C#: Good/best implementation of Swap method

Question

I read this post about card shuffling and in many shuffling and sorting algorithms you need to swap two items in a list or array. But what does a good and efficient Swap method look like?

Let's say for a T[] and for a List<T>. How would you best implement a method that swaps two items in those two?

Swap(ref cards[i], ref cards[n]);   // How is Swap implemented?

Answer 1

11 years later and we have tuples...

(foo, bar) = (bar, foo);