Guard APIs for .NET Standard 2.1

[Sergio0694]

Follow up to #3131 (separate PR to keep things simpler).

PR Type

What kind of change does this PR introduce?

• Feature

What is the current behavior?

The Guard APIs are in the Microsoft.Toolkit project, that currently only targets .NET Standard 2.0. This makes it necessary to always reference the System.Memory package, and it makes it impossible to decorate APIs with the new C# 8 nullability/flow attributes from .NET Core 3.0. This also results in a worse coding experience, as the code analysis tool don't have all the info it needs when invoking some calls (eg. Guard.IsNotNull won't notify it that the input variable will never be null afterwards).

What is the new behavior?

This PR makes a few changes, which don't alter the functionality at all or the API surface area. They're just changes in the project structure and code decorations. In particular:

• Added .NET Standard 2.1 as an additional target for Microsoft.Toolkit
• Made the System.Memory package only necessary when targeting .NET Standard 2.0. On .NET Standard 2.1, only the System.Runtime.CompilerServices.Unsafe package is referenced.
• Added attributes (eg. [NotNull] and [DoesNotReturnWhen]) on some of the Guard APIs

Here's a screenshot showing the difference in the code analysis info when using the .NET Standard 2.0 and 2.1 versions of the same Guard.IsNotNull API (with the new attributes):

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](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)
• 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

[Sergio0694]

@michael-hawker Thank you for the guidance with the merge, it worked! 😄🎉

[Sergio0694]

@michael-hawker Added those overloads for Guard.IsTrue and Guard.IsFalse with a custom message in b67e5b8 as we discussed, and did some last minute refactoring/tweaks following the suggestions from the review in #3273. This PR should be good to go now! 😊

[michael-hawker]

@azchohfi I can't find your old comment, we good?

[Sergio0694]

@michael-hawker All resolved.
Previous review threads for future reference:

• .NET Standard 2.1 target question
• Setup for .NET Core unit test project
• Last fix for CI errors

Outdated, Comments Addressed