When you implement an interface method with an optional parameter, specifying a different default value in the implementation has no effect when the method is called via the interface, so it's a potential bug. For instance:

interface IFoo { void Bar(string s = "hello"); }
class Foo : IFoo { public void Bar(string s = "hi") { Console.WriteLine(s); } }
IFoo foo = new Foo();
foo.Bar(); // will print "hello", not "hi"

I have written an analyzer for this (I know we're supposed to discuss it before actually doing it, but I wanted to check the feasibility before I mentioned it). I still have to write the code fix, but it shouldn't take long.

Severity : Warning
Category: Maintainability (I'm not really happy with that, but I can't find a more suitable category)
Diagnostic Id: CC0110

What do you think? Is this something we should include in Code Cracker?