How to chose between new and override in C#?

Question

It is advised to use override instead of new key word in C#. Why that rule?

Answer 1

"new" means you've got two completely different methods as far as the CLR is concerned - they happen to have the same name, but they're unrelated in terms of inheritance. That means that if you run:

Base b = new Derived();
Derived d = new Derived();
b.MyMethod(); // Calls Base.MyMethod
d.MyMethod(); // Calls Derived.MyMethod

This can make for some very hard-to-understand code.

Usually if you want different methods, call them different things. The normal reason to use the same name (and signature) is to override the behaviour of a base class's method. At that point you don't have any choice but to keep the name/signature the same.

Answer 2

I'm still trying to work out what the use case is for "new" aside from horrid hackery.