Monitoring Clipboard action that happens only in my application in a wpf application

Question

I want to add to my WPF application the ability to monitor Clipboard changes. But also I want to filter only Clipboard changes that happens in my app. So I wrote the following code in one of the main view models, this VM inherits from

Conductor<IScreen> 

of Caliburn.Micro.

The code:

protected override void OnActivate()
{
    base.OnActivate();
    ScreenExtensions.TryActivate(ChildWindow);

    BuildClipboard();

    GotoLogin();
}

private void BuildClipboard()
{
    _windowHandle = (new WindowInteropHelper(Application.Current.MainWindow)).EnsureHandle();
    _hWndSource = HwndSource.FromHwnd(_windowHandle);
    _hWndSource.AddHook(new HwndSourceHook(WndProc));

    SetClipboardViewer(_windowHandle);
}

private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam,      ref bool handled)
{
     try
     {
        if (_windowHandle == hwnd && msg == WM_DRAWCLIPBOARD)
        {
            IDataObject iData = Clipboard.GetDataObject();
            if (iData.GetDataPresent(DataFormats.Text))
            {
              /////
            }
        }
     }
     catch(Exception
     {
       ///
     }
 }

protected override void OnDeactivate(bool close)
{
   _hWndSource.RemoveHook(new HwndSourceHook(WndProc));
   _windowHandle = IntPtr.Zero;

   ScreenExtensions.TryDeactivate(ChildWindow, close);
   base.OnDeactivate(close);
   ....
}

I have some problems: First, the line

if (_windowHandle == hwnd && msg == WM_DRAWCLIPBOARD)

does not help to get the behavior I want. For some reason hwnd always have the same int value, even if I copy things from Word and not from my app. Second problem, each time I restart my app, when my app starts it gets to WndProc method with the value of a copy I made before I restarted my app. off curse It's not the behavior I want.

I hope you can help me, Thank you, Anat