C# Roslyn Extension for Zed

A Zed extension for C# with:

• Roslyn language server as the preferred backend
• OmniSharp-Roslyn as a compatibility fallback
• netcoredbg for debugging

Current backend strategy

This extension supports both language servers:

• Roslyn (roslyn-language-server) is preferred when a .NET 10+ runtime is available.
• OmniSharp-Roslyn is selected automatically for Unity projects or when .NET 10+ is not detected.
• A configured custom Roslyn binary is always honored.

omnisharp in settings refers to OmniSharp-Roslyn (OmniSharp/omnisharp-roslyn), which is Roslyn-based internally but different from roslyn-language-server.

Features

• Completion, hover, go to definition, find references, rename
• Diagnostics and analyzers
• Roslyn workspace configuration support
• Unity-oriented OmniSharp-Roslyn fallback
• Debugging via netcoredbg
• Automatic download of Roslyn/OmniSharp-Roslyn/netcoredbg when needed (platform-aware source selection)
• Support for custom language server binaries in Zed settings

Semantic highlighting

Zed now supports semantic tokens. For C#, the main requirement is that the active language server advertises semantic token support and that Zed has semantic tokens enabled.

This extension also ships default semantic token mappings for Roslyn custom token types such as regex, JSON, XML doc, punctuation, verbatim strings, and extension methods.

Recommended setting:

{
  "languages": {
    "CSharp": {
      "semantic_tokens": "full"
    }
  }
}

Use "combined" if you want tree-sitter highlighting with semantic overlays.

Notes:

• Roslyn is the preferred backend for semantic highlighting work.
• OmniSharp-Roslyn remains a compatibility path and may not match roslyn-language-server behavior.
• If highlighting does not update after changing this setting, run lsp: restart language servers.

Installation

This extension is intended for local development installation.

1. Clone this repository
2. Open Zed
3. Run zed: extensions: install dev extension
4. Select the cloned csharp_roslyn directory
5. Restart Zed

Configuration

Select language server priority

You can configure language-server priority for C#:

{
  "languages": {
    "CSharp": {
      "language_servers": ["roslyn", "!omnisharp", "..."]
    }
  }
}

To prefer OmniSharp-Roslyn explicitly:

{
  "languages": {
    "CSharp": {
      "language_servers": ["omnisharp", "!roslyn", "..."]
    }
  }
}

Even when roslyn is preferred, this extension may still fall back to OmniSharp-Roslyn automatically when Roslyn is not usable on the host.

Roslyn settings

Roslyn configuration goes under lsp.roslyn.settings:

{
  "lsp": {
    "roslyn": {
      "settings": {
        "csharp|completion": {
          "dotnet_show_completion_items_from_unimported_namespaces": true
        },
        "csharp|navigation": {
          "dotnet_navigate_to_decompiled_sources": true,
          "dotnet_navigate_to_source_link_and_embedded_sources": true
        },
        "csharp|inlay_hints": {
          "dotnet_enable_inlay_hints_for_parameters": true,
          "csharp_enable_inlay_hints_for_types": true
        }
      }
    }
  }
}

Custom Roslyn binary

If you installed roslyn-language-server yourself, you can point Zed to it:

{
  "lsp": {
    "roslyn": {
      "binary": {
        "path": "/path/to/roslyn-language-server",
        "arguments": ["--stdio", "--autoLoadProjects"]
      }
    }
  }
}

This is useful if you want to manage the Roslyn server outside the extension.

OmniSharp-Roslyn settings

OmniSharp-Roslyn configuration goes under lsp.omnisharp.

To set a solution path:

{
  "lsp": {
    "omnisharp": {
      "initialization_options": {
        "solution": "/absolute/path/to/your/solution.sln"
      }
    }
  }
}

To use a custom OmniSharp-Roslyn binary:

{
  "lsp": {
    "omnisharp": {
      "binary": {
        "path": "/path/to/OmniSharp",
        "arguments": ["-lsp"]
      }
    }
  }
}

Unity projects

Unity currently routes through OmniSharp-Roslyn.

See UNITY-SUPPORT.md for details.

Debugging

The extension provides netcoredbg support.

Notes:

• Linux, macOS x64, and Windows use official Samsung netcoredbg releases.
• macOS arm64 uses prebuilt arm64 assets from marcptrs/netcoredbg (upstream-style tag naming).

Task-based debugging

Create .zed/tasks.json:

[
  {
    "label": "Run MyApp",
    "command": "dotnet",
    "args": ["run", "--project", "src/MyApp/MyApp.csproj"],
    "use_new_terminal": false
  }
]

The extension can derive a debug scenario directly from dotnet run tasks.

Manual debug configuration

Create .zed/debug.json:

[
  {
    "label": "Debug MyApp",
    "adapter": "netcoredbg",
    "request": "launch",
    "program": "$ZED_WORKTREE_ROOT/src/MyApp/bin/Debug/$TARGET_FRAMEWORK/MyApp.dll",
    "args": [],
    "cwd": "$ZED_WORKTREE_ROOT",
    "stopAtEntry": false,
    "console": "internalConsole"
  }
]

$TARGET_FRAMEWORK is resolved from the corresponding .csproj when possible.

Troubleshooting

Enable debug logging

{
  "lsp": {
    "roslyn": {
      "settings": {
        "enable_debug_logging": true
      }
    },
    "omnisharp": {
      "initialization_options": {
        "enable_debug_logging": true
      }
    }
  }
}

Logs appear in the terminal when running Zed in the foreground.

Roslyn does not start

Common causes:

• No .NET 10+ runtime is installed.
• A Unity project was detected, so the extension intentionally fell back to OmniSharp-Roslyn.
• The custom Roslyn binary path is invalid.

In these cases, the extension should usually fall back to OmniSharp-Roslyn automatically. When debug logging is enabled, the extension now logs the backend decision explicitly.

Semantic highlighting not appearing

• Set semantic_tokens to "full" (or "combined") for CSharp.
• Run lsp: restart language servers after changing the setting.
• Prefer Roslyn when comparing backend behavior.

Debugger fails to launch

If you see startup errors (for example configurationDone failures), try:

• Ensure program points to an existing built DLL.
• Clear debugger cache so the adapter is re-downloaded:
  • ~/Library/Application Support/Zed/extensions/work/csharp_roslyn/cache/netcoredbg
• Restart Zed and retry.

No solution detected for OmniSharp-Roslyn

The extension first checks lsp.omnisharp.initialization_options.solution, then tries a simple heuristic for <workspace-name>.sln, .slnx, or .slnf in the workspace root.

If that still does not work, set it explicitly:

{
  "lsp": {
    "omnisharp": {
      "initialization_options": {
        "solution": "/absolute/path/to/project.sln"
      }
    }
  }
}

Development

Build:

cargo build --target wasm32-wasip2

Test:

cargo test

Install dev build:

cp target/wasm32-wasip2/debug/csharp_roslyn.wasm ~/Library/Application\ Support/Zed/extensions/installed/csharp_roslyn/extension.wasm

Links

• Roslyn language server: https://raw.githubusercontent.com/dotnet/roslyn/refs/heads/main/src/LanguageServer/Microsoft.CodeAnalysis.LanguageServer/README.md
• Zed C# language docs: ../zed/docs/src/languages/csharp.md
• OmniSharp-Roslyn: https://github.com/OmniSharp/omnisharp-roslyn
• netcoredbg (official): https://github.com/Samsung/netcoredbg
• netcoredbg (macOS arm64 release pipeline): https://github.com/marcptrs/netcoredbg