Question

Why doesnt this work?

 public class ClassOptions {}

 public interface Inode {
    ClassOptions Options {get;}
 }            

public class MyClass : Inode {
  public ClassOptions Options { get; set; }
}           

public class ClassDerivedOptions : ClassOptions {
}

public class MyDerivedClass : Inode {
    public ClassDerivedOptions Options { get; set; } << does not implement INode...
}

[ the compiler message tells me why it breaks but i’d like to know the reasoning behind why the compiler doesnt let this through – also if there are any work arounds? – thanks]

Answer 1

It doesn’t work because the INode interface explicitly calls for an Options property of type ClassOptions. C# doesn’t support return type covariance (which is what you’re asking for in this case).

For what it’s worth, there’s also a language feature request on Microsoft Connect specifically for return type covariance:

Need covariant return types in C# / all .NET languages

If you look at the page, they also mention that the common work-around is to use Explicit Interface Implementation:

public class MyDerivedClass : INode
{
    public ClassDerivedOptions Options { get; set; }
    public ClassOptions INode.Options { get { return Options; } }
}