Question

I have the following C# code in a WPF project:

private static void RunConfig(string owner)  
{  
    long ownerHandle;  
    var settingsWindow = new SettingsWindow();  
    if (long.TryParse(owner, out ownerHandle))  
    {  
        WindowInteropHelper helper = new WindowInteropHelper(settingsWindow);  
        helper.Owner = new IntPtr(ownerHandle);  
    }  
    settingsWindow.ShowDialog();  
}

The SettingsWindow isn’t properly modal to the owner window (i.e. I can focus on, interact with, and even close the owner window while the SettingsWindow is still open). What am I doing wrong?

For context, this code is part of a screen saver program, and the owner window is the Control Panel screen saver selection window (which passes in the handle to use as owner via command line parameter). I know the IF statement is evaluating true and correctly parsing the handle.

I have also tried using the SetWindowLongPtr method from user32.dll (compiling for x64, hence not using SetWindowLong) which is briefly described here and shown in use here. This method works in WinForms, but doesn’t seem to work here in WPF. Help me Obi-Wan Kenobi, you’re my only hope.

Answer 1

It turns out that using WindowInteropHelper to set the native window as owner of the WPF Window does work, it just doesn’t do the whole job. When set this way, the WPF Window will remain visible on top of the native window, even if the native window has focus. However, that is the only effect obtained. The WPF Window does not prevent interaction with the native Window, and the native window can even be closed, without the WPF Window closing or being affected.

In order to get the rest of the desired behaviour, we need to use the EnableWindow function in user32.dll to disable the native Window before calling ShowDialog on the WPF Window, and again to re-enable it once the WPF Window closes.

The modified code looks like this:

private static void RunConfig(string owner)
{
    long ownerHandle;
    var settingsForm = new SettingsWindow();
    if (long.TryParse(owner, out ownerHandle))
    {
        WindowInteropHelper helper = new WindowInteropHelper(settingsForm);
        helper.Owner = new IntPtr(ownerHandle);
        NativeMethods.EnableWindow(helper.Owner, false);
        settingsForm.ShowDialog();
        NativeMethods.EnableWindow(helper.Owner, true);
    }
    else
    {
        settingsForm.ShowDialog();
    }
}

(Note: The above code is correct in general, but incomplete in the case of screen savers, which is what this code is actually being used for. In the case that this code is being used for the config window of a screen saver, the string passed in for the owner handle is not the handle of the Control Panel window to be used as owner, but rather a handle for a control that is a child of the Control Panel window. The extra step in this case is to get the handle of the parent of that control. We can do this by calling GetParent, also in user32.dll, on the passed-in handle. This will return the real handle we want to use for the owner and EnableWindow calls.)

If anyone from Microsoft ever finds this, maybe consider modifying WindowInteropHelper to properly set all of this up when Owner is assigned and ShowDialog used, since this is the proper complete behavior for modal windows.