Question

I have the following scenario:

class Foo { }

class Foo<T> : Foo { }

And then two methods

void DoStuff(Foo foo) 
{
     DoStuffImpl(foo);
}

void DoStuffImpl(Foo foo) 
{ 
     Console.WriteLine("A");
}    
void DoStuffImpl<T>(Foo<T> foo) 
{ 
     Console.WriteLine("B");
} 

void Main() 
{
     DoStuff(new Foo<int>()); // prints A
}

(note, the code was written in the browser, but describes the situation I’m facing)

How can I get it to call the generic method, and print B?

Can this be done at all without reflection? I have some ideas on how it could be done with reflection, but I’m looking for a cleaner solution if one exists.

Note: I can’t make DoStuff generic because this will be used with WCF and open generic types are not allowed.

Answer 1

(I assume you already understand why this is happening. If not, read my overload resolution article and let me know if it’s still unclear.)

If you’re using C# 4 you could use dynamic typing:

void DoStuff(Foo foo) 
{
    dynamic d = foo;
    DoStuffImpl(d);
}

Note how this doesn’t just have a dynamic parameter – the idea is that by restricting foo to be of type Foo or a subclass, we’ll always have a valid DoStuffImpl to call… it’s just that the best method will be determined at execution time, not compile time.

If you’re stuck in pre-C# 4, you could potentially do it with double dispatch:

class Foo
{
    public virtual void CallStuffImpl(FooImplType x)
    {
        x.DoStuffImpl(this);
    }
}

class Foo<T> : Foo
{
    public override void CallStuffImpl(FooImplType x)
    {
        // Looks like it's redundant, but isn't! "this" is
        // known to be Foo<T> rather than Foo
        x.DoStuffImpl(this);
    }
}

Then:

void DoStuff(Foo foo) 
{
    foo.CallStuffImpl(this); // Let it dispatch appropriately
}