Permanently cast derived class to base

Question

Class A { } Class B : A { }  B ItemB = new B(); A ItemA = (A)B;  Console.WriteLine(ItemA.GetType().FullName); 

Is it possible to do something like above and have the compiler print out type A instead of type B. Basically, is it possible to permanently cast an object so it "loses" all the derived data?

Answer 1

What you ask for is impossible for two reasons:

1. ItemA.GetType() does not return the compile-time type of the variable ItemA - it returns the run-time type of the object referred to by ItemA.
2. There's no way you could make (A)B result in a representation-changing conversion (i.e. a new A object) because user-defined conversion operators (your only hope here) cannot convert from derived to base-classes. You're just going to get a normal, safe, reference-conversion.

That aside, what you ask for is very strange; one would think you're trying really hard to violate Liskov's substiution principle. There's almost certainly a serious design-flaw here that you should address.

If you still want to do this; you could write a method that manually constructs an A from a B by newing up an A and then copying data over. This might exist as a ToA() instance-method on B.

If you characterized this problem as "How do I construct an A from an existing A?", it makes a lot more sense: create a copy-constructor on A, whose declaration looks like public A(A a){...}, which is agnostic to subclass-specific details. This gives you a general means to create an A from an existing instance of A or one of its subclasses.

Answer 2

I've recently run into this migrating an old project to Entity Framework. As it was mentioned, if you have a derived type from an entity, you can't store it, only the base type. The solution was an extension method with reflection.

    public static T ForceType<T>(this object o)     {         T res;         res = Activator.CreateInstance<T>();          Type x = o.GetType();         Type y = res.GetType();          foreach (var destinationProp in y.GetProperties(System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.Instance))         {             var sourceProp = x.GetProperty(destinationProp.Name);             if (sourceProp != null)             {                 destinationProp.SetValue(res, sourceProp.GetValue(o));             }         }          return res;     } 

It's not too neat, so use this if you really have no other option.