TaskResultConverter returns default value when task not set

[michael-hawker]

Fixes #4505

Adds better testing around real-world x:Bind and converter scenarios for TaskResultConverter, also related to the nature of how this interacts with the MVVM Toolkit behavior (which would mimic most real-world app behavior as well).

Fixes the converter to better handle a wider variety of scenarios.

PR Type

What kind of change does this PR introduce?

• Bugfix

• Documentation content changes

What is the current behavior?

The TaskResultConverter will return UnsetValue when the expected Task has not been initialized yet (and is null).

The documentation specified the default value would be returned, but this was not true for value types.

What is the new behavior?

TaskResultConverter now will return the default value (null or otherwise for value types) whether a task is set or not.

It'll also passthru non-task values as a no-op.

PR Checklist

Please check if your PR fulfills the following requirements:

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• Tested code with current supported SDKs
• New component
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
  • If control, added to Visual Studio Design project
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• New major technical changes in the toolkit have or will be added to the Wiki e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information

See more details in accompanying issue.

[ghost]

Thanks michael-hawker for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[Arlodotexe]

Couldn't find any issues with it, and checking around this seems to be the best approach. Approved!

[michael-hawker]

@Sergio0694 had concerns on reflection bit for value type support. So instead updated the comment to reflect actual behavior (returning null) and took middle-ground approach for hotfix release.

Part of the discrepancy/confusion here is also due to the fact the base methods we use have slightly different behavior between the generic and non-generic versions from the TaskExtensions, so we may want to coordinate a resolution there if this doesn't solve the majority of common scenarios for folks. See: https://github.com/CommunityToolkit/dotnet/blob/main/CommunityToolkit.Common/Extensions/TaskExtensions.cs

[Sergio0694]

Ship it!

[ghost]

Hello @michael-hawker!

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