Отслеживание момента блокировки и разблокировки компьютера

private void SystemEvents_SessionSwitch(object sender, SessionSwitchEventArgs e) 
{ 
   //If the reason for the session switch is lock or unlock //send the message to mute or unmute the system volume 
   if (e.Reason == SessionSwitchReason.SessionLock)  
   { 
      SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle, (IntPtr)APPCOMMAND_VOLUME_MUTE); 
   } 
   else if (e.Reason == SessionSwitchReason.SessionUnlock) 
   { 
      SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle, (IntPtr)APPCOMMAND_VOLUME_MUTE); 
   } 
}