Tutorial: Containerize a .NET Core app

In this tutorial, you'll learn how to containerize a .NET Core application with Docker. Containers have many features and benefits, such as being an immutable infrastructure, providing a portable architecture, and enabling scalability. The image can be used to create containers for your local development environment, private cloud, or public cloud.

In this tutorial, you:

  • Create and publish a simple .NET Core app
  • Create and configure a Dockerfile for .NET Core
  • Build a Docker image
  • Create and run a Docker container

You'll understand the Docker container build and deploy tasks for a .NET Core application. The Docker platform uses the Docker engine to quickly build and package apps as Docker images. These images are written in the Dockerfile format to be deployed and run in a layered container.

Note

This tutorial is not for ASP.NET Core apps. If you're using ASP.NET Core, see the Learn how to containerize an ASP.NET Core application tutorial.

Prerequisites

Install the following prerequisites:

  • .NET Core 3.1 SDK
    If you have .NET Core installed, use the dotnet --info command to determine which SDK you're using.
  • Docker Community Edition
  • A temporary working folder for the Dockerfile and .NET Core example app. In this tutorial, the name docker-working is used as the working folder.

Create .NET Core app

You need a .NET Core app that the Docker container will run. Open your terminal, create a working folder if you haven't already, and enter it. In the working folder, run the following command to create a new project in a subdirectory named app:

dotnet new console -o App -n NetCore.Docker

Your folder tree will look like the following:

docker-working
    └──App
        ├──NetCore.Docker.csproj
        ├──Program.cs
        └──obj
            ├──NetCore.Docker.csproj.nuget.dgspec.json
            ├──NetCore.Docker.csproj.nuget.g.props
            ├──NetCore.Docker.csproj.nuget.g.targets
            ├──project.assets.json
            └──project.nuget.cache

The dotnet new command creates a new folder named App and generates a "Hello World" console application. Change directories and navigate into the App folder, from your terminal session. Use the dotnet run command to start the app. The application will run, and print Hello World! below the command:

dotnet run
Hello World!

The default template creates an app that prints to the terminal and then immediately terminates. For this tutorial, you'll use an app that loops indefinitely. Open the Program.cs file in a text editor.

Tip

If you're using Visual Studio Code, from the previous terminal session type the following command:

code .

This will open the App folder that contains the project in Visual Studio Code.

The Program.cs should look like the following C# code:

using System;

namespace NetCore.Docker
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
        }
    }
}

Replace the file with the following code that counts numbers every second:

using System;
using System.Threading.Tasks;

namespace NetCore.Docker
{
    class Program
    {
        static async Task Main(string[] args)
        {
            var counter = 0;
            var max = args.Length != 0 ? Convert.ToInt32(args[0]) : -1;
            while (max == -1 || counter < max)
            {
                Console.WriteLine($"Counter: {++counter}");
                await Task.Delay(1000);
            }
        }
    }
}

Save the file and test the program again with dotnet run. Remember that this app runs indefinitely. Use the cancel command Ctrl+C to stop it. The following is an example output:

dotnet run
Counter: 1
Counter: 2
Counter: 3
Counter: 4
^C

If you pass a number on the command line to the app, it will only count up to that amount and then exit. Try it with dotnet run -- 5 to count to five.

Important

Any parameters after -- are not passed to the dotnet run command and instead are passed to your application.

Publish .NET Core app

Before adding the .NET Core app to the Docker image, first it must be published. It is best to have the container run the published version of the app. To publish the app, run the following command:

dotnet publish -c Release

This command compiles your app to the publish folder. The path to the publish folder from the working folder should be .\App\bin\Release\netcoreapp3.1\publish\

From the App folder, get a directory listing of the publish folder to verify that the NetCore.Docker.dll file was created.

dir .\bin\Release\netcoreapp3.1\publish\

    Directory: C:\Users\dapine\App\bin\Release\netcoreapp3.1\publish

Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----        4/27/2020   8:27 AM            434 NetCore.Docker.deps.json
-a----        4/27/2020   8:27 AM           6144 NetCore.Docker.dll
-a----        4/27/2020   8:27 AM         171520 NetCore.Docker.exe
-a----        4/27/2020   8:27 AM            860 NetCore.Docker.pdb
-a----        4/27/2020   8:27 AM            154 NetCore.Docker.runtimeconfig.json

Create the Dockerfile

The Dockerfile file is used by the docker build command to create a container image. This file is a text file named Dockerfile that doesn't have an extension.

Create a file named Dockerfile in directory containing the .csproj and open it in a text editor. This tutorial will use the ASP.NET Core runtime image (which contains the .NET Core runtime image) and corresponds with the .NET Core console application.

FROM mcr.microsoft.com/dotnet/core/aspnet:3.1

Note

The ASP.NET Core runtime image is used intentionally here, although the mcr.microsoft.com/dotnet/core/runtime:3.1 image could have been used.

