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(); }
<         


        

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.