NullabilityInfoContext does not report the correct nullability for method in generic base class.

[madelson]

Description

Let's say I have the following class defined in nullable enabled context:

private class NonNullableValuesDictionary : Dictionary<string, object> { }

If I try to add a null value to the object, I get a compiler error:

var dictionary = new NonNullableValuesDictionary();
dictionary.Add("a", null); // CS Cannot convert null literal to non-nullable reference type

However, I can't replicate this compiler behavior with NullabilityInfoContext even though it seems to me that the compiler must be reading the same metadata in order to issue that warning.

Reproduction Steps

public static void Main()
{
    var context = new NullabilityInfoContext();
    var addMethod = typeof(NonNullableValuesDictionary).GetMethod("Add")!;
    var info = context.Create(addMethod.GetParameters()[1]);
    Console.WriteLine(info.WriteState); // Nullable
}

private class NonNullableValuesDictionary : Dictionary<string, object> { }

Expected behavior

I would expect the code above to print "NotNull" instead.

Actual behavior

Prints "Nullable".

Regression?

No response

Known Workarounds

No response

Configuration

VS 17.0.2, .NET 6, Windows 10 x64.

I don't have any reason to believe this is configuration specific.

Other information

Some other fun cases:

class DerivedFromNonNullableValuesDictionary : NonNullableValuesDictionary { }
class GenericDerivedFromDerivedFromNonNullableValuesDictionary<T> : DerivedFromNonNullableValuesDictionary { }

class GenericDerivedFromDictionary<T, U> : Dictionary<T, T> { }
class GenericDerivedFromGenericDerivedFromDictionary<U> : GenericDerivedFromDictionary<object> { }

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Issue Details

---

Description

Let's say I have the following class defined in nullable enabled context:

private class NonNullableValuesDictionary : Dictionary<string, object> { }

If I try to add a null value to the object, I get a compiler error:

var dictionary = new NonNullableValuesDictionary();
dictionary.Add("a", null); // CS Cannot convert null literal to non-nullable reference type

However, I can't replicate this compiler behavior with NullabilityInfoContext even though it seems to me that the compiler must be reading the same metadata in order to issue that warning.

Reproduction Steps

public static void Main()
{
    var context = new NullabilityInfoContext();
    var addMethod = typeof(NonNullableValuesDictionary).GetMethod("Add")!;
    var info = context.Create(addMethod.GetParameters()[1]);
    Console.WriteLine(info.WriteState); // Nullable
}

private class NonNullableValuesDictionary : Dictionary<string, object> { }

Expected behavior

I would expect the code above to print "NotNull" instead.

Actual behavior

Prints "Nullable".

Regression?

No response

Known Workarounds

No response

Configuration

VS 17.0.2, .NET 6, Windows 10 x64.

I don't have any reason to believe this is configuration specific.

Other information

No response

Author:	madelson
Assignees:	-
Labels:	area-System.Collections, untriaged
Milestone:	-

[ghost]

Tagging subscribers to this area: @dotnet/area-system-reflection
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

Let's say I have the following class defined in nullable enabled context:

private class NonNullableValuesDictionary : Dictionary<string, object> { }

If I try to add a null value to the object, I get a compiler error:

var dictionary = new NonNullableValuesDictionary();
dictionary.Add("a", null); // CS Cannot convert null literal to non-nullable reference type

However, I can't replicate this compiler behavior with NullabilityInfoContext even though it seems to me that the compiler must be reading the same metadata in order to issue that warning.

Reproduction Steps

public static void Main()
{
    var context = new NullabilityInfoContext();
    var addMethod = typeof(NonNullableValuesDictionary).GetMethod("Add")!;
    var info = context.Create(addMethod.GetParameters()[1]);
    Console.WriteLine(info.WriteState); // Nullable
}

private class NonNullableValuesDictionary : Dictionary<string, object> { }

Expected behavior

I would expect the code above to print "NotNull" instead.

Actual behavior

Prints "Nullable".

Regression?

No response

Known Workarounds

No response

Configuration

VS 17.0.2, .NET 6, Windows 10 x64.

I don't have any reason to believe this is configuration specific.

Other information

No response

Author:	madelson
Assignees:	-
Labels:	area-System.Reflection, untriaged
Milestone:	-

[krwq]

cc: @buyaa-n

[buyaa-n]

This is by design, at runtime there is no way to distinguish if the NonNullableValuesDictionary was Dictionary<string, object> or Dictionary<string, object?> at runtime the DeclaringType of refected method var addMethod = typeof(NonNullableValuesDictionary).GetMethod("Add")!; will be just Dictionary<string, object> without any nullability info we could determine object? was null.

Therefore in order to determine nullability of the Dictionary<string, object>.Add(string, object) we use the original generic definition of the type: Dictionary<TKey, TValue> .Add(TKey, TValue) where TKey not null and TValue is nullable

[madelson]

@buyaa-n if there is no way to tell, how does the compiler differentiate (or can the compiler only do it if the derived type is defined in the same compilation?)?

[buyaa-n]

I guess i should have said that there is no way to distinguish that in reflection, because compiler does not provide any nullability info for those generic parameters (reflection uses compiler provided NullableAttribute or NullableContextAttribute to determine nullability, and it is impossible to provide that info current design)

The compiler nullability analysis and this reflection API are do not work same

[madelson]

@buyaa-n but when I decompile my assembly with dotpeek it looks like the nullable metadata is there:

Original:

internal class NonNullableValuesDictionary : Dictionary<string, object> { }
internal class NullableValuesDictionary : Dictionary<string, object?> { }

Decompiled:

  [Nullable(new byte[] {(byte) 0, (byte) 1, (byte) 1})]
  internal class NonNullableValuesDictionary : Dictionary<string, object>
  {
  }

  [Nullable(new byte[] {(byte) 0, (byte) 1, (byte) 2})]
  internal class NullableValuesDictionary : Dictionary<string, object>
  {
  }

Aren't those attributes describing the nullability of the generic parameters for Dictionary?

[buyaa-n]

Hm, that actually looks feasible compared to, reopening the issue, thanks!

    var nonNull= typeof(Dictionary<string, object>);
    var Nullable = typeof(Dictionary<string, object?>);