The FROM keyword requires a fully qualified Docker container image name. The Microsoft Container Registry (MCR, mcr.microsoft.com) is a syndicate of Docker Hub - which hosts publicly accessible containers. The dotnet/core segment is the container repository, where as the aspnet segment is the container image name. The image is tagged with 3.1, which is used for versioning. Thus, mcr.microsoft.com/dotnet/core/aspnet:3.1 is the .NET Core 3.1 runtime. Make sure that you pull the runtime version that matches the runtime targeted by your SDK. For example, the app created in the previous section used the .NET Core 3.1 SDK and the base image referred to in the Dockerfile is tagged with 3.1.

Save the Dockerfile file. The directory structure of the working folder should look like the following. Some of the deeper-level files and folders have been omitted to save space in the article:

docker-working
    └──App
        ├──Dockerfile
        ├──NetCore.Docker.csproj
        ├──Program.cs
        ├──bin
        │   └──Release
        │       └──netcoreapp3.1
        │           └──publish
        │               ├──NetCore.Docker.deps.json
        │               ├──NetCore.Docker.exe
        │               ├──NetCore.Docker.dll
        │               ├──NetCore.Docker.pdb
        │               └──NetCore.Docker.runtimeconfig.json
        └──obj
            └──...

From your terminal, run the following command:

docker build -t counter-image -f Dockerfile .

Docker will process each line in the Dockerfile. The . in the docker build command tells Docker to use the current folder to find a Dockerfile. This command builds the image and creates a local repository named counter-image that points to that image. After this command finishes, run docker images to see a list of images installed:

docker images
REPOSITORY                              TAG                 IMAGE ID            CREATED             SIZE
counter-image                           latest              e6780479db63        4 days ago          190MB
mcr.microsoft.com/dotnet/core/aspnet    3.1                 e6780479db63        4 days ago          190MB

Notice that the two images share the same IMAGE ID value. The value is the same between both images because the only command in the Dockerfile was to base the new image on an existing image. Let's add three commands to the Dockerfile. Each command creates a new image layer with the final command representing the counter-image repository entry points to.

COPY bin/Release/netcoreapp3.1/publish/ App/
WORKDIR /App
ENTRYPOINT ["dotnet", "NetCore.Docker.dll"]

The COPY command tells Docker to copy the specified folder on your computer to a folder in the container. In this example, the publish folder is copied to a folder named App in the container.

The WORKDIR command changes the current directory inside of the container to App.

The next command, ENTRYPOINT, tells Docker to configure the container to run as an executable. When the container starts, the ENTRYPOINT command runs. When this command ends, the container will automatically stop.

From your terminal, run docker build -t counter-image -f Dockerfile . and when that command finishes, run docker images.

docker build -t counter-image -f Dockerfile .
Sending build context to Docker daemon  1.117MB
Step 1/4 : FROM mcr.microsoft.com/dotnet/core/aspnet:3.1
 ---> e6780479db63
Step 2/4 : COPY bin/Release/netcoreapp3.1/publish/ App/
 ---> d1732740eed2
Step 3/4 : WORKDIR /App
 ---> Running in b1701a42f3ff
Removing intermediate container b1701a42f3ff
 ---> 919aab5b95e3
Step 4/4 : ENTRYPOINT ["dotnet", "NetCore.Docker.dll"]
 ---> Running in c12aebd26ced
Removing intermediate container c12aebd26ced
 ---> cd11c3df9b19
Successfully built cd11c3df9b19
Successfully tagged counter-image:latest

docker images
REPOSITORY                              TAG                 IMAGE ID            CREATED             SIZE
counter-image                           latest              cd11c3df9b19        41 seconds ago      190MB
mcr.microsoft.com/dotnet/core/aspnet    3.1                 e6780479db63        4 days ago          190MB

Each command in the Dockerfile generated a layer and created an IMAGE ID. The final IMAGE ID (yours will be different) is cd11c3df9b19 and next you'll create a container based on this image.

Create a container

Now that you have an image that contains your app, you can create a container. You can create a container in two ways. First, create a new container that is stopped.

docker create --name core-counter counter-image
0f281cb3af994fba5d962cc7d482828484ea14ead6bfe386a35e5088c0058851

The docker create command from above will create a container based on the counter-image image. The output of that command shows you the CONTAINER ID (yours will be different) of the created container. To see a list of all containers, use the docker ps -a command:

docker ps -a
CONTAINER ID    IMAGE            COMMAND                   CREATED           STATUS     PORTS    NAMES
0f281cb3af99    counter-image    "dotnet NetCore.Dock…"    40 seconds ago    Created             core-counter

