Convert/Cast base type to Derived type

Question

I am extending the existing .NET framework class by deriving it. How do I convert an object of base type to derived type?

public class Results { //Framework methods }

public class MyResults : Results { //Nothing here }

//I call the framework method

public static MyResults GetResults()
{
    Results results = new Results();
    //Results results = new MyResults(); //tried this as well.

    results = CallFrameworkMethod();

    return (MyResults)results; //Throws runtime exception
}

I understand that this happens as I am trying to cast a base type to a derived type and if derived type has additional properties, then the memory is not allocated. When I do add the additional properties, I don't care if they are initialized to null.

How do I do this without doing a manual copy?

Answer 1

You can't. If results doesn't refer to a MyResults (e.g. if CallFrameworkMethod returns a base Results instance), then casting won't make it so: you'll need to create a new MyResults, based on the existing non-MyResults. Casting is about changing the compile-time type of the reference, not about changing the concrete type of the referenced object.

You can use tools such as Reflection or AutoMapper to help with the initialisation of the new MyResults object -- but a new MyResults object there must be, because you cannot tell a base Results object to become a MyResults object.

Answer 2

Results results = new MyResults();
   ...

return (MyResults)results;

should work. If not, then the problem is somewhere alse.

Answer 3

No, you can't avoid copying the content into a instance of the derived type. Well, if you can change CallFrameworkMethod to be a generic method, and MyResults has a zero-argument constructor, then CallFrameworkMethod could create a new instance of your derived type directly, then use only the members of the parent type.

But probably you'll have to end up copying to a new object. Remember that this copying code can certainly be reused in another method, you don't have to rewrite it everywhere you need it.