RoslynLens

How It Works

Overview

RoslynLens is an MCP (Model Context Protocol) server that uses Roslyn to provide semantic code navigation. Instead of Claude Code reading entire .cs files (500-2000+ tokens each), it queries this server and receives focused results (30-150 tokens).

Startup sequence

1. MSBuildLocator.RegisterDefaults()     — locate MSBuild SDK
2. SolutionDiscovery.FindSolutionPath()  — BFS for .sln/.slnx
3. SolutionDiscovery.BfsDiscoverAll()    — collect all solutions
4. Host starts MCP stdio transport       — stdin/stdout
5. WorkspaceInitializer (background)     — load solution
6. Tools become available                — "loading" until Ready

When multiple solutions are discovered at step 3, the server logs a warning and stores the full list. The list_solutions and switch_solution tools allow runtime switching without restart.

Solution loading

WorkspaceManager wraps MSBuildWorkspace with:

  • Lazy compilation: For solutions with 50+ projects, compilations are created on-demand instead of upfront
  • LRU cache: Keeps the 50 most recently used compilations in memory (~250-750 MB)
  • File watcher: .cs changes trigger incremental text updates; .csproj changes trigger full project reload (with 200ms debounce)

Solution switching

ReloadSolutionAsync disposes the current workspace (watchers, compilation cache, MSBuild workspace) and loads a new solution. If the new solution fails to load, it attempts to rollback to the previous solution. State transitions:

Ready → Loading → Ready (success) or Error (both loads failed)

Tool execution

Each tool class is auto-discovered via [McpServerToolType] and registered by WithToolsFromAssembly(). When Claude Code calls a tool:

Claude Code → JSON-RPC (stdin) → MCP SDK → Tool class → Roslyn API → JSON-RPC (stdout)

Tools access the workspace via dependency-injected WorkspaceManager.

Anti-pattern detection

Detectors implement IAntiPatternDetector:

public interface IAntiPatternDetector
{
    IEnumerable<AntiPatternViolation> Detect(
        SyntaxTree tree,
        SemanticModel? model,
        CancellationToken cancellationToken);
}

Two modes:

  • Syntax-only (model = null): Fast, works without compilation. Most detectors use this mode (pattern matching on the syntax tree).
  • Semantic (model provided): Required for type-aware analysis (e.g., checking if a method returns Task, resolving interface implementations).

Logging

All logs go to stderr via LogToStandardErrorThreshold = LogLevel.Trace. Stdout is reserved exclusively for JSON-RPC messages (MCP protocol requirement).