Does changing the return type of a function for a child type breaks binary compatibility?

Question

Let's go straight to it :

Old code :

public interface IFoo {}
public class Foo : IFoo {}
...
public static IFoo Bar() { return new Foo(); }

New code :

public static Foo Bar() { return new Foo(); }

Obviously there should be no problem here, everything you were doing on the old return type, you can still do on the new return type, any is, as or cast should behave the same as before...

So did I break binary compatibility, or can I just release it as a minor version without bothering users ?

Answer 1

This breaks binary compatibility, but not (most) compile-time compatibility issues, so it's typically an easy migration.

Note that it can even be a compile-time break if the client code constructs a delegate from the method.