C# call generic interface method from non generic parent interface without dynamic

Question

Say we have a non generic base interface, with a generic inheriting interface:

public interface IFoo { }
public interface IBar<T, K> : IFoo { 
    K Do(T t);
}

public class BarImpl : IBar<Type, AnotherType> {
    public AnotherType Do(Type type) {
        return new AnotherType(type);
    }
}

I need to create a factory which returns an IFoo instance, but using the returned instance I need to be able to call the derived types Do(T), which isn't available.

public class FooFactory() {
    IFoo Get() {
           // simplified, in reality i am returning the correct
           // type by checking the generic interface
           // types to get an object from a stored list 
           // of implementations
           return BarImpl();
    }
}

// Now in another class
public void DoFoo() {
    IFoo iFoo = new FooFactory().Get();
    // Need to be able to call iFoo.Do(Type) but cannot
}

The only way I have been able to get this to work is to create a dynamic object, instead of IFoo, and then call Do() - which does work in my case but I lose some type safety which i'd prefer to keep.

My question is can I re-engineer this to be able to get access to the derived interfaces method, whilst still being able to maintain a list (and subsequently factory method return type) of IFoo????

Answer 1

You expect or want type safety, but think about it this way:

• In order to be able to call Do , Get needs to return a type which defines that method. IFoo does not have it, but IBar<T, K> does. Get however returns an IFoo object which is not guaranteed to be a IBar<T, K> .
• Even if the implementation of Get would make sure that only a IBar<T, K> is returned, there is no way the type system would know this without actually returning that type .
• Assuming you could return a type that allowed you to call the Do method, the type would be unclear: You need to pass it an object of type T . But the returned IFoo or IBar<T, K> does not necessarily use the same type T that you wanted to pass to Do .
• Even if the implementation of Get would provide this (like “Get me an IBar<T, K> that accepts the type T ”) and the type system would have a way to reflect this, then this still wouldn't say anything about K . For a known T , it could still be an IBar<T, int> , or an IBar<T, string> . And there is actually no way to know that without actually having the concrete type.
• And still, assuming that this would work with the type system: What purpose would this actually serve? You could call the method of your generic type with your correct typed argument: But the return value still has no concrete type. You couldn't say anything about the returned type from Do .

My point is that you only need generic types, when you actually have a reason to maintain a concrete type. Usually if you call a generic method or a method of a generic type from another non-generic method, then you either have a discrete set of types you're working with, or you don't actually need the generic type information.

So maybe you're better off introducing a non-generic IBar type here:

interface IBar
{
    object Do(object t);
}
interface IBar<T, K> : IBar
{
    K Do(T t);
}

public class BarImpl : IBar<Type, AnotherType>
{
    public AnotherType Do(Type type)
    {
        return new AnotherType(type);
    }

    public object Do(object t)
    {
        return Do((Type) t);
    }
}

Then you could make Get return an IBar instead, and you have a way to call Do .

Btw. this pattern is used pretty commonly in the BCL, eg IEnumerable<T> and IEnumerable .

Answer 2

You could add a object DoIt() method to IFoo , then BarImpl would implement it:

public object DoIt()
{
    return Do(typeof(T));
}

The problem here is that you would then need to cast the return of DoIt to the actual type. But you won't know apriori what that is.