Generic wrapper class

Question

Given the following hierarchy:

class A
{
}
class B : A
{
    public void Foo() { }
}
class C : A
{
    public void Foo() { } 
}

This is a third-party library and I can't modify it. Is there a way I can write some kind of 'generic templated wrapper' which would forward the Foo() method to the apropriate object passed as constructor argument? I ended up writing the following, which uses no generics and seems rather ugly:

class Wrapper
    {
        A a;
        public Wrapper(A a)
        {
            this.a = a;
        }

        public void Foo()
        {
            if (a is B) { (a as B).Foo(); }
            if (a is C) { (a as C).Foo(); }
        }

    }

I'd love some template constraint like Wrapper<T> where T : B or C.

Answer 1

If A does not have Foo, you need to either use dynamic (see Jon Skeet's answer) or use a little trick with lambdas and overloading:

class Wrapper {
    private Action foo;
    public Wrapper(B b) {
        foo = () => b.Foo();
    }
    public Wrapper(C c) {
        foo = () => c.Foo();
    }
    public void Foo() {
        foo();
    }
}

Now you can do this:

var wb = new Wrapper(new B());
wb.Foo(); // Call B's Foo()
var wc = new Wrapper(new C());
wc.Foo(); // Call C's Foo()

This shifts the decision on what method to call from the moment the Foo is called to the moment the Wrapper is created, potentially saving you some CPU cycles.