Question

Basically my setting is this:

public abstract class BaseObject{
    public abstract BaseObject Clone();
}

public class DerivedObject : BaseObject{
    public DerivedObject Clone()
    {
        //Clone logic
    }
}

The above code doesn’t compile because it isn’t possible to change the return type when overriding a method.

Is it possible to achieve that every derived type’s Clone method returns an argument of it’s own type (maybe through generics)?

Answer 1

Well, C# doesn’t allow covariant return types as you’ve found… but you can use generics:

public abstract class BaseObject<T> where T : BaseObject<T>
{
    public abstract T Clone();
}

public class DerivedObject : BaseObject<DerivedObject>
{
    public override DerivedObject Clone()
    {
         // ...
    }
}

This solution can be a pain in various ways – not least because it’s hard to understand – but it can work reasonably well in many situations.

EDIT: The reason I’ve included the constraint on T is so that BaseObject can call “its own” methods on instances of T, which is usually very handy. If you don’t need this though, you can lose the constraint.