roslyn-csharp

claude-roslyn-lsp

A Claude Code plugin that brings full C# code intelligence via Microsoft's Roslyn language server (Microsoft.CodeAnalysis.LanguageServer) — the same engine powering the VS Code C# extension — instead of the lighter-weight csharp-ls.

The plugin ships a small stdio proxy that solves the one thing preventing Roslyn from working under a generic LSP client. The server itself is installed separately as a standard dotnet global tool.

Why Roslyn

Roslyn is the most accurate C# language server and handles file churn (edits, deletes, renames) far better than csharp-ls — no stale "predefined type System.Object is not defined" diagnostic storms after bulk changes.

The catch: Roslyn does not load a workspace from the standard LSP initialize/rootUri. It waits for the editor's proprietary solution/open notification (or project/open). A generic LSP client never sends those, so a directly-wired Roslyn server loads zero projects and returns empty results — actually worse than csharp-ls.

This proxy fixes that by injecting solution/open automatically. Measured against the bundled two-project fixture:

| Query | Without solution/open | With solution/open | | --- | --- | --- | | workspace/symbol "Greeter" | 0 | 1 | | references (cross-project) | 1 (same file only) | 3 |

How it works

Claude Code ──stdio──▶ roslyn-lsp-proxy.js ──stdio──▶ roslyn-language-server
                              │
                              └─ after the client's `initialized`, injects
                                 `solution/open` for the workspace's .sln
                                 (falls back to `project/open` over .csproj)

The proxy forwards all LSP traffic verbatim and injects solution/open right after initialized. Logs go to stderr and <tmp>/roslyn-lsp-proxy-logs/ only — never stdout, which is the LSP channel.

Requirements

• Node.js ≥ 18 (the proxy uses only built-ins — no npm install).
• dotnet ≥ 9 on PATH, used to install and run the language server.
• The roslyn-language-server dotnet global tool (see Install below).

Install

1. Install the language server

dotnet tool install -g roslyn-language-server --prerelease \
  --add-source https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-impl/nuget/v3/index.json

This installs the same build the VS Code C# extension uses. To update it later:

dotnet tool update -g roslyn-language-server --prerelease \
  --add-source https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-impl/nuget/v3/index.json

2. Install the plugin

/plugin marketplace add danielpivonka/claude-roslyn-lsp
/plugin install roslyn-csharp@claude-roslyn-lsp
/plugin disable csharp-lsp@claude-plugins-official   # avoid two servers on .cs files
/reload-plugins

To install from a local clone instead: /plugin marketplace add /absolute/path/to/clone

The first C# query triggers a one-time solution load (tens of seconds on a large solution); after that it's incremental.

Testing

A self-contained integration test drives the proxy against a two-project fixture (App → Lib) and asserts that cross-project semantics resolve correctly:

node test/run-test.js

Prints PASS/FAIL and exits non-zero on failure. Requires node, dotnet, and roslyn-language-server installed as a global tool (see Install above).

Layout

.
├── .claude-plugin/marketplace.json     # marketplace catalog entry
├── plugins/roslyn-csharp/
│   ├── .claude-plugin/plugin.json      # plugin manifest + LSP server declaration
│   └── bin/roslyn-lsp-proxy.js         # the proxy (solution/open injection)
└── test/
    ├── fixture/                        # minimal App→Lib C# solution
    └── run-test.js                     # integration test

Troubleshooting

• roslyn-language-server not found. The dotnet global tool isn't installed or ~/.dotnet/tools isn't on your PATH. Run the install command from the Install section above, then ensure ~/.dotnet/tools is in your shell's PATH.
• Empty results / no symbols. Check <tmp>/roslyn-lsp-proxy-logs/ and the proxy's stderr for injecting solution/open:. If there's no "injecting" line, no .sln or .csproj was found under the client's rootUri.
• Server exits immediately. Your .NET runtime may not meet the tool's requirements. Run dotnet --version and compare against the installed tool version.
• ${CLAUDE_PLUGIN_ROOT} not expanded. If your Claude Code build doesn't expand it for LSP args, edit plugins/roslyn-csharp/.claude-plugin/plugin.json and replace it with the absolute path to plugins/roslyn-csharp.

Licensing

MIT — see LICENSE. The roslyn-language-server tool installed separately is part of dotnet/roslyn and is also MIT-licensed.