FileSavePicker.FileTypeChoices throws InvalidCastException

Describe the bug

Trying to execute any methods of folderPicker.FileTypeFilter will throw an exception.

Steps to reproduce the bug

Steps to reproduce the behavior:

1. Clone https://github.com/microsoft/WinUI-3-Demos/
2. Edit 36-45 Lines of MainWindow.xaml.cs to

var savePicker = new FileSavePicker();

//Make folder Picker work in Win32
IntPtr windowHandle = (App.Current as App).WindowHandle;
InitializeWithWindowWrapper initializeWithWindowWrapper = InitializeWithWindowWrapper.FromAbi(savePicker.ThisPtr);
initializeWithWindowWrapper.Initialize(windowHandle);
folderPicker.FileTypeFilter.Add("*");

savePicker.FileTypeChoices.Add("Plain Text", new List<string>() { ".txt" });
savePicker.SuggestedFileName = "OUTPUT";
Windows.Storage.StorageFile file = await savePicker.PickSaveFileAsync();
textBox.Text = file!= null ? file.Name: string.Empty;

1. Run the app, click the Browse button.
2. The app will crash with exception System.InvalidCastException (“Specified cast is not valid”).

| Name | Value | Type – | – | – | – ▶ | savePicker.FileTypeChoices.Add(“Plain Text”, new List<string>() { “.txt” }); | ‘savePicker.FileTypeChoices.Add(“Plain Text”, new List<string>() { “.txt” })’ threw an exception of type ‘System.InvalidCastException’ | void {System.InvalidCastException}

Expected behavior

A file save picker is opened. User can pick a file, then it’s path is shown in textbox.

Screenshots

Version Info

NuGet package version: [Microsoft.WinUI 3.0.0-preview2.200713.0]

Windows app type:

UWP	Win32
No	Yes

Windows 10 version	Saw the problem?
Insider Build (20231)	Yes
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

Additional context

When trying to watch savePicker, I can see

| Name | Value | Type – | – | – | – ▶ | FileTypeChoices | ‘savePicker.FileTypeChoices’ threw an exception of type ‘System.InvalidCastException’ | System.Collections.Generic.IDictionary<string, System.Collections.Generic.IList<string>> {System.InvalidCastException}

I guees there is something abnormal with savePicker.FileTypeChoices.

[marb2000]

I forgot to say that the demos where updated: https://github.com/microsoft/WinUI-3-Demos/ and I’m no longer see this issue.

Thanks. I’m closing the issue, feel free to reopen it in case I miss something.

[marb2000]

@stevenbrix sure, demo samples will be updated. @KiruyaMomochi I will take a look to this issue to see if it repros in the next Preview.