为什么不创建此Grid / TextBlock？

Question

I'm adding a UI to some code I wrote for a messenger, some while ago. It seems to be going OK, apart from when I try to display new messages from a different thread.

The program flows thusly:

1. The MainWindow gets created and instantiated. The constructor for MainWindow :

    public MainWindow() { InitializeComponent(); _messageContainer = (StackPanel)FindName("Messages"); _messageStatus = (StatusBarItem)FindName("MessageStatus"); _messageCountElement = (StatusBarItem)FindName("MessageCount"); _messageBox = (TextBox)FindName("MessageToSend"); _sendButton = (Button)FindName("SendMessage"); _ipAddress = GetIPAddress(); try { Client.Initialize(_ipAddress, this); _sendButton.IsEnabled = true; _messageBox.IsEnabled = true; } catch (Exception e) { DisplayMessage("Could not connect to " + _ipAddress.ToString() + ". The error given was '" + e.Message + "'.", "Server", Colors.Red); } } 

2. The sending client ( Client class) is initialized using Client.Initialize from the MainWindow constructor. Client.Initialize :

    public static void Initialize(IPEndPoint endPoint, MainWindow window) { windowInstance = window; client = new TcpClient(); client.ReceiveTimeout = 500; listenerThread = new Thread(new ParameterizedThreadStart(Receiver.Start)); listenerThread.Start((object)new StartupData(endPoint, windowInstance)); client.Connect(endPoint); } 

3. The listener thread is started from Client.Initialize . The listener thread's Start method is long and complicated, but works fine. It boils down to calling another method, ProcessMessage to process what it receives. ProcessMessage :

    public static void ProcessMessage(string response) { response = response.Trim(); MessageBox.Show(response); if (response.StartsWith("[Message]")) { MessageBox.Show("Message"); response = response.Substring(9); int openIndex = response.IndexOf("<"); int closeIndex = response.IndexOf(">"); if (openIndex < 0 || closeIndex < 0 || closeIndex < openIndex) { throw new FormatException("Could not find ID tag in message"); } int diff = closeIndex - openIndex; int id = Int32.Parse(response.Substring(openIndex + 1, diff - 1)); if (id != Client.GetClientId()) { MessageBox.Show("Them"); string message = response.Substring(closeIndex + 1); window.DisplayMessage(message, "Them", Colors.Yellow); } else { MessageBox.Show("You"); string message = response.Substring(closeIndex + 1); window.DisplayMessage(message, "You", Colors.Blue); } } else if (response.Length == 5 && response.StartsWith("[") && response.EndsWith("]")) { MessageBox.Show("Response code"); Client.HandleResponse(ResponseCodes.GetResponse(response)); } else { try { Int32.Parse(response); MessageBox.Show("ID"); Client.SetClientId(Int32.Parse(response)); return; } catch (Exception) { window.DisplayMessage(response, "Server", Colors.Red); } } } 

4. The DisplayMessage method is called. DisplayMessage :

    public void DisplayMessage(string message, string name, Color nameColor) { MessageBox.Show("Called"); UpdateMessageStatus(ProcessingStatus.Processing); Grid fullMessage = new Grid(); fullMessage.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(50.00) }); fullMessage.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(600.00) }); Label nameLabel = new Label { Content = string.Format("[{0}]", name), Foreground = new SolidColorBrush(nameColor) }; Grid.SetColumn(nameLabel, 0); TextBlock textLabel = new TextBlock { Text = message, TextWrapping = TextWrapping.Wrap, Margin = new Thickness(0, 5, 0, 5) }; Grid.SetColumn(textLabel, 1); fullMessage.Children.Add(nameLabel); fullMessage.Children.Add(textLabel); UpdateMessageStatus(ProcessingStatus.Displaying); Dispatcher.BeginInvoke(new Action(delegate() { _messageContainer.Children.Add(fullMessage); })); _messageCount += 1; UpdateMessageCount(); UpdateMessageStatus(ProcessingStatus.Ready); } 

Here is the problem. When I call window.DisplayMessage from the listener thread, literally nothing happens. Not even an exception - but importantly, the message Grid doesn't get created.

What's up with this? I added Dispatcher.BeginInvoke on the advice of another SO question to make sure the thread ownership wasn't a problem, though the same thing happened before this.

(note: all the MessageBox es are just for debugging. Interestingly, the MessageBox at the top of DisplayMessage does show when called from the listener thread.)

Answer 1

I do suspect this is an issue of cross-thread UI element invokation. Since you are creating your new UI element ( fullMessage and its children) on a different thread.

If your window instance is a WPF UserControl or a WPF Window you can use it's synchronization context to perform the cross-thread marshaling.

• Capture its synchronization context in your MainWindow constructor

   _syncContext = SynchronizationContext.Current;  

• Add a method that uses the captured context, to marshal to the correct dispatcher.

  \npublic void InvokeDisplayMessage(string message, string name, Color nameColor) \n{ \n    // In the already started spirit of message box debugging ;-) \n    MessageBox.Show("InvokeDisplayMessage Called"); \n    this._syncContext.Post( \n        new SendOrPostCallback(x => DisplayMessage(message, name, nameColor)),  \n        null); \n} \n

• And finally change all of the window.DisplayMessage calls in your ProcessMessage to window.InvokeDisplayMessage .