Testing and debugging a containerized ASP.NET application


In my previous post I showed in detail how a node JS application running in a Docker container can be debugged and how we can run automated tests against this application also in a container. In this post I am going to investigate what is currently possible in this regard if the application is based on ASP.NET Core RC1 and is also running in a Docker container.

The code accompanying this post can be found here.

The Application

The application I want to use is a simple ASP.NET Core RC1 Web API. Phew, that’s quite a name, I know. But we are used to such monsters coming from Microsoft’s kitchen aren’t we? Basically I have one controller with an endpoint which allows us to determine if a given number is a prime. It’s the same functionality that we were having in our previous node JS application.


For this exercise to work make sure you have ASP.NET 5 RC installed. You can get it from here.

The Web API

So, now create a new folder api and add a file project.json to it. The content of the file looks like this

Note how we are using the .NET Core framework (line 23). Now add a Startup.cs file with the following content to the same folder

Next add a sub-folder Controllers to the api folder and add a file PrimesController.cs to it. The content of controller should look like this

That is all we need to do for the moment. In a console navigate to the project folder and execute dnu restore followed by dnu build. Since we’re using .NET core we need to make sure that our default (or active) framework is set to this. Use the following command

dnvm use 1.0.0-rc1-update1 -r coreclr -arch x64

If all goes well we can then start and test the application by executing dnx web. In the console we should see this output

Now open a browser and navigate to localhost:5000/api/primes/11. The value true should be shown in the browser indicating that indeed 11 is a prime number.

Manual Testing and Debugging

Now let’s look into how we can run the above application in a container and still test and debug it. First let’s run it in a container.

Docker-izing the application

It is easy and straight forward to run our Web API in a Docker container. We just need to add a Dockerfile to the project. The content of the file should look like this

We derive our Docker image from the Microsoft base image for ASP.NET 1.0 RC1. We then copy the project.json file into the image and do a restore of all the required nuget packages. We do this first to leverage the fact that the Docker image is layered. We will change the code much more frequently than we are going to change the list of required nuget packages. Thus when we’re rebuilding the image the restore does not have to happen all the time, which is good since it takes a considerable amount of time. Once we have done this we expose port 5000 and define the ENTRYPOINT. The command defined as entry point is executed whenever a container is started from our Docker image. Thus here the start command is

dnx -p project.json web --server.urls

Note how we need to define the URL as and not just using localhost:5000. Now we can build the container image using

docker build -t myapi .

and when done run a container from it

docker run -d --name api -p 5000:5000 myapi

Double check that the container runs using

docker ps

If for any reason the container api is not shown in the list try to find the root cause by displaying the logs

docker logs api

Open a browser and naviget to The browser should show true, indicating that indeed 11 is a prime number. Great, now we have a ASP.NET Web API based on .NET Core running in a Docker container on Linux. Make sure that after you’re done testing you remove the obsolete containers. Use the docker stop and docker rm commands to do so.

Enable edit and continue

Wouldn’t it be nice if I could edit the files of my project and not to have to rebuild the Docker image over and over? I wish we could have an edit and continue experience. Luckily this is possible. For this to work we need to craft a special Dockerfile to be used exclusively during development time. This Dockerfile will not copy the project files into the image but mount a volume containing all the project files. Add a file Dockerfile.debug to the project with the following content

Most of the content of this Dockerfile are similar to the one we presented above. We have to add a few tweaks to allow the containerized application to discover if some of the project files have changed and thus the application should be (automatically) restarted. The main change is on line 21 where we define a different ENTRYPOINT than in the standard Dockerfile. We are using dnx-watch to start the application instead of just dnx. dnx-watch monitors the files and restarts the application if a change is discovered. dnx-watch itself is installed with the command on line 13. We also set some additional environment variables, specifically DNX_TRACE to have DNX produce more (hopefully helpful) output.

Shared Folders

If we are on a Windows box and are using VirtualBox to run our Docker host then the folder c:\Users is automatically mapped to the Docker host at c:/user. As long as our project folder is a subfolder of c:\users we are good and don’t have to do anything else. Otherwise we need to make sure the project folder is shared with Docker host. Please follow the instructions here.

Running the application

Now we want to run our Web API in a Docker container. To do this we first have to build a Docker image. We use docker-compose for this task because docker-compose simplifies the building of images and creation of containers. Let’s create a docker-compose.debug.yml file with the following content

This docker compose file uses the new standard (version 2.0). We define our service to be built using the Dockerfile Dockerfile.debug. We than mount the current folder as /app folder to the image. This makes it possible that changes in the files can be discovered by the application running inside the container (via dnx-watch). To build the Docker image we use this command

docker-compose -f docker-compose.debug.yml build

and to run the application we can use

docker-compose -f docker-compose.debug.yml up -d

Now our Web API runs inside a container and Kestrel, our web server is configured to listen at port 5000. We can now open a browser and navigate to the following URL

and if all works fine the result should be true. Try to use another number instead of 11 to see if the algorithm is working as expected. If for some reason or not the application does not work then try to analyze the logs produced by the application in the container. Use this command to display the logs in the console

docker logs api

Now let’s try to modify some code and see whether or not the changes are reflected in the running application. Let’s just add the value the PrimesController controller is returning. Instead of only returning true or false let’s use this code

return "The number is " + (isPrime ? "" : " NOT ") + "a prime.";

Do not forget to change the return type of the Get method from bool to string. Now just refresh the browser and assert that the response has changed as expected.

Automated Tests

To create a test project using .NET core is at this point in time quite tricky. .NET core is a moving target (at the time of writing RC1 and RC2 are out in the field but the tooling does not yet support RC2) and documentation is sparse to say the least. Most third party libraries have not yet been ported to .NET core either. But let’s not complain and show the solution that I came up. Create a new folder docker-webapi-testing and move the folder containing the above Web API solution into this new folder. Add a new sub-folder called unit-tests to docker-webapi-testing. Add a file global.json to the docker-webapi-testing folder. The content of this file should look like this

As you can see this file basically contains a list of all projects that make up my solution. The Web API project is called api and the test project which is called unit-tests (basically the respective folder names).

The test project

At the time of writing XUnit is the only established test library that supports .NET core. I am going to use this library for my sample. To the folder unit-tests add a project.json file. The content looks as follows

Now we add actual tests. Add a file called primes-controller-spec.cs to the test project. It’s content should look like this

As you can see we have defined 2 test, one for a prime number and one for a non-prime number.

Containerizing the solution

Next to the global.json file we now add a Dockerfile since we want to run all those tests in a container. The content of this file should look like this

We base our image on the official Microsoft image for ASP.NET Core 1.0 RC1. On line 4 to 6 we are copying the global.json and the two project.json files into the image; we then run dnu restore which will restore the nuget packages of both projects. Finally on line 9 we are running the command test which is defined in the project.json of our unit-tests project. We can now build the test image as follows

docker build -t sut .

And then we can execute the tests

docker run -it --rm --name sut sut

The output in the console should look similar to this

With this we have container-ized our tests successfully.

Attaching a debugger

At the time of writing the tooling that Microsoft provides is not able to attach to an ASP.NET Core application running inside a Docker container in Linux. I hope that this will be possible in the not too far future. For the moment we have to rely on our tests to make sure that our code is correct or we need to debug the code on the host machine before we wrap the application into a container.


In this post I have shown how we can run an application based on ASP.NET Core in a Docker container and run it on Linux. I have also shown how we can create and run a container that executes tests against our code using XUnit. At the time of writing XUnit is the only test library that has been ported to .NET Core.

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