c# - Hook/detect windows language change even when app not focused

Question

Is there a way to detect if the windows/os language changed even when my app is not in focus?
So far I was able to achieve what I wanted only if the app was focused using:

string language = "";
System.Windows.Input.InputLanguageManager.Current.InputLanguageChanged +=
new System.Windows.Input.InputLanguageEventHandler((sender, e) =>
{
    language = e.NewLanguage.DisplayName;
    MessageBox.Show(language);
});

But as you can understand, this is not exactly what I want..

I was thinking about other solution such as hooking the keys that change the language (for example alt+shift) but I wont be able to know what language is currently in use and a user can change the default hotkey...

Would appreciate your help.

Answer 1

The problem you are facing is related with how WM_INPUTLANGCHANGE message works. This message is sent to programs by operating system in order to inform them about language changes. However, according to documentation this message is sent only to "to the topmost affected window". It means that you can even call a native method GetKeyboardLayout (it is used by InputLanguageManager by the way) but if an application is not active GetKeyboardLayout will always return the last known, outdated, language.

Taking this into account it might be a good idea to use the solution pointed by @VDohnal i.e. find the current topmost window and read keyboard layout for it. Here is a quick proof of concept how to do it inside WPF application. I used an additional thread that periodically finds the topmost window and ready keyboard layout for it. The code is far from being perfect but it works and it might help you to implement your own solution.

public partial class MainWindow : Window
{
    [DllImport("user32.dll")]
    static extern IntPtr GetKeyboardLayout(uint idThread);
    [DllImport("user32.dll")]
    private static extern IntPtr GetForegroundWindow();
    [DllImport("user32.dll")]
    static extern uint GetWindowThreadProcessId(IntPtr hWnd, IntPtr processId);

    private CultureInfo _currentLanaguge;

    public MainWindow()
    {
        InitializeComponent();

        Task.Factory.StartNew(() =>
            {
                while (true)
                {
                    HandleCurrentLanguage();
                    Thread.Sleep(500);
                }
            });
    }

    private static CultureInfo GetCurrentCulture()
    {
        var l = GetKeyboardLayout(GetWindowThreadProcessId(GetForegroundWindow(), IntPtr.Zero));
        return new CultureInfo((short)l.ToInt64());
    }

    private void HandleCurrentLanguage()
    {
        var currentCulture = GetCurrentCulture();
        if (_currentLanaguge == null || _currentLanaguge.LCID != currentCulture.LCID)
        {
            _currentLanaguge = currentCulture;
            MessageBox.Show(_currentLanaguge.Name);
        }
    }
}