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EmacsDotNet - Leverage the dotnet ecosystem from emacs

Call dotnet code from emacs.

NOTE: This is far from production quality, but it’s useful for me, and maybe it’s useful for someone else as well. Use at your own risk.

let helloFromFsharp (name : string) = $"F# greets %s{name}!"
public static class Greeter {
    public static string HelloFromCsharp(string name) {
        return $"C# greets {name}!"
(dotnet hello-from-fsharp "Jane Doe") ;; "F# greets Jane Doe"
(dotnet hello-from-csharp "John Doe") ;; "C# greets John Doe"


  • Basic types supported; int, float, bool, string
  • Automatic conversions between types
  • Lists of basic types
  • Hetrogenous lists


The project is only available from source, but should build out-of-the-box. These instructions assume you’re installing to ~/emacsdotnet.

Clone the repository

git clone ~/emacsdotnet

Start the server

dotnet run -p ~/emacsdotnet/emacsdotnet

You should see something like

Starting repl server at with 1 exposed functions
Press C-c to abort

Add to your emacs configuration:

(add-to-list 'load-path "~/emacsdotnet")
(require 'emacsdotnet)

You can test it by using the build-in echo function

(dotnet echo-sexpr 42) ;; evaluates to 42

Adding functions

The easiest way to test it is to use the CLI server to load functions from assemblies.

The other way is to embed the core library and exposing it yourself. It’s (too) easy to embed the server in an existing application and exposing everything, but remember that there are no security measures in place if you go this route.

emacsdotnet.exe --expose-assembly MyAssembly.dll true will expose all ExposeToEmacs functions from MyAssembly.dll, and is a good way to test.

If you want to embed, take a look at the emacsdotnet project.

USAGE: emacsdotnet.exe [--help] [--host <host>] [--port <port>] [--expose-assembly <fullpath> <justExposed>]


    --host <host>         Listen on host. Defaults to
    --port <port>         Listen on port. Defaults to 5000
    --expose-assembly <fullpath> <justExposed>
                          Load functions from assembly. Full path to the dll, and true/false indicated if only functions tagged ExposeToEmacs should be visible
    --help                display this list of options.


I’m not good at emacs, nor lisp, but I love using org-mode and customizing it. Certain things that is trivial for me in dotnet, is impossible for me in emacs-lisp. Being able to “extend” emacs by leveraging my existing dotnet skills and dotnet libraries is good for my productivity.

How it works

Reflection will look for functions (static methods) to expose. When a function is called, it’s called with a list of S-Expressions instead of Object arrays We’re converting a MethodInfo to a function SExpr list -> SExpr much like this simplified version:

let methodToLispFunction (m : MethodInfo) : (SExpr list -> SExpr) =
    fun (eParams : SExpr list) ->
        let iParams =
            |> sexprToValue
            |> Array.ofSeq
        let res = m.Invoke(null, iParams)
        valueToSexpr res

These functions are added in a map by their fully qualified name.

The dotnet console application listens for TCP connections, when it gets a connection, it starts a repl server for the connection. The repl server basically does

  • Read string from socket
  • Parse string to an S-Expression
  • If it’s a function-call (a list which starts with a symbol)
    • call function, use return as result
  • If not, just use parsed expression as result
  • Print S-Expression to string
  • Write string to socket

When emacs calls dotnet, it acs as a client which does much of the same

  • If not connected to server, connect
  • Print S-Expression to string (calling prin1-to-string)
  • Send to dotnet, wait for result
  • Parse string to an S-Expression (calling read)

Some design notes

License by using the CLI

If you supply your own attributes (I check by name, not type), you should be able to use the CLI to load your assembly and start the repl-server without being bound by any license of this project (AFAIK, but IANAL. At least that has been my intention, and it’s a permissive licence anyway).

Embedding vs CLI

It’s possible to embed this rather than using the console application, and that might be a better choice to expose functionality. The emacsdotnet application makes it simple to expose dlls, and at the same time you shouldn’t have to worry about licensing as you don’t use any of the code.

Socket vs stdin/stdout

Why use a socket instead of just stdin/stdout? I want to extend this to have multiple servers at the same time, maybe crossing various boundaries. My day job involves using Windows, but I run Emacs in WSL as I’m having a lots of performance problems and some show-stoppers with Emacs on Windows. By using a TCP socket, I can have a server running on the Windows side, and one on the WSL side.

Socket vs HTTP vs WebSocket

This is mostly just because a socket was simple and light-weight.


Secu..what? There’s not any security here. Everything the server exposes will become available to everyone. There’s no authentication nor authorization. Use the ExposeToEmacs attributes and only expose functions with that attribute.

Only expose what’s useful for emacs. You can easily expose core system libraries, and then (dotnet System/Exit 1) will exit the server! And that’s just a haha funny thing that can happen.


Leverage the dotnet ecosystem from emacs







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