Question

Is is possible to make the following compile without:

1. Making IFooCollection generic
2. Explicitly implementing IFooCollection.Items on FooCollection and performing an explicit cast.

---

public interface IFoo
{

}

public interface IFooCollection
{
    IEnumerable<IFoo> Items { get; }
}

public class FooCollection<T> : IFooCollection where T : IFoo
{
    public IEnumerable<T> Items { get; set; }
}

I’m happy enough with the second solution (implementing the interface explicitly) but would like to understand why I need to cast T as IFoo when we have a generic constraint specifying that T must implement IFoo.

Answer 1

The reason is the following:

IFooCollection.Items can contain any class that implements IFoo. So it can contain FooA, FooB, FooC at the same time.

FooCollection<FooA>.Items on the other hand can only contain elements of type FooA. Trying to cast FooB or FooC to FooA would yield an InvalidCastException although all implement IFoo.