Fix [ReadOnly]Span2D<T>.TryGetSpan on legacy frameworks

[Sergio0694]

Fixes #3947

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

[ReadOnly]Span2D<T>.TryGetSpan throws an exception when wrapping a T[] instance on legacy frameworks.

What is the new behavior?

[ReadOnly]Span2D<T>.TryGetSpan works correctly on legacy frameworks.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• 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

[ghost]

Thanks Sergio0694 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 🙌

[michael-hawker]

@Sergio0694 was the original code done for a performance thing, does that impact that?

[Sergio0694]

@michael-hawker Not sure I understand the question, could you clarify what you mean? Do you mean why do we have the TryGetSpan API at all, or why the line I changed was written that way instead of some other way (if so, how?), or something else?

As for the performance difference of this PR in general though, the new version with the fix is comparatively just as fast as the other one (note that that IndexOf method I'm calling is still O(1), it just calculates the index by calculating the pointer offset delta and dividing by element size, it's not actually enumerating anything), plus this isn't really a hot path anyway 😄

In particular, that IndexOf method being called is this extension below (not the IndexOf from LINQ or anything like that):

https://github.com/windows-toolkit/WindowsCommunityToolkit/blob/c31e049bc6b3120364abf1d8a6f3967e6e9eb927/Microsoft.Toolkit.HighPerformance/Extensions/SpanExtensions.cs#L161-L174

Does that help? 🙂

[michael-hawker]

Yeah, was just asking about the change in syntax and if it had an impact one the generation/code execution time, as I wasn't sure if the original code was optimized for that or not. So if it's all the same and you've done the analysis, that's all I wanted to know. 🙂 (I assumed you had, as always, but do like to check sometimes... 😋)

[ghost]

Hello @michael-hawker!

Because this pull request has the auto merge :zap: label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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