Question

I need a way to be notified when my application is activated (through Alt-Tab task switching, clicking its icon on the taskbar, etc.) in order to display a reload “file” dialog box.

I tried MainForm.Activated but this event is called whenever I close an application modal dialog.

Ideally, what I would like is something akin to the Application.Activated event found in WPF, but that I can use from a WinForms application.

Answer 1

The Activated and Deactivate events seem to work fine for me. When you tab away, Deactivate is called. When you tab too the form, Activated is called:

   public partial class Form1 : Form
   {
      public Form1()
      {
         InitializeComponent();
         this.Activated += new EventHandler(Form1_Activated);
         this.Deactivate += new EventHandler(Form1_Deactivate);
      }

      void Form1_Deactivate(object sender, EventArgs e)
      {
         System.Diagnostics.Trace.WriteLine("DeActivated");
      }

      void Form1_Activated(object sender, EventArgs e)
      {
         System.Diagnostics.Trace.WriteLine("Activated");
      }
   },

Just found the solution from here

protected void OnActivateApp(bool activate) {
  Console.WriteLine("Activate {0}", activate);
}
protected override void WndProc(ref Message m) {
  // Trap WM_ACTIVATEAPP
  if (m.Msg == 0x1c) OnActivateApp(m.WParam != IntPtr.Zero);
  base.WndProc(ref m);
}

Then inside OnActivateApp, activate will be true if the Application is activated, and false if deactivated.