I thought I'd used this sort of construct before. But the compiler insists that the type of Parent in Test must be BaseClass.

public interface ITest
    {
        BaseClass Parent { get; }
    }

    public class BaseClass {}

    public class DerivedClass : BaseClass{}

    public class Test : ITest
    {
        public DerivedClass Parent
        {
            get {return new DerivedClass(); }
        }
    }

Am I cracking up?