Capturing active window title failed when WPF application's control focused

Question

I am trying to add to my time tracker window caption tracking using SetWindowsHookEx, but it is works only partially. Here is my code I using to provide subscribers with coresponding event:

public class Hooks
{
    #region DllImport

    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_SKIPOWNPROCESS = 2;
    private const uint WINEVENT_SKIPOWNTHREAD = 1;
    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); 

    #endregion DllImport

    public static event EventHandler<EventArgs<string>> WindowActivated;

    public void Bind()
    {
        var listener = new WinEventDelegate(EventCallback);
        var result = SetWinEventHook(EVENT_SYSTEM_FOREGROUND,
                        EVENT_SYSTEM_FOREGROUND,
                        IntPtr.Zero,
                        listener,
                        0,
                        0,
                        (WINEVENT_OUTOFCONTEXT));
    }

    private static void EventCallback(IntPtr hWinEventHook, uint eventType, IntPtr hwnd,
                                      int idObject, int idChild, uint dwEventThread, uint dwmsEventTime)
    {
        System.Diagnostics.Debug.Write("EventCallback enter!");

        if (eventType == EVENT_SYSTEM_FOREGROUND)
        {
            var buffer = new StringBuilder(300);
            var handle = GetForegroundWindow();

            System.Diagnostics.Debug.Write(string.Format("EventCallback GetWindowText: {0}, '{1}'",
                GetWindowText(handle, buffer, 300),
                buffer));

            if (GetWindowText(handle, buffer, 300) > 0 && WindowActivated != null)
            {
                System.Diagnostics.Debug.Write(string.Format(
                    "Calling handlers for WindowActivated with buffer='{0}'",
                    buffer));
                WindowActivated(handle, new EventArgs<string>(buffer.ToString()));
            } 
        }

        System.Diagnostics.Debug.Write("EventCallback leave!");
    }
}

I have main ui WPF application with single Textbox and I bind Hook's event to text box content. When I run it it looks work fine, until itself focused. I.e. if I clicked texbox of WPF window becomes active hook stops working for about 30-40 seconds. After that events capturing becomes work, but again - until main WPF windows becomes active.

Any ideas what is it and how to fix it?

Answer 1

I created a new WPF application using the standard Visual Studio template (VS2012; .NET 4.5). I then replaced the XAML and code-behind as follows:

MainWindow.xaml

<Window x:Class="stackoverflowtest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow">
    <Grid>
        <StackPanel VerticalAlignment="Top" Margin="15">
            <TextBox x:Name="_tb" />
            <Button Content="Does Nothing" HorizontalAlignment="Left" Margin="0,15" />
        </StackPanel>
    </Grid>
</Window>

MainWindow.xaml.cs

using System;
using System.Runtime.InteropServices;
using System.Text;
using System.Windows;
using System.Windows.Controls;

namespace stackoverflowtest
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            _hooks = new Hooks(_tb);
            _hooks.Bind();
        }

        readonly Hooks _hooks;
    }

    public class Hooks
    {
        public Hooks(TextBox textbox)
        {
            _listener = EventCallback;
            _textbox = textbox;
        }

        readonly WinEventDelegate _listener;
        readonly TextBox _textbox;
        IntPtr _result;

        public void Bind()
        {
            _result = SetWinEventHook(
                EVENT_SYSTEM_FOREGROUND,
                EVENT_SYSTEM_FOREGROUND,
                IntPtr.Zero,
                _listener,
                0,
                0,
                WINEVENT_OUTOFCONTEXT
                );
        }

        void EventCallback(IntPtr hWinEventHook, uint eventType, IntPtr hwnd, int idObject, int idChild,
            uint dwEventThread, uint dwmsEventTime)
        {
            var windowTitle = new StringBuilder(300);
            GetWindowText(hwnd, windowTitle, 300);
            _textbox.Text = windowTitle.ToString();
        }

        #region P/Invoke
        const uint WINEVENT_OUTOFCONTEXT = 0;
        const uint EVENT_SYSTEM_FOREGROUND = 3;

        [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 int GetWindowText(IntPtr hWnd, StringBuilder text, int count);

        delegate void WinEventDelegate(IntPtr hWinEventHook, uint eventType, IntPtr hwnd, int idObject,
            int idChild, uint dwEventThread, uint dwmsEventTime);
        #endregion
    }
}

For me, this works fine. I can switch between different applications and their window titles are reflected in the textbox of the WPF app. There is no delay even when the textbox is focused. I added a do-nothing Button control just to prove that whether the texbox or the button is focused it does not change the behaviour.

I can only assume the problem you are having is related to something you are doing in your event handler, or failing that some kind of threading issue.

Answer 2

Probably the reason is that the delegate was garbage collected since it is assigned into a local variable. That's why it stopped to work after 30-40 sec.