detect foreground window switch in C#

Main.cs

using System;
using System.Runtime.InteropServices;
using System.ComponentModel;

namespace ShellEventsTest
{
    class Program
    {
        static void Main(string[] args)
        {
            HookManager.SubscribeToWindowEvents();

            EventLoop.Run();
        }
    }

    public static class HookManager
    {
        public static void SubscribeToWindowEvents()
        {
            if (windowEventHook == IntPtr.Zero)
            {
                windowEventHook = SetWinEventHook(
                    EVENT_SYSTEM_FOREGROUND, // eventMin
                    EVENT_SYSTEM_FOREGROUND, // eventMax
                    IntPtr.Zero,             // hmodWinEventProc
                    WindowEventCallback,     // lpfnWinEventProc
                    0,                       // idProcess
                    0,                       // idThread
                    WINEVENT_OUTOFCONTEXT | WINEVENT_SKIPOWNPROCESS);

                if (windowEventHook == IntPtr.Zero)
                {
                    throw new Win32Exception(Marshal.GetLastWin32Error());
                }
            }
        }

        private static void WindowEventCallback(IntPtr hWinEventHook, uint eventType, IntPtr hwnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime)
        {
            Console.WriteLine("Event {0}", hwnd);
        }

        private static IntPtr windowEventHook;

        private delegate void WinEventProc(IntPtr hWinEventHook, uint eventType, IntPtr hwnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime);

        [DllImport("user32.dll", SetLastError = true)]
        private static extern IntPtr SetWinEventHook(int eventMin, int eventMax, IntPtr hmodWinEventProc, WinEventProc lpfnWinEventProc, int idProcess, int idThread, int dwflags);
        [DllImport("user32.dll", SetLastError = true)]
        private static extern int UnhookWinEvent(IntPtr hWinEventHook);

        private const int WINEVENT_INCONTEXT = 4;
        private const int WINEVENT_OUTOFCONTEXT = 0;
        private const int WINEVENT_SKIPOWNPROCESS = 2;
        private const int WINEVENT_SKIPOWNTHREAD = 1;

        private const int EVENT_SYSTEM_FOREGROUND = 3;
    }

    public static class EventLoop
    {
        public static void Run()
        {
            MSG msg;

            while (true)
            {
                if (PeekMessage(out msg, IntPtr.Zero, 0, 0, PM_REMOVE))
                {
                    if (msg.Message == WM_QUIT)
                        break;

                    TranslateMessage(ref msg);
                    DispatchMessage(ref msg);
                }
            }
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct MSG
        {
            public IntPtr Hwnd;
            public uint Message;
            public IntPtr WParam;
            public IntPtr LParam;
            public uint Time;
            public System.Drawing.Point Point;
        }

        const uint PM_NOREMOVE = 0;
        const uint PM_REMOVE = 1;

        const uint WM_QUIT = 0x0012;

        [DllImport("user32.dll")]
        private static extern bool PeekMessage(out MSG lpMsg, IntPtr hwnd, uint wMsgFilterMin, uint wMsgFilterMax, uint wRemoveMsg);
        [DllImport("user32.dll")]
        private static extern bool TranslateMessage(ref MSG lpMsg);
        [DllImport("user32.dll")]
        private static extern IntPtr DispatchMessage(ref MSG lpMsg);
    }
}