement)
CollectionAssert.AreEquivalent[T](IEnumerable`1 expected, IEnumerable`1 actual, IEqualityComparer`1 comparer, String message)
CollectionAssert.AreEquivalent(ICollection expected, ICollection actual)
Test.AssertDictionary() line 22
MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

Regression?

This worked well in .NET8.

Known Workarounds

None

Configuration

.NET 10, Windows11, x64

Other information

I get a NullReferenceException if the struct reads the interface's static property.

Using the debugger, I see that the runtime uses System.__Canon instead of my type.

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[EgorBo]

Most likely a regression from either #97590 or #98754 cc @huoyaoyuan

copilot's suggested fix:

--- a/src/coreclr/vm/comutilnative.cpp
+++ b/src/coreclr/vm/comutilnative.cpp
@@ -1918,8 +1918,11 @@ static ValueTypeHashCodeStrategy GetHashCodeStrategy(MethodTable* mt, QCall::Obje
              else
              {
-                // got another value type. Get the type
-                TypeHandle fieldTH = field->GetFieldTypeHandleThrowing();
+                // got another value type. Get the exact type (the field type may be
+                // a shared generic instantiation, e.g. Id<__Canon>, but we need the
+                // exact type here so that boxing the field for an overridden GetHashCode
+                // recovers the correct per-instantiation static data, generic context, etc.)
+                TypeHandle fieldTH = field->GetExactFieldType(TypeHandle(mt));
                 _ASSERTE(!fieldTH.IsNull());
                 MethodTable* fieldMT = fieldTH.GetMethodTable();
                 if (CanCompareBitsOrUseFastGetHashCode(fieldMT)

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[huoyaoyuan]

The exception is happening in KeyValuePair<Id<Target>, Target>.GetHashCode.

I'm not sure whether the Copilot fix is related. GetFieldTypeHandleThrowing was already used before #97590.

Looks like exposed by #98754. The old implementation wasn't calling overridden GetHashCode at all.

---

Interestingly, the code will not work correctly on .NET 8 either. Replace the StringComparer.Ordinal with StringComparer.OrdinalIgnoreCase, and test the following code:

        var target = new Target(new("a"));
        actual.Add(new Id<Target>("A"), target);
        expected.Add(new Id<Target>("a"), target);

The assert will fail, despite "A" and "a" are treated as the same key and actual.Add(new Id<Target>("a"), target) fails.