Detect active window changed using C# without polling

Create a new windows forms project, add a textbox, make it multiline, and set the textbox Dock property to fill, name it Log and paste in the following code (you'll need to add System.Runtime.InteropServices to your usings)...

    WinEventDelegate dele = null;    public Form1()    {        InitializeComponent();        dele = new WinEventDelegate(WinEventProc);        IntPtr m_hhook = SetWinEventHook(EVENT_SYSTEM_FOREGROUND, EVENT_SYSTEM_FOREGROUND, IntPtr.Zero, dele, 0, 0, WINEVENT_OUTOFCONTEXT);    }    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);    private const uint WINEVENT_OUTOFCONTEXT = 0;    private const uint EVENT_SYSTEM_FOREGROUND = 3;    [DllImport("user32.dll")]    static extern IntPtr GetForegroundWindow();    [DllImport("user32.dll")]    static extern int GetWindowText(IntPtr hWnd, StringBuilder text, int count);    private string GetActiveWindowTitle()    {        const int nChars = 256;        IntPtr handle = IntPtr.Zero;        StringBuilder Buff = new StringBuilder(nChars);        handle = GetForegroundWindow();        if (GetWindowText(handle, Buff, nChars) > 0)        {            return Buff.ToString();        }        return null;    }    public void WinEventProc(IntPtr hWinEventHook, uint eventType, IntPtr hwnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime)    {        Log.Text += GetActiveWindowTitle() + "\r\n";    } 

I know this thread is old, but for sake of future use: when running the code you'll notice a crash after a while. This is caused from the line in the Form constructor:

public Form1()    {        InitializeComponent();        WinEventDelegate dele = new WinEventDelegate(WinEventProc);//<-causing ERROR        IntPtr m_hhook = SetWinEventHook(EVENT_SYSTEM_FOREGROUND, EVENT_SYSTEM_FOREGROUND, IntPtr.Zero, dele, 0, 0, WINEVENT_OUTOFCONTEXT);    }

Instead of the above make the following modification:

public Form1()        {            InitializeComponent();            dele = new WinEventDelegate(WinEventProc);             IntPtr m_hhook = SetWinEventHook(EVENT_SYSTEM_FOREGROUND, EVENT_SYSTEM_FOREGROUND, IntPtr.Zero, dele, 0, 0, WINEVENT_OUTOFCONTEXT);        }WinEventDelegate dele = null;

..works now as expected!

You can use SetWinEventHook and listen for the EVENT_SYSTEM_FOREGROUND event. Use the WINEVENT_OUTOFCONTEXT flag to avoid the global-hook problem.