onmousemove listener on WPF WebBrowser control stops working after few seconds

Question

I Have a Windows WPF application that uses the default WebBrowser control (System.Windows.Controls.WebBrowser). I have the need to intercept MouseMove events that happen on the browser. In order to subscribe to the event I use the following code fragment (these are methods of a UserControl that wraps the WebBrowser):

public void HookInputElementForKeyboard()
{
    HTMLDocument htmlDocument = (HTMLDocument)webBrowserControl.Document;

    htmlDocument.attachEvent("oncontextmenu", new ContextMenuDisablerEventHandler());
    HTMLDocumentEvents_Event documentEvents = htmlDocument as HTMLDocumentEvents_Event;
    documentEvents.onmousemove += DocumentOnMouseMove;
}

private void DocumentOnMouseMove()
{
    HTMLDocument document = webBrowserControl.Document as HTMLDocument;
    var window = document.parentWindow as IHTMLWindow2;
    var currentEvent = window.@event;
    MouseMoveOnDocumentEventArgs args = new MouseMoveOnDocumentEventArgs(currentEvent.clientX, currentEvent.clientX);
    var now = DateTime.Now;
    logger.Debug(now.Second + "." + now.Millisecond + ": " + args.ToString());
    // fires the event of the wrapping UserControl to notify the MouseMove to external clients
    OnMouseMoveOnDocument(args);
}

HookInputElementForKeyboard gets called on the LoadCompleted event of the WebBrowser. The problem is that after a seemingly random amount of time (usually some seconds), the callback DocumentOnMouseMove stops being called. This an extract of my log with the timestamps of the events (format is <timestampSeconds>: [<clientX>; <clientY>] ):

2015-07-14 10:30:22,005 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - onmousemove event subscribed at 22.5
[some other unrelated logs]
2015-07-14 10:30:22,568 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 22.568: [4; 4]
2015-07-14 10:30:22,584 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 22.584: [15; 15]
2015-07-14 10:30:22,584 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 22.584: [24; 24]
[...]
2015-07-14 10:30:22,599 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 22.599: [33; 33]
2015-07-14 10:30:22,599 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 22.599: [45; 45]
2015-07-14 10:30:22,615 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 22.615: [63; 63]
2015-07-14 10:30:22,615 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 22.615: [92; 92]
[...]
2015-07-14 10:30:23,849 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 23.849: [565; 565]
2015-07-14 10:30:23,849 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 23.849: [571; 571]
2015-07-14 10:30:23,865 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 23.865: [580; 580]
2015-07-14 10:30:23,865 [8] DEBUG Common.GUI.UserControls.WebBrowserView [(null)] - 23.865: [587; 587]
[from now on, the MouseMove event is not captured any more, even if I keep moving the pointer]

There is no error traced in the application log nor in the Event Viewer.

Anybody knows what could be the cause of this behaviour? Is there any possible workaround?

Answer 1

Just found another way to implement this behavior and it works. I still do not know exactly why. The solution might seem a bit convoluted, but it's the only way I found so far. First, define a class that will act as a collector of the mousemove event generated by the WebBrowser control. I called this class DomMouseMoveEventManager :

 [ComVisible(true)]
 [ClassInterface(ClassInterfaceType.AutoDispatch)]
 public class DomMouseMoveEventManager
 {
     public event DomMouseMoveEventHandler DomMouseMove;
     [DispId(0)]
     public void CallbackFunction(mshtml.IHTMLEventObj arg)
     {
         //Console.WriteLine(String.Format("[{0}, {1}]", arg.clientX, arg.clientY));
         OnDomMouseMove(arg.clientX, arg.clientY);
     }

     private void OnDomMouseMove(int clientX, int clientY)
     {
         if(DomMouseMove != null)
         {
             var args = new DomMouseMoveEventArgs(clientX, clientY);
             DomMouseMove(this, args);
         }
     }
 }

Note that the class has an event called DomMouseMove: use this event instead of a direct listener in the client class. Where previously we had:

  private void HookOnMouseMove()
  {
      var document = WebBrowserControl.Document as HTMLDocument;
      var documentEvents = document as HTMLDocumentEvents_Event;
      documentEvents.onmousemove += documentEvents_onmousemove;
  }

now we register to the event using a member of type DomMouseMoveEventManager , like this:

  private DomMouseMoveEventManager mouseMoveManager;
  private void HookOnMouseMove()
  {
      var document = WebBrowserControl.Document as HTMLDocument;
      document.attachEvent("onmousemove", mouseMoveManager);
  }

Obviously, you will also need an event args class:

  public class DomMouseMoveEventArgs : RoutedEventArgs
  {
      public int ClientX { get; set; }
      public int ClientY { get; set; }

      public DomMouseMoveEventArgs(int clientX, int clientY)
      {
          ClientX = clientX;
          ClientY = clientY;
      }
  }

And, in order to complete the picture, this is how I attach the client class to the DomMouseMoveEventManager:

// The constructor of the client class that must be notified the mousemove event
  public WebBrowserAdapter()
  {
      InitializeComponent();
      this.Loaded += WebBrowserAdapter_Loaded;
      mouseMoveManager = new DomMouseMoveEventManager();
      mouseMoveManager.DomMouseMove += mouseMoveManager_DomMouseMove;
  }

  void mouseMoveManager_DomMouseMove(object source, DomMouseMoveEventArgs args)
  {
      diagnosticBlock.Text = "[" + args.ClientX + "; " + args.ClientY + "]";
      OnDomMouseMove(args.ClientX, args.ClientY);
  }