Allow IntPtr and UIntPtr in attributes

[jkrejcha]

Currently using [Foo(0x0)] as an attribute fails with CS0181

public class FooAttribute
{
   public FooAttribute(IntPtr num)
   {
       // ...
    }
}

In addition, §22.2.4 only constrains types allowed in attributes to...

One of the following types: bool, byte, char, double, float, int, long, sbyte, short, string, uint, ulong, ushort.

• The type object.
• The type System.Type.
• Enum types.
• Single-dimensional arrays of the above types.

and my proposal would allow the usage of nint and nuint in attributes.

[HaloFour]

I don't think it makes sense to use IntPtr in attributes as the actual data embedded in the assembly metadata needs to be a known predictable size at compile time. It can't change to accommodate a different size based on the native word size. So at best it would still be a long or ulong, which you could then cast back to an IntPtr in code anyway.

[jkrejcha]

Ah right I see, this doesn't work for AnyCPU, etc.

[HaloFour]

Yeah, it'd be a little weird. You'd have to pick a fixed size to store the number in the assembly metadata (or have a flag to indicate the number of bits), but the only safe value you could store is 32-bit. It's not impossible, but it would probably be a bit awkward.

[tannergooding]

It's also worth noting that this is a runtime limitation:

CLS Rule 34: The CLS only allows a subset of the encodings of custom attributes. The only types that shall appear in these encodings are (see Partition IV): System.Type, System.String, System.Char, System.Boolean, System.Byte, System.Int16, System.Int32, System.Int64, System.Single, System.Double, and any enumeration type based on a CLS-compliant base integer type.

There is an interesting bit in that the runtime does allow nint based enums and therefore technically nint is allowed (albeit indirectly) in an attribute. However, much like nint constants, they are restricted to 32-bits and are sign-extended by the runtime up to the actual target size at runtime.

[HaloFour]

There is an interesting bit in that the runtime does allow nint based enums and therefore technically nint is allowed (albeit indirectly) in an attribute.

That is interesting, and it doesn't seem like C# supports that today.