Question

(This is .Net 3.5) I have a class FooList which implements IList and a class FooClass which implements IFoo. A user requires IList<IFoo>. In my implementation, I create a FooList<FooClass>, called X. How do I code my return so that my FooList<FooClass> X becomes his IList<IFoo>?

If I try

return X.Cast( ).ToList( );

he gets an IList<IFoo>, but it is not my FooList; it is a List, and a new one at that.

Answer 1

This isn’t going to work out, because a FooList<FooClass> is not an IList<IFoo>. This is why:

var myList = new FooList<FooClass>();
IFoo obj = new SomeOtherFooClass();
IList<IFoo> result = (IList<IFoo>)myList; // hypothetical, wouldn't actually work
result.Add(obj); // uh-oh, now myList has SomeOtherFooClass

You need to either make a copy or use an interface that is actually covariant on the contained type, like IEnumerable<T> instead of IList<T>. Or, if appropriate, you should declare your FooList<FooClass> as an FooList<IFoo> from the get-go instead.

Here is a small implementation that demonstrates my second suggestion:

public interface IFoo { }
public class FooClass : IFoo { }

public class FooList<T> : IList<T>
{
    public void RemoveAt(int index) { /* ... */ }
    /* further boring implementation of IList<T> goes here */
}

public static void ListConsumer(IList<IFoo> foos)
{
    foos.RemoveAt(0); // or whatever
}

public static IList<IFoo> ListProducer()
{
    // FooList<FooClass> foos = new FooList<FooClass>(); // would not work
    FooList<IFoo> foos = new FooList<IFoo>();

    foos.Add(new FooClass());

    return foos; // a FooList<IFoo> is an IList<IFoo> so this is cool
}

public static void Demo()
{
    ListConsumer(ListProducer()); // no problemo
}