Roslyn Analyzer : Analyzing Comments

One of the things one might encounter quite early while writing a Roslyn Code analyzer for comments is it is slightly different from detection of Syntax Nodes. In fact, it is not a node at all.

Syntax nodes represent syntactic constructs, for example declaration, statements, clauses and expressions. A Comment do not quite fall in the same category. It is rather a Syntax Trivia. Syntax Trivia are, as Microsoft states, largely non-significant components such as whitespaces, comments and preprocessor directives. Due to their rather insignificant presence, they are not included as child nodes of Syntax Tree. However, since they are still important in their own ways, they are still part of the Syntax Tree.

For the same reason, you would need to register a SyntaxTreeAction using RegisterSyntaxTreeAction rather SyntaxNodeAction using RegisterSyntaxNodeAction. For analyzing comments, our typical Initialize() method would like the following.

public override void Initialize(AnalysisContext context)
    context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.Analyze | GeneratedCodeAnalysisFlags.None);


The next step involves parsing the SingleLineCommentTrivia and MultiLineCommentTrivia trivias from the syntax tree. You could achieve this with a simple Linq Query.

private void AnalyzeComment(SyntaxTreeAnalysisContext context)
    SyntaxNode root = context.Tree.GetCompilationUnitRoot(context.CancellationToken);
    var commentTrivias = root.DescendantTrivia()
                            .Where(x => x.IsKind(SyntaxKind.SingleLineCommentTrivia) || x.IsKind(SyntaxKind.MultiLineCommentTrivia));

    // Rest of the code

That’s all you need. Didn’t that turn up quite easy. I wish there was a easy to parse the actual comment out of Trivia, however, unfortunately, I haven’t found one yet (not sure if one exist). At the moment, the most likely way to use the ToString() method. This would in fact, include comment characters as well, which you can parse out using Regex or simple string transformations.