Extension methods on a static class?

You can't have extension methods on static classes because extension methods are only applicable to instantiable types and static classes cannot be instantiated.

Check this code..

    public static bool IsEmail(this string email)
    {
        if (email != null)
        {
            return Regex.IsMatch(email, "EmailPattern");
        }

        return false;
    }

First parameter to IsEmail() is the extending type instance and not just the type itself. You can never have an instance of a static type.

You can't extend static classes in C#. Extension methods work by defining static methods that appear as instance methods on some type. You can't define an extension method that extends a static class.