How to upcast an object of type IEnumerable<Foo> to IEnumerable<IFoo> if passed to the function as object?

Question

I'm trying to upcast a parameter passed as an object (which is an upcast of an object of type IEnumerable) to IEnumerable where Foo implements IFoo.

Here is an example of what I'd like to do but it does not work.

public void F(object o)
{
    //I know the object o is of type IEnumerable<Foo> where Foo implements IFoo
    IEnumerable<IFoo> ifoos = (IEnumerable<IFoo>) o);
}

Is there a work around? I don't want to make the function F Foo specific but I cannot get it to cast to the Interface Unless I do:

IEnumerable<IFoo> ifoos = (IEnumerable<Foo>) o).Select( f => (IFoo) f);

Thanks

Giuseppe

Answer 1

Before .NET 4.0, you can't - IEnumerable<T> is invariant before .NET 4.0.

With .NET 4.0, the code you've posted will work fine.

In .NET 3.5 you could use Enumerable.Cast:

public void F(object o)
{
    IEnumerable<IFoo> ifoos = ((IEnumerable) o).Cast<IFoo>();
}

Basically this uses the weakly typed IEnumerable (which both IEnumerable<Foo> and IEnumerable<IFoo> extend, so the cast will be fine) and then applies the Cast<T> method, which will basically chain another iterator to perform a cast on each item. That cast will certainly work if Foo : IFoo, so you'll be fine.

In .NET 2.0 you'd have to basically write Cast yourself, which is easy enough - particularly if you don't need to do any shortcuts:

public static IEnumerable<TResult> Cast<TSource, TResult>
    (IEnumerable<TSource> source)
{
    foreach(TSource item in source)
    {
        yield return (TResult) (object) item;
    }
}

The double cast is slightly awkward, but it works...

Answer 2

Would a generic method be acceptable?

interface IFoo { }
class Foo : IFoo { }
static void F<T>(IEnumerable<T> data) where T : IFoo
{ 
    foreach(T item in data) {
       // compiler knows that `item` is an `IFoo` etc
    }
}

...
List<Foo> foos = new List<Foo>();
F(foos);

Otherwise; wait until .NET 4.0 / C# 4.0 / VS2010.