Need api to determine if a type is "byref-like."

[ghost]

Proposal for New API:

   class Type
   {
        public virtual bool IsByRefLike => throw new NotSupportedException();
   }

With Span<>, we're now introducing a mainstream type that has "by-ref-like" or "stack-only" restrictions. We also have TypedReference in this bucket, but Span<> takes this concept out of the "eccentric uncle in the closet" realm.

There is, however, no robust way to know if a type has these restrictions. Apis like Array.CreateInstance() need to guard against these types (dotnet/corert#3000) and could use a formalized way to catch these types. It's likely that checks like these could go viral into app code or higher level libraries that call apis like this.

[karelz]

Looks somewhat related to recent #19493. Will we need more similar APIs for Span?
cc @jkotas

[jkotas]

This should wait for the design for byref-like types (e.g. dotnet/csharplang#264).

I expect that the byref-like types will be explicitly marked by a custom attribute. So this property won't be inferred, but explicitly specified instead and this API will be essentially just a fetch of this custom attribute (or more efficient equivalent).

[MichalStrehovsky]

This would be an API that should be used in Microsoft.CSharp as well (in isSpecialByRefType). The current implementation of that seems to have been hollowed out when TypedReference and friends weren't in the .NET Core profile. It likely wants the special behavior for Span<T> as well.

Cc @JonHanna

[JonHanna]

@MichalStrehovsky From taking a look at a disassembly of the version in the GAC it seems that that version is implementing isSpecialByRefType by checking against a few specified names, which aren't names the corefx version knows about. Making use of the API proposed here would likely be better than any attempt to just restore the old logic.

[MichalStrehovsky]

This should wait for the design for byref-like types (e.g. dotnet/csharplang#264).

These now have an approved attribute (System.Runtime.CompilerServices.IsByRefLikeAttribute) that marks them. Maybe we can drop the blocked tag?

[EDIT] Fixed attribute name to IsByRefLikeAttribute by @karelz

[terrajobst]

Video

We should align this with the attribute we approved earlier, which is named IsByRefLikeAttribute. Since the proposal matches that, it's approved as proposed.

[karelz]

FYI: The API review discussion was recorded - see https://youtu.be/VppFZYG-9cA?t=5306 (4 min duration)

[bergmeister]

@terrajobst The IsByRefLike property on System.Type is only available in .Net Core and not in .Net Standard or full .Net. Could I use the IsByRefLikeAttribute instead on full .net as an equivalent implementation of my own for IsByRef or is there another way of using this property on full .net (4.7.2)? The documentation says only This attribute is used by the compiler for tracking metadata. It should not be used by application developers.

[jkotas]

@bergmeister Correct, the workaround is to check for presence of IsByRefLikeAttribute. The first class support for byref-like types is present in .NET Core only.

Also see https://github.com/dotnet/corefx/issues/33149