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.