C# Analyzer and Code Fix Developer

This skill provides comprehensive guidance for developing Roslyn analyzers and code fix providers in the OpenRewrite C# project.

Project Structure and Placement

File Organization

• •Analyzers: Place in src/Rewrite.RoslynRecipe/ project
• •Code Fix Providers: Place in src/Rewrite.RoslynRecipe/ project
• •Tests: Place in tests/Rewrite.RoslynRecipe.Tests/
• •Helper Utilities: Place in src/Rewrite.RoslynRecipe/Helpers/

Naming Conventions

• •Analyzers: [Category][Action]Analyzer.cs (e.g., NamingConventionAnalyzer.cs, ActionContextAccessorObsoleteAnalyzer.cs)
• •Code Fix Providers: [Category][Action]CodeFixProvider.cs (e.g., NamingConventionCodeFixProvider.cs)
• •Test Files: [AnalyzerName]Tests.cs and [CodeFixProviderName]Tests.cs
• •If analyzer targets an interface (such as IActionContextAccessor), the analyzer name should drop the I from the prefix as that clashes with naming convention identying interfaces - an analyzer is not an interface. Instead, it should name it like ActionContextAccessorObsoleteAnalyzer.
• •Analyzers shall define DiagnosticId starting with characters OR (meaning OpenRewrite developed recipes). The next set of characters shall identify the target framework or package that the diagnostic is targeting. For example for recipes that help identify / fix issues when migration .NET framework versions, it would include NETX. The X is the roman numeral of the version 10 of .NET implying that the recipes targets migration to version 10. Roman numerals are used here as to not clash with the last digits of diagnostic id which are sequentially imported. The last number are 4 digits incrementing number for that specific issue for the target version. Full example for analyzer targeting issues in .NET 10: ORNETX0001.

Analyzer Implementation Pattern

When targeting specific APIs, semantic analysis must ALWAYS be performed to determine if the targeted node correctly corresponds to fully qualified semantic target. Use helper method Rewrite.RoslynRecipes.Helpers.SemanticAnalysisUtil.IsSymbolOneOf for this purpose (public static bool IsSymbolOneOf(this SyntaxNode node, SemanticModel semanticModel, params IEnumerable<string> symbolNames)). This method operates on strings that are mapped to symbol.GetDocumentationCommentId() and will have one of the corresponding prefixes (ex: T:Microsoft.AspNetCore.Mvc.Infrastructure.IActionContextAccessor)

Code Fix Provider Implementation Pattern

1. •

   All editing of the syntax tree MUST be done using DocumentEditor, which provides a simplified and robust way to make multiple edits to the document and access to the original semantic model. Remember that semantic model is tied to the original syntax, so all decision need to be made on the original nodes, not any subsequent mutations.

2. •

   If the codefix only targets the immediate code marked by the analyzer, semantic confirmation is not required since it's already guaranteed to have been done in the analyzer. However, if the refactoring requires nodes outside of what was selected by the diagnostic (other areas of the document), semantic model should always be used to ensure accurate targeting.

3. •

   When introducing symbol usages from other namespaces that are not guaranteed to be in the document, they an annotation must be placed on the node to indicate which namespace(s) must be imported. Use WithRequiredNamespace extension method for this. Example

   csharp

   editor.ReplaceNode(memberAccess.Node, (current, gen) =>
   	SyntaxFactory.ParseExpression($"HttpContext?.GetRouteData()")
       .WithRequiredNamespace("Microsoft.AspNetCore.Routing")
       .FormatterAnnotated());
   

4. •

   We need to ensure new code is properly formatted but not try to format existing user code. To do this, whenever doing any operation on the DocumentEditor (ex. replace/insert), call .FormatterAnnotated() on the new node. After all edits are done, format only edited blocks via call like this when obtaining the final document:

editor.ReplaceNode(oldNode, (current, gen) =>
                        SyntaxFactory.ParseExpression($"HttpContext?.GetRouteData()")
                            .WithRequiredNamespace("Microsoft.AspNetCore.Http")
                            .FormatterAnnotated());
var changedDocument = await editor.GetChangedDocumentFormatted(cancellationToken);

1. •

   When removing nodes from the syntax tree, these operations MUST be done after all other edits are complete (exception formatting). This is to ensure that any edits do not try to operate on nodes that have been subsequently removed.

