扩展方法可以有通用参数吗？

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, ie params TName[] is okay, but params List<TName> isn't.

Answer 2

There are limits on type inferance (as in when the compiler can guess the generic types from the parameters. The extension methods have one limitation less: you can "dereference" null pointers (you can write and extension method bool IsNullorEmpty(this string) and use it on a null string.

For examples: Linq is full of nice generic extension methods, and you can see a lot of great examples (with smart use of generics) in this question:

What are your favorite extension methods for C#? (codeplex.com/extensionoverflow)