What actually "label.visble = true" did?

Posted on 2004-10-13
Last Modified: 2010-04-15
The code below is extracted from one of my project, it will hang when there a client is connected. When I make some changes in the showConnected() function, the program work well. I would like to know why!!

//The code below is at the main page

private void btOpenPort_Click(object sender, System.EventArgs e)
      Connector = new TcpServer(Int32.Parse(tbPort.Text), this);
      Thread runSrv = new Thread(new ThreadStart(Connector.OpenPort));

public void showConnected()
                //lbConnected is a label
      lbConnected.visible = true;

//These the code run is another thread

public TcpServer(int port, Server theBoss)
      PortNo = port;
      boss = theBoss;

public void OpenPort()
      Listener = new TcpListener(PortNo);;
      Socket ConnectionSocket;
      NetworkStream socketStream;
      BinaryReader reader;
      BinaryWriter writer;

            ConnectionSocket = Listener.AcceptSocket();
            socketStream = new NetworkStream(ConnectionSocket);

            writer = new BinaryWriter(socketStream);
            reader = new BinaryReader(socketStream);
            ClientReader = reader;

When I initially set the visible of lbConnected.visible = false and lbConnected.Text = "" and modified the showConnected function as below, the program work well, why?

public void showConnected()
      lbConnected.Text = "Client Connected !!";
Question by:Kahhoe
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Accepted Solution

CrazyIvan007 earned 125 total points
ID: 12306787
You are accessing the UI Thread from another Thread. That can makes problem. You should ever use Delegates to access the UI Thread from within Threads.

public delegate void DelegateSetLabelVisible(string strControlName, bool boolVisible);

public class YourClass
     public DelegateSetLabelVisible m_DelegateSetLabelVisible;

     public YourClass()
           m_DelegateSetLabelVisible = new DelegateSetLabelVisible(this.DelegateSetLabelVisible);

     private void DelegateSetLabelVisible(string strControlName, bool boolVisible)
      Control ctrl = GetControlAtForm(this, strControlName);

      if (ctrl != null)
            Object objCtrl = ctrl;
            if (objCtrl.GetType().ToString() == "System.Windows.Forms.Label")
                  Label _label = (Label)objCtrl;
                  _label.Visible = boolVisible;

    private Control GetControlAtForm(Control ctrl, string strControlName)
      Control ListViewExCtrl = null;

      foreach (Control _ctrl in ctrl.Controls)
            if (_ctrl.Name == strControlName)
            if (_ctrl.HasChildren)
                  Control ctrl_Temp = GetControlAtForm(_ctrl, strControlName);
                  if (ctrl_Temp != null)      { ListViewExCtrl = ctrl_Temp; break; }

                return (ListViewExCtrl);


then use

BeginInvoke(m_DelegateSetLabelVisible, new object[] { "your_label", true });

to Invoke the LabelChange from the Worker Thread to the UI Thread

Author Comment

ID: 12315557
Thank you for this very important concept... The clue is the BeginInvoke method!! This also helped me in solving the problem while I need to add a control in another thread. For example need to add a new row to a grid..

Expert Comment

ID: 12317042
As i said you should use Delegates with Invoke/BeginInvoke if you access the UI Thread from other Threads. That means every Control you wan't to modify/change/add should be Invoked. Without that it can run, but it can throw Exceptions. So its safer to use it.

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