Question

In general, given a class A, and a derived class DerivedFromA, and an instance of A, is there a way to construct DerivedFromA from that instance?

E.g., let’s say that DerivedFromA simply overrides one or more methods or properties of A. Let’s say I have some Container class with a CreateA() method that returns an instance of A, and I either can’t or don’t want to override or otherwise mess with that Container class. What I want to do is take the instance returned by Container.CreateA(), pass it to a DerivedFromA( A instanceOfA) constructor and have the resulting object be treated just like instanceOfA and use all of instanceOfA’s methods, properties, and data except for the ones I have overridden.

BTW, the language in question is C#, it that makes a difference.

Thanks.

Answer 1

You’ll have to make a new DerivedFromA instance, and use that instance instead of your original. If you do that, however, it will work fine.

Just add a constructor like you showed:

public DerivedFromA(A instanceOfA)
{
    // Copy members from instanceOfA into here as necessary
    this.Foo = instanceOfA.Foo;
    this.Bar = instanceOfA.Bar;

    // Setup unique values
    this.Baz = 42;
}

Then, when you call, you can just do:

DerivedFromA instance = new DerivedFromA(container.CreateA());