Manage the container

The container was created with a specific name core-counter, this name is used to manage the container. The following example uses the docker start command to start the container, and then uses the docker ps command to only show containers that are running:

docker start core-counter
core-counter

docker ps
CONTAINER ID    IMAGE            COMMAND                   CREATED          STATUS          PORTS    NAMES
2f6424a7ddce    counter-image    "dotnet NetCore.Dock…"    2 minutes ago    Up 11 seconds            core-counter

Similarly, the docker stop command will stop the container. The following example uses the docker stop command to stop the container, and then uses the docker ps command to show that no containers are running:

docker stop core-counter
core-counter

docker ps
CONTAINER ID    IMAGE    COMMAND    CREATED    STATUS    PORTS    NAMES

Connect to a container

After a container is running, you can connect to it to see the output. Use the docker start and docker attach commands to start the container and peek at the output stream. In this example, the Ctrl+C keystroke is used to detach from the running container. This keystroke will end the process in the container unless otherwise specified, which would stop the container. The --sig-proxy=false parameter ensures that Ctrl+C will not stop the process in the container.

After you detach from the container, reattach to verify that it's still running and counting.

docker start core-counter
core-counter

docker attach --sig-proxy=false core-counter
Counter: 7
Counter: 8
Counter: 9
^C

docker attach --sig-proxy=false core-counter
Counter: 17
Counter: 18
Counter: 19
^C

Delete a container

For the purposes of this article you don't want containers just hanging around doing nothing. Delete the container you previously created. If the container is running, stop it.

docker stop core-counter

The following example lists all containers. It then uses the docker rm command to delete the container, and then checks a second time for any running containers.

docker ps -a
CONTAINER ID    IMAGE            COMMAND                   CREATED          STATUS                        PORTS    NAMES
2f6424a7ddce    counter-image    "dotnet NetCore.Dock…"    7 minutes ago    Exited (143) 20 seconds ago            core-counter

docker rm core-counter
core-counter

docker ps -a
CONTAINER ID    IMAGE    COMMAND    CREATED    STATUS    PORTS    NAMES

Single run

Docker provides the docker run command to create and run the container as a single command. This command eliminates the need to run docker create and then docker start. You can also set this command to automatically delete the container when the container stops. For example, use docker run -it --rm to do two things, first, automatically use the current terminal to connect to the container, and then when the container finishes, remove it:

docker run -it --rm counter-image
Counter: 1
Counter: 2
Counter: 3
Counter: 4
Counter: 5
^C

The container also passes parameters into the execution of the .NET Core app. To instruct the .NET Core app to count only to 3 pass in 3.

docker run -it --rm counter-image 3
Counter: 1
Counter: 2
Counter: 3

With docker run -it, the Ctrl+C command will stop process that is running in the container, which in turn, stops the container. Since the --rm parameter was provided, the container is automatically deleted when the process is stopped. Verify that it doesn't exist:

docker ps -a
CONTAINER ID    IMAGE    COMMAND    CREATED    STATUS    PORTS    NAMES

Change the ENTRYPOINT

The docker run command also lets you modify the ENTRYPOINT command from the Dockerfile and run something else, but only for that container. For example, use the following command to run bash or cmd.exe. Edit the command as necessary.

In this example, ENTRYPOINT is changed to cmd.exe. Ctrl+C is pressed to end the process and stop the container.

docker run -it --rm --entrypoint "cmd.exe" counter-image

Microsoft Windows [Version 10.0.17763.379]
(c) 2018 Microsoft Corporation. All rights reserved.

C:\>dir
 Volume in drive C has no label.
 Volume Serial Number is 3005-1E84

 Directory of C:\

04/09/2019  08:46 AM    <DIR>          app
03/07/2019  10:25 AM             5,510 License.txt
04/02/2019  01:35 PM    <DIR>          Program Files
04/09/2019  01:06 PM    <DIR>          Users
04/02/2019  01:35 PM    <DIR>          Windows
               1 File(s)          5,510 bytes
               4 Dir(s)  21,246,517,248 bytes free

C:\>^C

Essential commands

Docker has many different commands that create, manage, and interact with containers and images. These Docker commands are essential to managing your containers:

Clean up resources

During this tutorial, you created containers and images. If you want, delete these resources. Use the following commands to

  1. List all containers

    docker ps -a
    
  2. Stop containers that are running by their name.

    docker stop counter-image
    
  3. Delete the container

    docker rm counter-image
    

Next, delete any images that you no longer want on your machine. Delete the image created by your Dockerfile and then delete the .NET Core image the Dockerfile was based on. You can use the IMAGE ID or the REPOSITORY:TAG formatted string.

docker rmi counter-image:latest
docker rmi mcr.microsoft.com/dotnet/core/aspnet:3.1

Use the docker images command to see a list of images installed.

Tip

Image files can be large. Typically, you would remove temporary containers you created while testing and developing your app. You usually keep the base images with the runtime installed if you plan on building other images based on that runtime.

Next steps