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(); }
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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.

Answer 2

You could have problems with people who have created unit tests around your code. Because it's static people could have created a FooAdapater as follows:-

public class FooAdapater(){ 
public IFoo GetFoo() { Return your static Bar; }
}

They could then create a mocked foo adapter than returns a mocked foo. Your code would break this scenario.

So, no it's not binary compatible :-)