Can extension methods have generic parameters?

Question

Can an extension method have a say: List as one of the parameters?

public static IEnumerable<XElement> GetSequenceDescendants(this IEnumerable<XElement> elements, params List<XName> names)
        {
            //do something
        }

Is there any restriction on the types of parameters an extension method can have?

Answer 1

The short answer is that an extension method is just a public static method which can be accessed like an instance method of the first parameter (thanks to the this keyword). This means you can use the same parameters you could use in any static method.

But if you want the parameters to actually be generic you'd need to change your method to this:

public static IEnumerable<TElement> GetSequenceDescendants<TElement, TName>(this IEnumerable<TElement> elements, List<TName> names)
{
    //do something
}

You have to specify all the generic arguments in your method definition.

Also, you can't use the params keyword with anything but an array, i.e. params TName[] is okay, but params List<TName> isn't.