Overriding an abstract property with a derived return type in c#

Question

I have four classes. Request, DerivedRequest, Handler, DerivedHandler. The Handler class has a property with the following declaration:

public abstract Request request { get; set; } 

The DerivedHandler needs to override this property so that it returns DerivedRequest instead:

public override DerivedRequest request { get; set; } 

Does anyone have any ideas about how to make this work?

Answer 1

This isn't really a good way to structure things. Do one of the following

1) Just don't change the return type, and override it normally in the subclass. In DerivedHandler you can return an instance of DerivedRequest using the base class signature of Request. Any client code using this can choose to cast it to DerivedRequest if they want to.

2) Use generics instead if they are not supposed to be polymorphic.

public abstract class HandlerBase<T> where T: Request {     public abstract T Request {get;set;} }  public class Handler: HandlerBase<Request>()  public class DerivedHandler: HandlerBase<DerivedRequest>() 

Answer 2

In the C# language you are not allowed to change the signature of an inherited method, unless you substitute it with another method with the same name. This technique is referred to as "member hiding" or "shadowing".

If you are using .NET 2.0 or later, you could solve this problem by turning the return type of the Request property into a generic type parameter of the Handler class. The DerivedHandler class would then specify the DerivedRequest class as argument for that type parameter.

Here's an example:

// Handler.cs public class Handler<TRequest> where TRequest : Request {     public TRequest Request { get; set; } }  // DerivedHandler.cs public class DerivedHandler : Handler<DerivedRequest> { }