> But in C# for example, if the system was not designed with dependency injection and everything being an interface it's very hard to build a test harness since

> But in C# for example, if the system was not designed with dependency injection and everything being an interface it's very hard to build a test harness since you can't mock anything. Which means everything has to be tested manually. I wouldn't have put it like that, but I think I know what you mean. Mocks for unit testing do require that you have defined an interface to implement, which means that every class that you want to be mocked out needs to have an interface extracted. It is extra work. Overall I think the tradeoff is worth it, myself, especially if your IDE can automated extracting an interface. But it is dumb work that a smarter language/type system could avoid.