Setting up a C# Language Server (csharp-ls) on an Air-Gapped Windows System

For a little while now, I have been developing C# applications at work and on Windows!, possibly the perfect storm for Emacs, but actually, it copes pretty well, especially with eglot and the LSP server OmniSharp.

But recently my work setup has become even more air-gapped, and unfortunately, I reached a point where I couldn’t get eglot and OmniSharp working together. I gave it a really good try, but I think the mishmash and dependency installation madness finally took a toll.

It has something to do with trying to find MSBuild.exe, but due to my potentially wonky installation, I couldn’t figure out how to detect it. If Visual Studio is installed on the system, there are no problems; however, offline installation, even of the Community edition, is not an easy task.

So, the solution? Well, I thought I would try another C# LSP in the form of csharp-ls. Its deliverable form is a NuGet package, which can be extracted, placed in the relevant location, and then you’re pretty much off and running!

So here is how I did it:

Step 1: Download csharp-ls nuget Components

On a machine with internet access (which fortunately I do have access to):

Download csharp-ls

Step 2: Install csharp-ls (Airgapped Machine)

  1. Create a directory in your Emacs installation:

          mkdir "c:\path\to\emacs\bin\csharp-ls"

  2. Copy the extracted csharp-ls contents to this directory. The key file you need is:

          tools\net9.0\any\CSharpLanguageServer.dll

Step 3: Configure Emacs

Add this configuration to your Emacs init file:

;; Configure csharp-ls as the language server for C#
(setq eglot-server-programs
      '((csharp-mode . ("dotnet" "c:/path/to/emacs/bin/csharp-ls/tools/net9.0/any/CSharpLanguageServer.dll"))))

Step 4: Set Up Your C# Project Structure

Basically defining a source code collection as a project in Emacs. This can be a solution file, a source code control directory or as I generally have set up, an empty .project file

Step 5: Set up paths

This may not be needed, but for my own sanity and just in case, absorb the new directory in to the PATH and exec-path variables.

(when (eq system-type 'windows-nt)
  (let ((xPaths
         `(
           ,(expand-file-name "~/bin/csharp-ls/tools/net9.0/any")
           )))
    (setenv "PATH" (concat
                    (mapconcat 'identity xPaths ";")))
    (setq exec-path (append (split-string winPaths ";") xPaths (list "." exec-directory)))))

So it works as I had it with OmniSharp and that really is all I can ask for, lets see how I get on!

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