C# - generic methods vs. non-generic methods

Question

I'm a bit confused about why/when I'd ever want to use a generic method since a non-generic method can access the generic members of its containing class and be passed generic arguments anyway.

So, using a canned example that likely misses the point (yet highlights why I'm asking this question), why would I do this:

public class SomeGeneric<T>
{
    public T Swap<T>(ref T a, ref T b)
    {
        T tmp = a;
        a = b;
        b = tmp;
    }
}

over

public class SomeGeneric<T>
{
    public T Swap(ref T a, ref T b)
    {
        T tmp = a;
        a = b;
        b = tmp;
    }
}

this?

Or, really, why would I want to use a generic method at all?

Answer 1

A common scenario for method level type parameters are extension methods because they must be declared in a non-generic static class. But they are required for every generic member in a non-generic type.

public static class Extensions
{
    public static void Foo<A, B>(this A a, B b) { [...] }

    public static T Bar<T>(this String input) { [...] }

    public static U FooBar<V, W>(this V v, W w) { [...] }
}

Answer 2

You'd typically use a generic method in a type that isn't generic.

For example, look at the Enumerable class. It defines the generic extension methods for most of the LINQ fucntionaltiy, but itself isn't generic.

You also might want a generic method within a generic type, but only if the generic method used a different generic type specifier.

This lets you write something like the following:

 class Foo<T> where T : IConvertible, IComparable<T>
 {
      int CompareTo<U>(U other) where U : IConvertible
      {
           // Convert to this
           T otherConverted = Convert.ChangeType(other, typeof(T));
           return this.CompareTo(otherConverted);
      }
 }

(Granted, this is a bit contrived, but does compile and work correctly for Foo<int> comparing to a double, etc)

Answer 3

What if the containing class is not generic? What if it has different generic type parameters?