WindowsAppSDK: FileOpenPicker PickMultipleFilesAsync Break in WinUI3 Desktop

I’d like to share the workaround of using Interop GetOpenFileName API call.

By researching several links and verifying some experiments:

https://docs.microsoft.com/en-us/windows/win32/dlgbox/open-and-save-as-dialog-boxes https://stackoverflow.com/questions/9088227/using-getopenfilename-instead-of-openfiledialog https://social.msdn.microsoft.com/forums/vstudio/en-US/2f4dd95e-5c7b-4f48-adfc-44956b350f38/getopenfilename-for-multiple-files

I can workaround this issue as below:

Notes: This works in Project Reunion 0.8 Preview. If you are using WinUI3 Preview or Project Reunion 0.5, may hit this bug:

https://github.com/microsoft/microsoft-ui-xaml/issues/4952

And need to upgrade the project to 0.8 by going through: https://docs.microsoft.com/en-us/windows/apps/project-reunion/update-existing-projects-to-the-latest-release

The helper class is as attached: Libwrap.txt

The calling sample code snippet: ` string[] files = LibWrap.ShowOpenFileDialog(“Select Files”,“c:\users\freistli\documents”, “All files\0 .\0”, true, true);

        if (files != null)
        {
            FPS.Text = "";
            foreach (var file in files)
            {
                Debug.WriteLine("Selected file with full path: {0}", file);
                FPS.Text += file + "\r\n";
            }
        }

`

Hope this helps.

@harvinders you’ll have to statically store the current Window somewhere you can access it globally. If your app only has one single window, the easiest thing to do is, in your App.xaml.cs, refactor…

private Window m_window;

to…

public static readonly Window MainWindow;

And then when you need to reference it, you can do…

var hwnd = WinRT.Interop.WindowNative.GetWindowHandle(App.MainWindow);

Yeah, I used the IInitializeWithWindow interface. The issue was that I didn’t specify a file type filter. Looking through the docs, I discovered that not specifying it generates an exception. It’s all resolved now. Thanks for the swift response!

@eleanorleffler thanks opening the bug and share the code. I could repro this.

I noticed that the doc of this API has the UWP label. I don’t know if this means that this API won’t work in Win32 apps.

Shows the file picker so that the user can pick multiple files. (UWP app)

I need help from the Project Reunion folks, this API is out of the WinUI domain. @jonwis, can you help us to know what is happening here?

We get:

COMException: The application called an interface that was marshalled for a different thread. (0x8001010E (RPC_E_WRONG_THREAD))

Below you can find the code snipped:

 public sealed partial class MainWindow : Window
    {
        public MainWindow()
        {
            this.InitializeComponent();
        }

        private async void fileOpenPickerMultipleButton_Click(object sender, RoutedEventArgs e)
        {
            FileOpenPicker picker = new FileOpenPicker();

            IInitializeWithWindow initializeWithWindowWrapper = picker.As<IInitializeWithWindow>();
            var hwnd = this.As<IWindowNative>().WindowHandle;
            initializeWithWindowWrapper.Initialize(hwnd);

            picker.ViewMode = PickerViewMode.List;
            picker.SuggestedStartLocation = PickerLocationId.Desktop;
            picker.FileTypeFilter.Add(".txt");

            var filesToOpen = await picker.PickMultipleFilesAsync();
            string text = string.Empty;
            foreach (StorageFile storageFile in filesToOpen)
            {
                text += storageFile.Path + Environment.NewLine;
            }
            fileOpenPickerMultipleTextBlock.Text = text;
        }
        public static void InitializeWithWindow(Object obj, Window window)
        {
            // When running on win32, FileOpenPicker needs to know the top-level hwnd via IInitializeWithWindow::Initialize.
            if (Window.Current == null)
            {
                IInitializeWithWindow initializeWithWindowWrapper = obj.As<IInitializeWithWindow>();
                var hwnd = window.As<IWindowNative>().WindowHandle;
                initializeWithWindowWrapper.Initialize(hwnd);
            }
        }

    }

    [ComImport]
    [Guid("3E68D4BD-7135-4D10-8018-9FB6D9F33FA1")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IInitializeWithWindow
    {
        void Initialize([In] IntPtr hwnd);
    }
    [ComImport]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    [Guid("EECDBF0E-BAE9-4CB6-A68E-9598E1CB57BB")]
    internal interface IWindowNative
    {
        IntPtr WindowHandle { get; }
    }

Is m_hWndOwner correct at : (the window handle passed as 1st parameter of ShowDialog)

        public void OnButtonClicked([In][MarshalAs(UnmanagedType.Interface)] IFileDialogCustomize pfdc, [In] int dwIDCtl)
        {
            IntPtr pClassPtr = Marshal.GetIUnknownForObject(this);
            SendMessage(parentCFD.m_hWndOwner, WM_APP_FILEDIALOG, (IntPtr)dwIDCtl, (IntPtr)pClassPtr);
            return;
        }

PickMultipleFilesAsync is unfortunately not supported right now. You can instead use the Win32 API GetOpenFileName, we’re working on posting a workaround sample for that.

@Aloento make sure when you’re calling this.As<IWindowNative>(), the this object is a Window object. If you’re still seeing problems, you can open a new issue, and please provide a full example of the code you’re calling, what specifically is failing, etc. Thanks!

I got the same issue on PickMultipleFilesAsync, which crashes WinUI/Reunion WPF desktop app. But PickSingleFolderAsync works well. Do we have workaround or fix now? Thanks in advance.

See microsoft/ProjectReunion#466 for more information on why this doesn’t work and the workaround.