Update guidance for C# 8/9/10

[bkoelman]

I've reviewed the language changes in C# 8, 9 and 10 in an attempt to update CSharpGuidelines.

I'm reluctant to add rules that recommend using the various syntax enhancements in recent C# versions. These are already being suggested by Rider/Resharper (or Visual Studio itself) and can be configured to produce warnings. While it's good for teams to standardize on such syntax, there's little point in spelling them out here. Amongst these are:

• Pattern matching enhancements
• Switch statements vs switch expressions
• Using declarations (example: using var stream = ... vs nested statements)
• Asynchronous disposable (await using)
• Indices and ranges
• Init-only property setters (example: public int Name { get; init; })
• Target typed new (example: List<string> items = new())
• Target type resolution of conditional expressions (example: var account = isAdmin ? new AdminAccount() : new UserAccount();)
• Global using directives
• File-scoped namespaces
• Deconstruction enhancements (example: int x = 0; (x, int y) = point;)
• Static local functions and lambda expressions

Then there's a category of features that I suspect most application developers aren't likely to care much about:

• Struct enhancements: ref structs, disposable ref structs, readonly structs, readonly struct members
• Enhancements for unmanaged generic constraint
• Nested stackalloc
• Native sized integers (nint and nuint)
• Covariant return types
• Source generator support (non-void partial methods and module initializers)
• AsyncMethodBuilder attribute (for custom async/await handlers)
• CallerArgumentExpression attribute
• Enhanced #line pragma (used by debuggers)

I've skipped nullable reference types, as there's already great guidance and tooling for that.

Then there's record types, which I haven't used yet, so nothing to report there from me.

What's left are features that I wouldn't know any guidance to provide for:

• Default interface methods
• Asynchronous streams (allows to yield return in methods that return IAsyncEnumerable<T>)
• Top-level statements
• Attributes on local functions and lambda expressions
• Natural (return) types in lambda expressions (used in ASP.NET Core Minimal APIs)

Based on the above, I've created https://github.com/dennisdoomen/CSharpGuidelines/pull/241 and https://github.com/dennisdoomen/CSharpGuidelines/pull/244, which concludes my investigation.

Once these are merged, I think we can extend the supported version range to C# 10.

Feedback and suggestions welcome!

[bkoelman]

I noticed https://github.com/dennisdoomen/CSharpGuidelines/commit/34949fae383678e2a6bea80b30105e034a4f453e, which increments to C# 9, but I believe it should be C# 10, which was released last year.

[dennisdoomen]

I knew I should have used a PR for that ;-)

[dennisdoomen]

Is there anything else you want to do with this issue?

[bkoelman]

I'm okay with closing this, but perhaps keep it open a little longer to allow community members to chime in?

[bkoelman]

CLosing due to inactivity.

@dennisdoomen can you create a new release, so I can align CSharpGuidelinesAnalyzer with the latest changes?