Adding a Detector

1. Create the detector

Create src/RoslynLens/Analyzers/MyDetector.cs:

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

namespace RoslynLens.Analyzers;

public class MyDetector : IAntiPatternDetector
{
    public IEnumerable<AntiPatternViolation> Detect(
        SyntaxTree tree,
        SemanticModel? model,
        CancellationToken cancellationToken)
    {
        var root = tree.GetRoot(cancellationToken);

        foreach (var node in root.DescendantNodes().OfType<InvocationExpressionSyntax>())
        {
            if (node.ToString().Contains("BadPattern"))
            {
                var line = node.GetLocation().GetLineSpan();
                yield return new AntiPatternViolation(
                    Id: "MY-001",
                    Message: "Avoid BadPattern — use GoodPattern instead",
                    FilePath: tree.FilePath,
                    Line: line.StartLinePosition.Line + 1,
                    Severity: AntiPatternSeverity.Warning);
            }
        }
    }
}

Conventions

• ID format: APxxx for general .NET, GR-* for domain-specific
• Handle null SemanticModel: If your detector needs semantic analysis, check if (model is null) yield break; at the top
• Use CancellationToken: Pass it to GetRoot() and check it in long loops

2. Create the test

Create tests/RoslynLens.Tests/Analyzers/MyDetectorTests.cs:

using RoslynLens.Analyzers;
using Microsoft.CodeAnalysis.CSharp;
using Shouldly;

namespace RoslynLens.Tests.Analyzers;

public class MyDetectorTests
{
    private readonly MyDetector _detector = new();

    [Fact]
    public void Detects_BadPattern()
    {
        const string source = """
            public class Foo
            {
                public void Run() { BadPattern(); }
            }
            """;

        var tree = CSharpSyntaxTree.ParseText(source);
        var violations = _detector.Detect(tree, null, TestContext.Current.CancellationToken).ToList();
        violations.ShouldContain(v => v.Id == "MY-001");
    }

    [Fact]
    public void Ignores_GoodPattern()
    {
        const string source = """
            public class Foo
            {
                public void Run() { GoodPattern(); }
            }
            """;

        var tree = CSharpSyntaxTree.ParseText(source);
        var violations = _detector.Detect(tree, null, TestContext.Current.CancellationToken).ToList();
        violations.ShouldBeEmpty();
    }
}

Test conventions

• Use TestContext.Current.CancellationToken (xUnit v3 requirement)
• Use CSharpSyntaxTree.ParseText() — no full compilation needed for syntax detectors
• Test both positive (detects the bad pattern) and negative (ignores good code) cases

3. Verify

dotnet build
dotnet test

The detector is automatically picked up by DetectAntiPatternsTool via reflection — no registration needed.