namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
try
{
User user =null;
user.ToHelloWorld ();
if (user == null)
{
Console.Write("world");
Console.Read();
}
else
{
Console.Write(user.UserName);
Console.Read();
}

        }
        catch
        {
            Console.Write("hello world");
            Console.Read();
        }
    }
}

public class User
{
    public string UserId { get; set; }
    public string UserName { get; set; }
}

public static class UserExtention
{
    public static void ToHelloWorld(this User user)
    {
        if (user == null)
        {
            user = new User() { UserName = "hello" };
        }
    }
}

}

这段代码的输出结果为WORLD。调试发现在执行完 user.ToHelloWorld ();后User还是为NULL。请问大神们这是为什么？