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

Answer 1

Change your method signature on Derived class to:

 public override BaseReturnType PolymorphicMethod() 
 {
    return new DerivedReturnType();     
 }

C# doesn't support variant return types. You can check out this post for a way to do this using Generics...http://srtsolutions.com/blogs/billwagner/archive/2005/06/17/covaraint-return-types-in-c.aspx

Here's a sample using Generics in your model:

public class BaseReturnType
{
}
public class DerivedReturnType : BaseReturnType
{
}

public abstract class BaseClass where T : BaseReturnType
{
    public abstract T PolymorphicMethod();

}

public class DerviedClass : BaseClass
{
    public override DerivedReturnType PolymorphicMethod()
    {
        throw new NotImplementedException();
    }
}