2. •

   When calling editor.ReplaceNode, use ONLY overload that uses a delegate for computeReplacement:

   code

   editor.ReplaceNode(memberAccess.Node, (current, gen) => MutateMyNode(current));
   // or
   editor.ReplaceNode(memberAccess.Node, (current, gen) => replacementNode);
   

Common refactoring scenarios and helper methods

Replace type in document

var actionContextAccessorType = semanticModel.Compilation.GetTypeByMetadataName("Microsoft.AspNetCore.Mvc.Infrastructure.IActionContextAccessor") 
                            ?? throw new InvalidOperationException($"Can't resolve type Microsoft.AspNetCore.Mvc.Infrastructure.IActionContextAccessor");
var httpContextAccessorType = semanticModel.Compilation.GetTypeByMetadataName("Microsoft.AspNetCore.Http.IHttpContextAccessor")
                              ?? throw new InvalidOperationException($"Can't resolve type Microsoft.AspNetCore.Http.IHttpContextAccessor");
editor.ReplaceType(semanticModel, actionContextAccessorType, httpContextAccessorType);

Rename symbol

Accurately renames a symbol using semantic model

var nodeSymbol = semanticModel.GetDeclaredSymbol(someNode, cancellationToken);
editor.RenameSymbol(semanticModel, nodeSymbol, "NewName");

Semantic type matching for syntax node

Ensures that symbol is of a particular semantic type. Ideally

var propertiesOfTypeActionDescriptor = containingType.DescendantNodes()
    .OfType<PropertyDeclarationSyntax>()
    .Where(x => x.IsSymbolOneOf(semanticModel, "P:Microsoft.AspNetCore.Mvc.ActionContext.ActionDescriptor"))
    .ToList();

Testing Requirements

1. Test Structure

Every analyzer and code fix MUST have comprehensive tests including:

For Analyzers:

• •Positive scenarios: Where diagnostics should be reported
• •Negative scenarios: Where diagnostics should NOT be reported

For Code Fixes:

• •Basic transformations: Simple cases
• •Syntax permutations: When multiple ways to write the same thing exists, test each permutation (ex. delegates may take form of a simple lambda expression, a block with multiple statements, or a method group)

2. Test Implementation Pattern

Analyzer Tests

using Microsoft.CodeAnalysis.Testing;
using Rewrite.RoslynRecipe.Tests.Verifiers;
using TUnit.Core;
using Verifier = Rewrite.RoslynRecipe.Tests.Verifiers.CSharpAnalyzerVerifier<
    Rewrite.RoslynRecipe.[Name]Analyzer>;

namespace Rewrite.RoslynRecipe.Tests
{
    public class [Name]AnalyzerTests
    {
        /// <summary>
        /// Tests that [specific scenario] triggers diagnostic.
        /// </summary>
        [Test]
        public async Task [Scenario]_TriggersDiagnostic()
        {
            const string source = """
                // Code with issue
                {|ORNETX[Number]:ProblemCode|}
                """;

            await Verifier.VerifyAnalyzerAsync(source,
                Assemblies.AspNet100 // Or appropriate reference assemblies
                    .AddPackage("PackageName", "Version"))
                .ConfigureAwait(false);
        }

        /// <summary>
        /// Tests that [valid scenario] does not trigger diagnostic.
        /// </summary>
        [Test]
        public async Task [ValidScenario]_NoDiagnostic()
        {
            const string source = """
                // Valid code
                ValidCode
                """;

            await Verifier.VerifyAnalyzerAsync(source,
                Assemblies.AspNet100)
                .ConfigureAwait(false);
        }
    }
}

Code Fix Tests

using Microsoft.CodeAnalysis.Testing;
using Rewrite.RoslynRecipe.Tests.Verifiers;
using TUnit.Core;
using Verifier = Rewrite.RoslynRecipe.Tests.Verifiers.CSharpCodeFixVerifier<
    Rewrite.RoslynRecipe.[Name]Analyzer,
    Rewrite.RoslynRecipe.[Name]CodeFixProvider>;

namespace Rewrite.RoslynRecipe.Tests
{
    public class [Name]CodeFixTests
    {
        /// <summary>
        /// [Tests that [specific transformation] is correctly applied.]
        /// </summary>
        [Test]
        public async Task [Scenario]_CorrectlyFixed()
        {
            const string source = """
                // Code before fix
                {|ORNETX[Number]:ProblemCode|}
                """;

            const string fixedSource = """
                // Code after fix
                FixedCode
                """;

            await Verifier.VerifyCodeFixAsync(source, fixedSource,
                Assemblies.AspNet100
                    .AddPackage("PackageName", "Version"))
                .ConfigureAwait(false);
        }
    }
}

