this 扩展类方法

    interface IUserService
    {
        string GetUserName(string name);
    }

    public class UserService : IUserService
    {

        public IUserService GetUserName(string name)
        {
            return new UserService();
        }
    }

    public static class UserClient
    {
        public static int GetUserAge(this IUserService user)
        {
            return 12;
        }
    }

   static void Main(string[] args)
        {
            var servicet = new UserService();
            servicet.GetUserName("a").GetUserAge();
        }