//Send Text
 void Send()
 {
     if (RTBText.Text == string.Empty)
     {
         MessageBox.Show("Tin nhắn rỗng. Thử lại!");
     }
     else
     {
         ChatMessage CM = new ChatMessage();
         ChatUsers      = UserClient;
         CM.Sender      = UserClient;
         CM.Content     = " " + txtusername.Text + " : " + RTBText.Text + "\n";
         CM.CurrentTime = DateTime.Now;
         if (cbChatMat.Checked)
         {
             AppendText(RTBPrivate, Color.Blue, CM.Content);
             AppendText(RTBRoom, Color.Blue, CM.Content);
             RTBPrivate.SelectionStart = RTBPrivate.Text.Length;
             RTBPrivate.ScrollToCaret();
             RTBRoom.SelectionStart = RTBRoom.Text.Length;
             RTBRoom.ScrollToCaret();
             proxy.SendPrivateMessage(CM, _toUser);
         }
         else
         {
             AppendText(RTBRoom, Color.Black, CM.Content);
             RTBRoom.SelectionStart = RTBRoom.Text.Length;
             RTBRoom.ScrollToCaret();
             proxy.SendPublicMessage(CM, CM.Sender.username);
         }
     }
     RTBText.Select(0, RTBText.TextLength);
     RTBText.SelectedText = "";
 }
 //user log out
 public void Userloggedout(User user)
 {
     //listBox2.Items.Add(name);
     lbfriend.Items.Remove(user.username);
     ChatUsers = user;
     AppendText(RTBRoom, Color.Red, user.username + " vừa thoát hệ thống\n");
     RTBRoom.AppendText("\n");
 }
 // Khi user log in
 public void UserLoggedin(User user)
 {
     lbfriend.Items.Add(user.username);
     ChatUsers = user;
     AppendText(RTBRoom, Color.Red, user.username + " vừa đăng nhập vào hệ thống\n");
     RTBRoom.AppendText("\n");
     //listBox2.Items.Remove(name);
 }
 //Send a message ra mọi đối tượng đang online tren mạng
 public void NewPublicMessage(ChatMessage CM)
 {
     ChatUsers = CM.Sender;
     AppendText(RTBRoom, Color.Black, CM.Content);
     RTBRoom.SelectionStart = RTBRoom.Text.Length;
     RTBRoom.ScrollToCaret();
     ChatUsers = UserClient;
 }