Allow DynamicallyAccessedMembersAttribute on methods

[vitek-karas]

Background and Motivation

The DynamicallyAccessedMembersAttribute is used to annotate "data things" (method parameters, fields, return values, ...) of type System.Type and System.String. Adding the attribute means that the code may access some members on the type in question (represented either as System.Type or as the type name in System.String) using reflection or similar APIs. We plan to use these attribute to annotate reflection APIs themselves, so for example Activator.CreateInstance(Type type) will have the attribute on the type parameter marking it to need default constructor.

But some reflection APIs will need to put this annotation onto the "this" parameter. For example Type.GetFields() - should have DynamicallyAccessedMembers[DynamicallyAccessedMemberType.PublicFields] on the "this" parameter.

C# doesn't have a way to add attribute to the "this" parameter, so there needs to be some other way to this.

The DynamicallyAccessedMembersAttribute addition has been discussed here: #33861.

Proposed API

Allow the DynamicallyAccessedMembersAttribute to be applied to methods, the meaning would be that it applies to the "this" parameter. Tools would only recognize the attribute on methods on types derived from System.Type and would issue a warning if such attribute is found on a method on any other type.

    [AttributeUsage(
        AttributeTargets.Field | AttributeTargets.ReturnValue | AttributeTargets.GenericParameter |
        AttributeTargets.Parameter | AttributeTargets.Property |
        AttributeTargets.Method, // This is the proposed API change
        Inherited = false)]
    public sealed class DynamicallyAccessedMembersAttribute : Attribute
    {
    }

Usage Examples

namespace System
{
    public class Type
    {
         [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicFields)]
         public FieldInfo[] GetFields();
    }
}

Alternative Designs

Alternative is to introduce a new attribute type just for this case which would otherwise look exactly like the DynamicallyAccessedMembersAttribute. Possible names could be DynamicallyAccessedMembersOnThisAttribute. The semantics of such attribute would be exactly the same as those described above for DynamicallyAccessedMembersAttribute on method.

Risks

Allowing the DynamicallyAccessedMembersAttribute on methods may lead to users incorrectly adding it onto the method when they meant to put it onto either a method parameter or the return value of a method since compiler would not complain about such usage.

This will be partially mitigated by the tools (IL Linker) warnings about such cases.

In the future we can also mitigate this better by having a Roslyn analyzer detect such cases and warn about them directly in the IDE.

[vitek-karas]

/cc @jkotas @MichalStrehovsky @eerhardt

[terrajobst]

Video

Looks good as proposed

 [AttributeUsage(
        AttributeTargets.Field |
        AttributeTargets.ReturnValue |
        AttributeTargets.GenericParameter |
        AttributeTargets.Parameter |
        AttributeTargets.Property |
+       AttributeTargets.Method,
        Inherited = false)]
    public sealed class DynamicallyAccessedMembersAttribute : Attribute
    {
    }