Expose nullability info

[buyaa-n]

Fixes #29723

Approved API shape:

namespace System.Reflection
{
    public sealed class NullabilityInfoContext
    {
        public NullabilityInfo Create(ParameterInfo parameterInfo);
        public NullabilityInfo Create(PropertyInfo propertyInfo);
        public NullabilityInfo Create(EventInfo eventInfo);
        public NullabilityInfo Create(FieldInfo parameterInfo);
    }

    public sealed class NullabilityInfo
    {
        public Type Type { get; }
        public NullabilityState ReadState { get; }
        public NullabilityState WriteState { get; }
        public NullabilityInfo? ElementType { get; }
        public NullabilityInfo[] GenericTypeArguments { get; }
    }

    public enum NullabilityState
    {
        Unknown,
        NotNull,
        Nullable
    }
}

• [MaybeNullWhen] should work same as [MaybeNull].

• if the NullablePublicOnlyAttribute set in the module we return NullabilityState.Unknown for private and/or internal members

• The nullability for generic type T should be tracked by the user, a field declaration List<string?> list or List<string> list will have type parameter nullability info but nullability for other List<T> API calls should be tracked by the user. For example for list.Add(T item) the nullability of item will be evaluated as follows:

  1. T is Nullable if the declaration has ? i.e. GenericType<T?> (even if T is value type it is nullable value type)
  2. T is Nullable for GenericType<T> when nullability context enabled and the concrete type of T is ref type or nullable value type. (In this case the nullability of parameter T of List.Add(T item) will be Nullable for both List<string> and List<string?> instance, we might want 4th nullability state for this scenario) CC @terrajobst
  3. T is NotNull for GenericType<T> when concrete T is non-nullable value type
  4. T is Unknown for GenericType<T> when nullability context disabled and the concrete type of T is ref type
  5. For open generics the behavior is the same as ref type of T

• For value type nullability:

  • int? is nullable
  • int is not null no matter nullability context
  • AllowNull/DisallowNull/NotNull/MaybeNull atributes affect write/read state of int? type, but not int

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[steveharter]

Should isFamilyOrAssembly (aka protected internal) also be considered? Are there tests for that?

[buyaa-n]

I will test that, as far as i know, protected members was not affected

[buyaa-n]

Added some test, could not test exactly as could not find an assembly that removed nullability attribute for the internal member's too, it only applied for private members, for now i just guess that because of protected modifier protected internal members will not be affected

[n9]

Does this PR provide information about generic arguments of implemented interfaces? For instance:

interface IBar<T> { }

class Bar : IBar<string> { }
class BarN : IBar<string?> { }

Context: #49900

[buyaa-n]

Does this PR provide information about generic arguments of implemented interfaces?

No, and I don't think there is a way to distinguish IBar<string>, IBar<string?> from your example

If there is a member declared with a generic interface it would have the nullability of the generic type

interface IBar<T> { }
class C
{
    public  IBar<string> bar;  // {ReadState=NotNull, GenericTypeArguments = {ReadState=NotNull, ...}, ... } 
    public  IBar<string?> nBar;  // {ReadState=NotNull, GenericTypeArguments = {ReadState=Nullable, ...}, ... } 
}

[n9]

@buyaa-n There are different flags in the IL:

.class private auto ansi beforefieldinit Bar
    extends [System.Private.CoreLib]System.Object
    implements .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8[]) = (
        01 00 02 00 00 00 00 01 00 00
    )
    class IBar`1<string>

.class private auto ansi beforefieldinit BarN
    extends [System.Private.CoreLib]System.Object
    implements .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8[]) = (
        01 00 02 00 00 00 00 02 00 00
    )
    class IBar`1<string>

[buyaa-n]

@n9 I don't see that info in runtime, anyway, the API is not for Type nullability, it is only for FieldInfo, ParameterInfo, PropertyInfo, EventInfo nullability, and the NullabilityInfo returned by the API has no info of its Type's interface implementation cc @terrajobst

[steveharter]

Per discussion, looks good pending known open items (Mono test failures, readonly properties should return "unknown")

[buyaa-n]

Even NullablePublicOnlyAttribute is set the nullability attributes not being removed for private events (not sure if this is Roslyn bug or by design), therefore not checking that info for EventInfo