Running .NET apps on Docker

This blog post covers running simple .NET apps in Docker lightweight containers using Mono. I run Docker in a Vagrant/VirtualBox VM on Windows. This works great and is fast. Installation instructions are available on the Docker site.

Building the base image

First order of business is to create a Docker image that has Mono installed. We will use this as the base image for containers that actually run apps. To get the most recent Mono version (3.2.6 at the time of writing) I use packages created by Timotheus Pokorra installed on a Ubuntu 12.04 LTS Docker image. Here’s the Dockerfile for that:

FROM ubuntu:12.04

RUN apt-get -y -q install wget
RUN wget -q -O- | apt-key add -
RUN apt-get remove -y --auto-remove wget
RUN sh -c "echo 'deb /' >> /etc/apt/sources.list.d/mono-opt.list"
RUN apt-get -q update
RUN apt-get -y -q install mono-opt

Here’s what’s going on:

  1. Install wget
  2. Add repository key to apt-get
  3. remove wget
  4. Add openSUSE repository to sources list
  5. Install Mono from there

At first I did all of the above in one command since these steps represent the single logical step of installing Mono and it seems like they should be just one commit. Nobody will be interested in the commit after wget was installed, for example. I ended up splitting it up into separate RUN commands since that’s what other people seem to do.
With that Dockerfile, we can build an image:

$ docker build -t friism/mono .

We can then run a container using the generated image and check our Mono installation:

vagrant@precise64:~/mono$ docker run -i -t friism/mono bash
root@0bdca65e6e8e:/# /opt/mono/bin/mono --version
Mono JIT compiler version 3.2.6 (tarball Sat Jan 18 16:48:05 UTC 2014)

Note that Mono is installed in /opt and that it works!

Running a console app

First, we’ll deploy a very simple console app:

using System;

namespace HelloWorld
	public class Program
		static void Main(string[] args)
			Console.WriteLine("Hello World");

For the purpose of this example, we’ll pre-build apps using the VS command prompt and msbuild and then add the output to the container:

msbuild /property:OutDir=C:\tmp\helloworld HelloWorld.sln

Alternatively, we could have invoked xbuild or gmcs from within the container.

The Dockerfile for the container to run the app is extremely simple:

FROM friism/mono

ADD app/ .
CMD /opt/mono/bin/mono `ls *.exe | head -1`

Note that it relies on the friism/mono image created above. It also expects the compiled app to be in the /app folder, so:

$ ls app

The CMD will simply use mono to run the first executable found in the build output. Let’s build and run it:

$ docker build -t friism/helloworld-container .
$ docker run friism/helloworld-container
Hello World

It worked!

Web app

Running a self-hosting OWIN web app is only slightly more work. For this example I used the sample code from the OWIN/Katana-on-Heroku post.

$ ls app/
HelloWorldWeb.exe  Microsoft.Owin.Diagnostics.dll  Microsoft.Owin.dll  Microsoft.Owin.Host.HttpListener.dll  Microsoft.Owin.Hosting.dll  Owin.dll

The Dockerfile for this exposes port 5000 from the container, and CMD is used to start the web app and specify port 5000 to listen on:

FROM friism/mono

ADD app/ .
CMD ["/opt/mono/bin/mono", "HelloWorldWeb.exe", "5000"]

We start the container and map port 5000 to port 80 on the machine running Docker:

$ docker run -p 80:5000 -t friism/mono-hello-world-web

And with that we can visit the OWIN sample site on http://localhost/.

If you’re a .NET developer this post will hopefully have helped you place Docker in the context of your everyday work. If you’re interested in more ideas on how to use Docker to deploy apps, check out this Automated deployment with Docker – lessons learnt post.

3 thoughts on “Running .NET apps on Docker”

  • Thanks for this – I have been wondering how a Mono container would be built.

    Do you have or know of any example apps that could be built *solely* on the Mono side of things – I would like to get an example app running on octohost – which is our web focused Docker based mini-PaaS for web apps.

    I created a mono container (from your instructions) and would just like to be able to build / link to an example mono web app.

    If you’re interested in help – I’d gladly give you access to try build – it’s pretty simple – all interaction happens within git – kind of like Heroku.


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