Added: C# Language Design Notes for May 31, 2017

[MadsTorgersen]

C# Language Design Notes for May 31, 2017

1. Default interface members: overriding or implementing?
2. Downlevel poisoning of ref readonly in signatures
3. Extension methods with ref this and generics
4. Default in operators

Please discuss.

[Thaina]

Extension methods with ref this and generics

I think we can safe to allow constraint interface only with struct constraint

void A<T>(ref this T data) where T : struct,InterfaceA,InterfaceB,InterfaceC
{
    DoSomethingT;
}

[JeroenBos]

Regarding the weird syntax like
base(I3).(I1.M1)<string>(1, 2, 3); // Call I3's implementation of I1's M1

Isn't base(I3).M<...>(...) already unambiguous?

And why has been chosen for the syntax base(IType).M? That's not idiomatic C#. I know this requires more parentheses, but I prefer ((IType)base).M. It doesn't require new syntax.

[sirgru]

@JeroenBos

Isn't base(I3).M<...>(...) already unambiguous?

I was just thinking the same thing. Also, maybe it's in the other proposals, but what is the result of base(T4) where interface I4: I2, I3 { ... }? This seems a little ambiguous.
As far calling base(T) this has been discussed here https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-06-14.md and I personally prefer this style over the alternatives.

[svick]

@JeroenBos

And why has been chosen for the syntax base(IType).M? That's not idiomatic C#.

Of course it's not idiomatic in the current version of C#. It's doing something that's not possible in the current version of the language, so it pretty much can't use existing syntax.

I know this requires more parentheses, but I prefer ((IType)base).M. It doesn't require new syntax.

It does. base is not an expression on its own, according to the specification, so the current syntax does not allow you to write that code and allowing it would be introducing new syntax. (Though base is an expression according to Roslyn.)

[alrz]

Using a cast (although, just syntactically) doesn't make sense, because virtual dispatch doesn't care about static type of an expression, same goes with casting this. So you're not communicating what is happening here with a cast.