I have a User object, UserID (int) and UserName (string)

Now I have 2 web method called
GetUser(int userID);
UpdateUser (User user);

then I invoke those 2 methods in my client application, I found I can pass correct int userID through the GetUser(), but couldn't pass correct user.UserID in UpdateUser(), the userName is always correct, for example:
User myUser = new User();
myUser.UserID=200;
myUser.UserName = "abc";
proxy.UpdateUser(myUser);

in UpdateUser()
the UserID= 0
UserName = "abc";

Anybody can give me a hint?

thanks!