.NET Compiler Platform ("Roslyn") Analyzers VS Roslynator

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The other comment here is probably a good start. There may be some guidance over at https://github.com/dotnet/roslyn-analyzers on how to write high-performance analyzers. But your code is simple enough that I'm not sure it's a huge concern.

Moreover, in the Roslyn Analyzers issue tracker, was created the task of developing a new diagnostic that would detect such cases.

Microsoft.CodeAnalysis.PublicApiAnalyzers: Helps library authors monitor changes to their public APIs (more info).

Like the other person said, one way is to look at code analyzers, e.g. https://github.com/dotnet/roslyn-analyzers/blob/a0522afd52b25ad211298e308fd5bbc7734865e0/src/PerformanceSensitiveAnalyzers/CSharp/CodeFixes/AvoidAllocationWithArrayEmptyCodeFix.cs

There's also this project https://github.com/JosefPihrt/Roslynator that can add even more analyzers.

It does. Nearly every style guide I've seen for any language that allows this prefers this style. I personally prefer it when such structures are needed, but would suggest following it just for consistency and the practical benefits.

Roslynator.Analyzers

The reason I say 'similar' above is that though you may treat it as production code, there are some things in the nature of writing tests that may require different coding standard to be enforced. A good example of this is the RCS1046 analyzer, enforcing names of asynchronous methods to be suffixed with 'Async'; in this case it would be acceptable to override this rule (with an .editorconfig for example) so that you're method name continues to describe the behaviour of the application, and we don't have tests named like public async Task My_descriptive_method_name_async().

You can write Roslyn analyzer but you can also simply use this set of analyzers: https://github.com/JosefPihrt/Roslynator. It has analyzers for both async method not ending with Async and non-async methods ending with Async.

My suggestion is to look into installing Roslynator or Sonarlint. Both are free and work with either Visual Studio or VS Code. With them you can right click on your solution and Run Analysis and it will give you a list of "code smells" and automatic fixes for many of them. They are great tools for any dev.

Roslynator while not exactly as good as full R# does a pretty good job for the analysis and refactoring hints imo. Paired with Intellicode + NCrunch (paid) so far has been a good enough experience for us.

I prefer the second and roslynator would give RCS1124: Inline local variable1 for the first one.

Roslynator.Analyzers