Detect active window changed using C# without polling

Question

How might one invoke a callback whenever the current active window changes. I've seen how it might be done using CBTProc. However, global events aren't easy to hook into with managed code. I'm interested in finding a way that doesn't require polling. I'd prefer an event driven approach.

Regards

Solution 1

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";
    } 

Solution 2

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!

Solution 3

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

Comments

[Ehsan Zargar Ershadi]

Can you give some examples , please.

[Raymond Chen]

You can search stackoverflow.

[Kenneth K.]

As per Savvas Sopiadis' answer, the scope of dele needs to be widened to prevent the delegate from being disposed. See the comments on the documentation page for more info.

[Anthony Raimondo]

This doesnt seem to work with a console application, does anyone know why? I removed InitializeComponent().

[public wireless]

Additional info: Your out of context function must be fast. (msdn.microsoft.com/en-us/library/windows/desktop/…)

[public wireless]

@AnthonyRaimondo Apparently it 'doesn't fire if you have a Console.ReadLine active'. See stackoverflow.com/questions/8840926/…

[Wobbles]

To reduce interop calls and clean up code a bit; GetForegroundWindow() is not needed as WinEventProc already receives the foreground handle as as hwnd which can be used to get the title.

[NtFreX]

You should also call UnhookWinEvent in the end.

[IneedHelp]

Too bad this doesn't work properly when resorting windows (from the minimized state)..

[Walter Stabosz]

It crashes because when WinEventDelegate dele is scoped inside of the Form1 constructor, it is a candidate for garbage collection since the reference is lost at the end of the constructor. Scoping the at the class level will keep the reference alive until the class object is disposed.

[A Petrov]

This does not work in a windows service.