• •Note that if the package comes from a Microsoft package that is part of the same version scheme as the runtime, explicit version number can be ommited. For example Assemblies.Net100.AddPackage("Microsoft.Extensions.Options") the version will be set to 10.0.0 automatically and should be omitted. However, if the package version is does not follow the same version train as the target framework, then explicit version should be specified like this: Assemblies.Net90.AddPackage("Microsoft.AspNetCore.HttpOverrides","2.3.9"). If you see an error in tests that a particular package is not available when using version-less overload, find the latest version of the package on nuget.org (ex: https://www.nuget.org/packages/microsoft.aspnetcore.httpoverrides) and use explicit version number when calling AddPackage
• •Test code should target code and packages in the BEFORE state. For example if the recipe is intended to migrate code from .net 9 to 10, use Net90 reference assemblies.

3. Special Test Cases for Delegates

When refactoring delegates, test ALL variations:

• •Expression lambdas: x => x.Method()
• •Statement lambdas: x => { return x.Method(); }
• •Method groups: obj.Method
• •Async variations: async x => await x.Method()
• •Non-async Task returns: x => Task.FromResult(x.Method())

4. Use Minimal Code in Tests

Keep test code concise:

// GOOD: Minimal, focused test
const string source = """
    using Microsoft.AspNetCore.Builder;

    var app = WebApplication.Create();
    app.MapGet("/", () => "test")
       .{|ORNETX0010:ProblemMethod|}();
    """;

// BAD: Unnecessary complexity
const string source = """
    using System;
    using System.Collections.Generic;
    using Microsoft.AspNetCore.Builder;
    using Microsoft.AspNetCore.Hosting;

    namespace MyApp
    {
        public class Startup
        {
            public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
            {
                if (env.IsDevelopment())
                {
                    app.UseDeveloperExceptionPage();
                }

                app.UseRouting();
                app.UseEndpoints(endpoints =>
                {
                    endpoints.MapGet("/", () => "test")
                        .{|ORNETX0010:ProblemMethod|}();
                });
            }
        }
    }
    """;

• •Unless explicitly being tested for as part of the testcase, use top level statements when doing entrypoint code

1. •All test case code must be written under assumption that nullable types are enabled, meaning that all null checks must be performed and all non nullable types must be initialized. Analyzer base classes ensure that the code is fully compilable - NEVER alter these rules. If there's a specific warning / error that shows up by analyzer related to the test code that may need to be added to an exclusion list, stop and prompt user on what to do.

XML Documentation Requirements

ALL public APIs MUST include complete XML documentation:

/// <summary>
/// Analyzes C# code for [specific issue] and reports diagnostics.
/// </summary>
/// <remarks>
/// This analyzer detects [detailed explanation of what it finds].
/// </remarks>
public class [Name]Analyzer : DiagnosticAnalyzer
{
    /// <summary>
    /// Registers analysis actions for [specific syntax kinds].
    /// </summary>
    /// <param name="context">The analysis context to register actions with. Must not be null.</param>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="context"/> is null.</exception>
    public override void Initialize(AnalysisContext context)
    {
        // Implementation
    }
}

When creating analyzer / codefix from a URL article, link URL must appear in the xml comments

Checklist for New Analyzer/Code Fix

• • Analyzer placed in src/Rewrite.RoslynRecipe/
• • Code fix provider in same project
• • Follows naming conventions
• • Uses semantic model when targeting types and their members
• • Uses IsSymbolOneOf extension method which uses symbol.GetDocumentationCommentId to determine symbol match
• • Comprehensive test suite created
• • Tests cover positive and negative cases
• • Tests use minimal code samples
• • XML documentation on all public members
• • Using directives managed with UsingsUtil
• • New syntax formatted with Formatter.Annotation
• • DiscardFormatting() called on parsed syntax
• • Document is formatted after transformation
• • Test for all delegate variations (if applicable)
• • Code compiles and all tests pass