Question

I would like make an extension method for the generic class A which takes yet another generictype (in this example TC), but i guess that aint possible?

class Program
{
    static void Main(string[] args)
    {
        var a = new A<B, B>();
        a.DoIt<B>();
    }
}

static class Ext
{
    public static A<TA, TB> DoIt<TA, TB, TC>(this A<TA, TB> a)
    {
        return a;
    }
}

class A<TA, TB> { }
class B { }

Answer 1

If you can accept a slight syntax change, then it would be possible.

Change it to:

var a = new A<B, B>(); 
a.Do().It<B>(); 

The trick is that the Do method is an extension method on A<TA, TB>:

public static Doer<TA, TB> Do<TA, TB>(this A<TA, TB> a)
{
    return new Doer<TA, TB>(a);
}

The trick is that this signature lets type inferincing pick up TA and TB from a so that you don’t have to specify them explicitly.

The Doer class provides the generic method you need:

public class Doer<TA, TB>
{
    public void It<TC>() { }
}