extend a class to conform to an interface?

Question

I have an interface:

interface IFoo {
  int foo(int bar);
}

Can I now extend an existing class to conform to the interface? Say class String. I know I can define the foo() method on strings. But is it possible to go further and tell the compiler that strings can be cast to IFoo?

Answer 1

You can do it with other classes, but not with System.String, because it is sealed.

If you wanted to do this to a non-sealed class, you could simply derive from it, add appropriate constructors, and put the interface as something that your new class implements.

interface IFoo {
    int Size {get;}
}
// This class already does what you need, but does not implement
// your interface of interest
class OldClass {
    int Size {get;private set;}
    public OldClass(int size) { Size = size; }
}
// Derive from the existing class, and implement the interface
class NewClass : OldClass, IFoo {
    public NewCLass(int size) : base(size) {}
}

When the class is sealed, your only solution of presenting it as some interface through composition: write a wrapper class implementing your interface, give it an instance of the sealed class, and write method implementations that "forward" calls to the wrapped instance of the target class.