Whitebox.setInternalState(Foo.class, b);
Works as long as you set a non-null value, and if theres only one field with the class of B. If you can’t rely on that luxury, you have to provide the field-name and cast the null to the type you want to set. In that case you would need to write something like this:
Whitebox.setInternalState( Foo.class, "b", (B)null );