AD0001: NullReferenceException

[csuzw]

I'm trying to create a code analyzer for a project. I've been following various guides I've found online but am now completely stuck. Whatever I do I get an AD0001: Analyzer 'Infuriating.CodeAnalysis.MyRule' threw an exception of type 'System.NullReferenceException' with message 'Object reference not set to an instance of an object.' error. I have created a minimal reproduction of this error here: https://github.com/csuzw/curly-doodle . I'm assuming I'm doing something wrong rather than this being a bug but I have no idea how to go about fixing it or getting more information. Can anyone help? Thanks.

[RikkiGibson]

What OS/IDE are you using for development? For Windows+Visual Studio, the default analyzer template should include a project suffixed ".Vsix". If you "Debug>start new instance" on that, it should launch+attach debugger to a second instance of VS which has your analyzer loaded as an extension. Then, as long as you have the debugger set up to break on the particular exception type (which it should by default), you can just navigate to some code which was crashing your analyzer, and have the debugger break, and you'll be able to inspect what is going on.

Ideally, if you have some suspicion that a particular coding pattern is crashing the analyzer, you would create a unit test for the scenario and repro/fix that way. That would probably be somewhat easier to do. See Analyzers.Test for samples of that.

[csuzw]

Windows/VS2022. I'm not sure what analyzer template you're talking about as I don't see any options for that when creating a new project.

If you look at the example I linked, the analyzer does almost nothing and in fact you can comment out the code that is there and it still throws the exception so this is not a problem with the analyzer itself or the project that it's analyzing.

[RikkiGibson]

You may be missing a VS optional component which is recommended in this scenario: https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/tutorials/how-to-write-csharp-analyzer-code-fix . I recommend you at least take a look at the template and extract out bits that look useful, as it is intended to address these basic quality of life issues for analyzer development.

Glancing at your code the following looks broken to me:
https://github.com/csuzw/curly-doodle/blob/d892f633b85cd07e1b3b09bd0460742acd324b3f/Infuriating.CodeAnalysis/MyRule.cs#L21

Unfortunately ImmutableArray due to being a struct has some weird ergonomics. Your code is creating a new ImmutableArray<DiagnosticDescriptor>(), which is equivalent to default(ImmutableArray<DiagnosticDescriptor>). Calling Add on that through collection-initializer will throw NRE. This is something we really should have an analyzer for..

Instead, you need to use ImmutableArray.Create(item) or just [item] if your language version supports it.

[csuzw]

Aha ImmutableArray.Create(item) appears to be the way - this seems to have got rid of the error. [item] doesn't work as it needs a later language version than seems to be possible for .net standard 2.0 (which as far as I'm aware you're stuck with?).

[RikkiGibson]

You're right, netstandard2.0 is needed to run the analyzer everywhere the compiler runs, and there's no official support for recent language versions in that case.