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 immediately when the user changes focus??
I keep thinking I may just have to do polling :( :( every 1 second and call GetForegroundWindow but I really don't want to do that.
SetWinEventHook() is probably your best bet; you can listen to either the EVENT_SYSTEM_FOREGROUND to listen for foreground window changes - or even EVENT_OBJECT_FOCUS to listen for more fine-grain focus changes within apps and within controls.
You'll need to use this with the WINEVENT_OUTOFCONTEXT flag; this means that the change notification will be delivered asynchronously to your own app, so you won't need a separate DLL - you'll still need to P/Invoke though. But the notification won't be instant - there may be a small delay - but that's implied with asynchronous. If you want to do something absolutely immediately with no delay whatsoever, you're going to need to use C++ and an in-process hook (either SetWinEventHook with WINEVENT_INCONTEXT or the SetSetWindowsHookEx-style hook.)
Here's a sample that seems to do what you're looking for:
using System;
using System.Windows;
using System.Windows.Forms;
using System.Runtime.InteropServices;
class ForegroundTracker
{
// Delegate and imports from pinvoke.net:
delegate void WinEventDelegate(IntPtr hWinEventHook, uint eventType,
IntPtr hwnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime);
[DllImport("user32.dll")]
static extern IntPtr SetWinEventHook(uint eventMin, uint eventMax, IntPtr
hmodWinEventProc, WinEventDelegate lpfnWinEventProc, uint idProcess,
uint idThread, uint dwFlags);
[DllImport("user32.dll")]
static extern bool UnhookWinEvent(IntPtr hWinEventHook);
// Constants from winuser.h
const uint EVENT_SYSTEM_FOREGROUND = 3;
const uint WINEVENT_OUTOFCONTEXT = 0;
// Need to ensure delegate is not collected while we're using it,
// storing it in a class field is simplest way to do this.
static WinEventDelegate procDelegate = new WinEventDelegate(WinEventProc);
public static void Main()
{
// Listen for foreground changes across all processes/threads on current desktop...
IntPtr hhook = SetWinEventHook(EVENT_SYSTEM_FOREGROUND, EVENT_SYSTEM_FOREGROUND, IntPtr.Zero,
procDelegate, 0, 0, WINEVENT_OUTOFCONTEXT);
// MessageBox provides the necessary mesage loop that SetWinEventHook requires.
MessageBox.Show("Tracking focus, close message box to exit.");
UnhookWinEvent(hhook);
}
static void WinEventProc(IntPtr hWinEventHook, uint eventType,
IntPtr hwnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime)
{
Console.WriteLine("Foreground changed to {0:x8}", hwnd.ToInt32());
}
}
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