Would allowing extension methods to be defined in nested classes make sense?

[JohanLarsson]

public class Foo
{
    ...
    private static class Bar
    {
        internal static string Baz(this string s)
        {
            ...
        }
    }
}

My thinking is that allowing ^ would be a way to scope extension methods to only be visible in Foo.
Could be nice for extensions on types like string when a limited scope is desired.
I don't remember the reason why extension methods are not allowed in nested classes.
I have not though much about and it may be a bad idea, wrote this to get some feedback.
Would this add much complexity to parsing etc?

[jnm2]

Yes! This restriction is frustrating once in a while in general and frustrating frequently in C# scripts like Cake, where there is no way to declare an extension method.

[filipw]

@jnm2

C# scripts have supported extension methods for a while. This is a valid script for example:

static void Hi(this object o) {
    Console.WriteLine("hi " + o);
}

"foo".Hi();

[filipw]

here is the PR #3472

[jnm2]

Hmm... thanks for the heads up. The most recent version of Roslyn that Cake uses is 1.0.0-rc2, so that's why I'm still getting CS1106.

[filipw]

that's the last version that supported .NET 4.5. it's also almost 2 years old, with 2.0.0 going RTM soon and 2.0.0-rc4 being the latest

[svick]

I think this is a duplicate of #4565 (which was moved to dotnet/csharplang#301).

[OJacot-Descombes]

Why declare it in a nested static class instead of directly in the class?

public class Foo
{
    ...
    protected static string Baz(this string s) // Always protected or private
    {
        ...
    }
}

[jnm2]

Would you close the issue, please, since all language design issues have migrated to https://github.com/dotnet/csharplang?