inheritance - How to return subtype in overridden method of subclass in C#?

Question

I have a subclass with an over-ridden method that I know always returns a particular subtype of the return type declared in the base class. If I write the code this way, it won't compile. Since that probably doesn't make sense, let me give a code example:

class BaseReturnType { }
class DerivedReturnType : BaseReturnType { }

abstract class BaseClass {
    public abstract BaseReturnType PolymorphicMethod();
}

class DerivedClass : BaseClass {
    // Compile Error: return type must be 'BaseReturnType' to match 
    // overridden member 'BaseClass.PolymorphicMethod()'
    public override DerivedReturnType PolymorphicMethod() { 
        return new DerivedReturnType(); 
    }
}

Is there any way to accomplish this in C#? If not, what's the best way to achieve something similar? And why isn't it allowed? It doesn't seem to allow any logical inconsistency, since any object returned from the over-ridden method still is BaseReturnType. Maybe there is something I hadn't considered though. Or maybe the reason is technological or historical.

Answer 1

Unfortunately no, covariant return types aren't supported in C# for method overriding. (Ditto contravariant parameter types.)

If you're implementing an interface you can implement it explicitly with the "weak" version and also provide a public version with the stronger contract. For simple overriding of a parent class, you don't have this luxury I'm afraid :(

(EDIT: Marc has a reasonable solution - although it's pretty ugly, and method hiding is generally a bad thing for readability. No offence meant, Marc ;)

I believe this is actually a CLR restriction, not just a language one - but I could well be wrong.

(As a matter of history, Java (the language) had the same restriction until 1.5 - but it gained covariance at the same time as generics.)