Fix Gtk open file dialog returns folder

[zii-dmg]

What does the pull request do?

Gtk open file dialog shows folders in recent list. User can select folder and Gtk will return its path. Avalonia open file dialog should return only files. Fix is checking if Gtk path is real file.

What is the current behavior?

Gtk open file dialog returns folder path. Avalonia creates FileInfo from this path and returns IStorageFile to user. Then user can call IStorageFile.Open* methods or manually call any .NET file API and will get exception because path is folder.

What is the updated/expected behavior with this PR?

Avalonia checks if path is file and returns only files.

How was the solution implemented (if it's not obvious)?

Fix is checking if Gtk path is real file by File.Exists().

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053270-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

According to their documentation, GTK_FILE_CHOOSER_ACTION_OPEN should only allow picking files.
While I don't see issues with current File.Exist approach, are we sure something else is not broken? Just double checking.

[zii-dmg]

I don't see other problems with Gtk dialog. With select folder recent list contains only folders. With save file there is no recent list.

GTK_FILE_CHOOSER_ACTION_OPEN should only allow picking files

I think it's bug in Gtk. To reproduce user must select folder from recent list but not double click it. Then click "open" button. Double click will go to folder in dialog but not accept it.