They can smuggle it in via attributes. But if they don't make this part of the CLR spec, then not really cross-platform and more like C#'s extension methods. I

They can smuggle it in via attributes. But if they don't make this part of the CLR spec, then not really cross-platform and more like C#'s extension methods. Ideally these features would be designed at a higher level, in the CLR, aimed to target as many languages as possible. I.e., they shouldn't be proposed as C# extensions.