Hooking the Windows Message Loop in a WinForms Application

Provided by Dave Juth, Senior Systems Architect

The days of Windows API programming are just a memory for most, but .NET WinForms applications still allow you to hook Windows messages and respond to operating system events like the good old days.

The WndProc method of a form provides access to the Windows message loop for your application. You can monitor these messages in Visual Studio’s output window with the following code in a form:

protected override void WndProc(ref Message m)
{
  base.WndProc(ref m);

  Trace.WriteLine(m.Msg.ToString() + “: “ + m.ToString());
}

If you want respond to a particular message, such as a Windows shutdown event, the following code example illustrates how to do this:

protected override void WndProc(ref Message m)
{
  base.WndProc(ref m);

  if (m.Msg == 0×11)   // WM_QUERYENDSESSION
  {
    // TODO: Do something, then close the form.
    this.Close();
  };
}

There are many events exposed through the WinForms object model itself, but not all events are. Knowing how to hook into the operating system’s messaging is a powerful capability and, thanks to .NET, easily done.