asynchronously GetForegroundWindow via SendMessage or something?

Question

Is there a way to be notified of when focus changes from any window to another window(even between windows applications) such that I can just have my delegate called immedia

Answer 1

The example above works like a champ. I refactored it a little to make a class that might be useful. I didn't define all the constants, so you'd need to add a few if you want to trap other events.

using System;
using System.Runtime.InteropServices;

#pragma warning disable 1591
// ReSharper disable InconsistentNaming

namespace MosaiqPerformanceMonitor {
     public class EventHook {
          public delegate void WinEventDelegate(
                IntPtr hWinEventHook, uint eventType,
                IntPtr hWnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime);

          [DllImport("user32.dll")]
          public static extern IntPtr SetWinEventHook(
                uint eventMin, uint eventMax, IntPtr hmodWinEventProc, WinEventDelegate lpfnWinEventProc,
                uint idProcess, uint idThread, uint dwFlags);

          [DllImport("user32.dll")]
          public static extern bool UnhookWinEvent(IntPtr hWinEventHook);

          public const uint EVENT_SYSTEM_FOREGROUND = 3;
          public const uint WINEVENT_OUTOFCONTEXT = 0;
          public const uint EVENT_OBJECT_CREATE = 0x8000;

          readonly WinEventDelegate _procDelegate;
          readonly IntPtr _hWinEventHook;

          public EventHook(WinEventDelegate handler, uint eventMin, uint eventMax) {
                _procDelegate = handler;
                _hWinEventHook = SetWinEventHook(eventMin, eventMax, IntPtr.Zero, handler, 0, 0, WINEVENT_OUTOFCONTEXT);
          }

          public EventHook(WinEventDelegate handler, uint eventMin)
                : this(handler, eventMin, eventMin) {
          }

          public void Stop() {
                UnhookWinEvent(_hWinEventHook);
          }

          // Usage Example for EVENT_OBJECT_CREATE (http://msdn.microsoft.com/en-us/library/windows/desktop/dd318066%28v=vs.85%29.aspx)
          // var _objectCreateHook = new EventHook(OnObjectCreate, EventHook.EVENT_OBJECT_CREATE);
          // ...
          // static void OnObjectCreate(IntPtr hWinEventHook, uint eventType, IntPtr hWnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime) {
          //    if (!Win32.GetClassName(hWnd).StartsWith("ClassICareAbout"))
          //        return;
          // Note - in Console program, doesn't fire if you have a Console.ReadLine active, so use a Form
     }
}