🧩 codedotnet

A browser-based C# playground — write, compile, and run C# entirely client-side using the .NET 10 WebAssembly runtime and Roslyn. No server. No account. No install.

🚀 Live demo →

🔌 Also includes an MCP Playground — a Model Context Protocol server + in-browser client that exposes compile_and_run as a tool for VS Code, Claude Desktop, and any other MCP-aware client.

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✨ What it does

📝	Monaco-powered editor for a single Program.cs file with C# syntax highlighting, inline diagnostics, and error navigation.
▶️	Click Run to compile and execute your code inside a dedicated Web Worker running Roslyn and the .NET WebAssembly runtime, entirely in your browser.
⌨️	Supply buffered STDIN before running, and see captured stdout/stderr, exceptions, and timing breakdowns (compile / execute / total) in the output pane.
📊	Automatic time/space Big-O complexity estimate (with rationale) for the code you just ran.
🧱	Resizable, swappable editor/I-O panes, focus modes, and fullscreen support.
💾	Editor preferences, theme (light/dark/system), and layout are persisted locally so your workspace is restored next time you visit.
⚡	Keyboard shortcuts: Ctrl/Cmd+Enter to run, Shift+F5 to stop, Ctrl/Cmd+S to force-save locally, F11 to toggle fullscreen, plus Monaco's native Find/Replace and Undo/Redo.
🛑	Graceful recovery from runaway programs (e.g. infinite loops): Stop hard-terminates the worker and a fresh one takes its place.
🌐	A clear "unsupported browser" message if required capabilities (WebAssembly, Web Workers, Blob URLs, local storage) are missing, instead of a silent failure.

🔌 MCP Playground

codedotnet also ships a Model Context Protocol (MCP) server (codedotnet-sandbox/) that exposes the exact same compile_and_run capability as a tool any MCP-aware client (VS Code, Claude Desktop, Cursor, Continue, the browser, or a custom JSON-RPC-over-stdio/WebSocket script) can call — reusing the same CodeDotNet.Engine-backed WebAssembly artifact as the main playground, but isolated per-call in a disposable, hard-timeout-enforced child process.

The MCP Playground tab in the web app is an in-browser client for this server over a WebSocket transport (ws://127.0.0.1:7900 by default):

🔗	Connect to any MCP WebSocket endpoint (defaults to the local sandbox) and see its Connected/Disconnected status live.
🧰	Discovered tools panel lists every tool the server advertises via tools/list, with its description.
📝	Edit a single-file C# program in the same Monaco-powered editor used by the main playground.
▶️	Call compile_and_run invokes the tool over JSON-RPC 2.0 and renders the parsed result: status (e.g. ExecutionCompleted, ExecutionCompletedWithWarnings), captured stdout, diagnostics/warnings with file/line/column, and a build / run / total timing breakdown.
🔍	A collapsible Raw JSON-RPC tools/call response section shows the exact wire payload for debugging or integrating your own MCP client.

⚠️ Note: The live demo is a static GitHub Pages site and cannot run the MCP WebSocket sandbox server, so the MCP Playground tab will show Disconnected there. The sandbox (codedotnet-sandbox/) is a Node.js process that forks a pool of disposable child processes — each cold-booting the .NET WebAssembly runtime (~1.5-2s) — to isolate and hard-timeout every compile_and_run call. That's intentionally heavy for a hosted/serverless environment, so it is not deployed; you must run it locally (see below) to get a live MCP connection.

To try it locally:

# Terminal 1: start the MCP WebSocket server (ws://127.0.0.1:7900)
cd codedotnet-sandbox
npm install
npm run serve

# Terminal 2: start the web app
cd codedotnet.web
npm install
npm run dev

Requirements: Node.js 22+ for both processes, and the .NET 10 SDK to publish the codedotnet.compiler browser-wasm artifact the sandbox reuses (one-time step: dotnet publish -c Release codedotnet.compiler/codedotnet.compiler.csproj, or run scripts/copy-worker-assets.ps1).

Open the printed dev server URL, switch to the MCP Playground tab, click Connect, and run the sample program. See codedotnet-sandbox/README.md for the stdio transport, worker-pool isolation model, and instructions for registering the server with VS Code, Claude Desktop, Cursor, Continue, or any other MCP-aware/JSON-RPC client.

🚦 Quick start

Prerequisites: .NET 10 SDK and Node.js 22+.

# Frontend (dev server with hot reload)
cd codedotnet.web
npm install
npm run dev

# Publish the compiler worker once so the frontend can load it locally
npm run build:worker

Then open the printed local URL in your browser.

To produce a production build (as used for GitHub Pages deployment):

cd codedotnet.web
npm run build:worker:release
npm run build

🗂️ Repository layout

Path	Purpose
🖥️ codedotnet.web/	React + TypeScript + Vite frontend (editor, shell, worker manager, complexity estimator).
⚙️ codedotnet.compiler/	.NET 10 browser-wasm project: Roslyn compile host, execution host, JS interop.
✅ codedotnet.compiler.tests/	xUnit tests for the compiler/execution host logic (linked source, runs natively).
🧰 codedotnet-sandbox/	Node.js MCP server: stdio + WebSocket transports for a sandboxed compile_and_run tool.
🤖 .github/workflows/deploy.yml	CI validation + GitHub Pages deployment workflow.
📦 .github/dependabot.yml	Automated weekly dependency updates (npm, NuGet, GitHub Actions).
📚 docs/	Architecture, testing, deployment, security, privacy, limitations, and ADRs.

📖 Documentation

• 🏗️ docs/architecture.md — system architecture and component responsibilities.
• 🧪 docs/testing.md — how to run and what each test suite covers.
• 🚀 docs/deployment.md — CI/CD and GitHub Pages deployment flow.
• 🌍 docs/custom-domains.md — DNS/CNAME guidance for custom domains.
• 🔒 docs/security.md — Content-Security-Policy, sandboxing model, dependency scanning.
• 🕵️ docs/privacy.md — what happens to your code, and how to verify it yourself.
• ⚠️ docs/known-limitations.md — buffered STDIN, single-file constraint, no NuGet restore, browser resource limits.
• 📝 docs/adr/ — Architecture Decision Records (ADR-001 through ADR-008).
• 🧩 codedotnet-sandbox/README.md — MCP server: architecture, isolation model, tool contract, and MCP client registration.
• ©️ THIRD-PARTY-NOTICES.md — third-party license notices.

📜 License

MIT — see LICENSE.