C# 14 Extension Members & Generics

[TonyValenti]

How do you implement generic extension members in C# 14?

var C = new Base<string>();
C.Test();
C.GenericTest<object>(); //This does not work.
//'Base<string>' does not contain a definition for 'GenericTest' and no accessible extension method 'GenericTest' accepting a first argument of type 'Base<string>' could be found (are you missing a using directive or an assembly reference?)

public static class ExtensionMembers {

    extension<T>(Base<T> This){

        public void Test() {
            Console.WriteLine("Test");
        }

        public void GenericTest<TOther>() {
            Console.WriteLine("TOther");
        }

    }

}



public record Base {

}

public record Base<T> : Base {

}

[HaloFour]

Generic extension methods on generic extensions have the combination of the generic type parameters between the two declarations, so:

public static class ExtensionMembers {
    extension<T>(Base<T> This) {
        public void GenericTest<TOther>() {
            Console.WriteLine("TOther");
        }
    }
}

// must be called like this:

Base<int> b = ...;
b.GenericTest<int, string>();

This was already the case with extension methods, but more obvious given the way they are declared:

public static class ExtensionMembers {
    public static void GenericTest<T, TOther>(this Base<T> This) {
        Console.WriteLine("TOther");
    }
}

[TonyValenti]

Oh bummer! I knew that was the case with extension methods but I was hopeful that the extension syntax would allow me a way to provide only the last generic parameter instead of also requiring the first one.

[jnm2]

This choice was made so that all classic extension methods can be migrated without binary- or source-breaking changes. Another concern would be that if the new style of extension method behaved differently than the classic style, it could be confusing and you'd have to be asking yourself the question "is this an old-style or new-style extension method" whenever you have to use explicit type arguments. It's nice to not have to ask that question every time, and just stick with 18 years of muscle memory.