Please note that this article is very badly outdated since the release of .NET Core and Visual Studio for Mac. I'm leaving it up for historical accuracy.

Mono is the open source .NET runtime for Windows, Linux, and OS X. It consists of the Mono runtime environment, libraries, and C# and F# compilers. Recently Mono has gained extra popularity due to Microsoft's purchase of Xamarin, the makers of a cross-platform toolkit of the same name.

If you just want to create command-line .NET applications on the Mac, and don't need Xamarin.Forms or the mobile tools, you can just install Mono and start hacking away.

The Mono Project home page advises you to download and install Mono as a Mac package, but you also do a a Homebrew-based installation. If you don't yet have Homebrew (“the missing package manager for OS X”), install it by following the instructions on its home page.

Once you have Homebrew installed, you can install Mono:

$ brew install mono
==> Downloading https://homebrew.bintray.com/bottles/mono-4.2.3.4.yosemite.bottl
######################################################################## 100.0%
==> Pouring mono-4.2.3.4.yosemite.bottle.tar.gz
==> Caveats
To use the assemblies from other formulae you need to set:
  export MONO_GAC_PREFIX="/usr/local"
Note that the 'mono' formula now includes F#. If you have
the 'fsharp' formula installed, remove it with 'brew uninstall fsharp'.
==> Summary
?  /usr/local/Cellar/mono/4.2.3.4: 1,280 files, 205.2M

You can probably pick up that I'm still using OS X Yosemite on this machine, but there shouldn't be any difference with El Capitan. If you upgraded from Yosemite to El Capitan, and had Homebrew installed, you may have run into an issue with the OS X security restrictions – read the solution.

C# support for Visual Studio Code

Microsoft Visual Studio Code, or VSCode for short, is a relatively new programmer's text editor, but already quite mature. Typically I use it for Python, Clojure and JavaScript. Now I wanted to use it to edit C# source files on the Mac, but surprisingly it does not have C# syntax highlighting support out of the box. You need to install an extension and restart VSCode.

Hello, .NET world!

Just a simple C# source file to get you started:

using System;

namespace Hello
{
    class Hello
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello, .NET world!");
        }
    }
}

Save it as Hello.cs. Compile with:

mcs Hello.cs

Here, mcs is the Mono C# compiler. You should get a file named Hello.exe, but you can't execute it directly. Instead, use the Mono runtime:

mono Hello.exe

You should see the greeting printed out by System.Console.WriteLine.

Why C#?

I'm dusting off the C# tools on my Mac because I envision that C# and .NET will become more important on OS X because of the Xamarin acquisition. I like C#, sometimes better than Java, and have programmed many applications for Windows Phone with it.

Why F#?

F# intrigues me as a language that embraces many of the good things about functional programming, but lets you leverage the .NET ecosystem. I've started to learn F# in earnest several times during the last few years, but have not made a concentrated attempt yet. Hopefully soon.