Tutorial: Containerize a .NET app
In this tutorial, you learn how to containerize a .NET 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 app
- Create and configure a Dockerfile for .NET
- Build a Docker image
- Create and run a Docker container
You explore the Docker container build and deploy tasks for a .NET 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 8+ SDK.
If you have .NET installed, use thedotnet --infocommand to determine which SDK you're using. - Docker Community Edition.
- A temporary working folder for the Dockerfile and .NET example app. In this tutorial, the name docker-working is used as the working folder.
Create .NET app
You need a .NET app that the Docker container runs. 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 DotNet.Docker
Your folder tree looks similar to the following directory structure:
📁 docker-working
└──📂 App
├──DotNet.Docker.csproj
├──Program.cs
└──📂 obj
├── DotNet.Docker.csproj.nuget.dgspec.json
├── DotNet.Docker.csproj.nuget.g.props
├── DotNet.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. Now, you change directories and navigate into the App folder from your terminal session. Use the dotnet run command to start the app. The application runs, and prints Hello World! below the command:
cd App
dotnet run
Hello World!
The default template creates an app that prints to the terminal and then immediately terminates. For this tutorial, you 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:
Console.WriteLine("Hello World!");
Replace the file with the following code that counts numbers every second:
var counter = 0;
var max = args.Length is not 0 ? Convert.ToInt32(args[0]) : -1;
while (max is -1 || counter < max)
{
Console.WriteLine($"Counter: {++counter}");
await Task.Delay(TimeSpan.FromMilliseconds(1_000));
}
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. Consider the following 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 app
In order for the app to be suitable for an image creation it has to be built. The dotnet publish command is most apt for this, as it builds and publishes the app. For an in-depth reference, see dotnet build and dotnet publish commands documentation.
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\net8.0\publish\.
From the App folder, get a directory listing of the publish folder to verify that the DotNet.Docker.dll file was created.
dir .\bin\Release\net8.0\publish\
Directory: C:\Users\default\App\bin\Release\net8.0\publish
Mode LastWriteTime Length Name
---- ------------- ------ ----
-a--- 9/22/2023 9:17 AM 431 DotNet.Docker.deps.json
-a--- 9/22/2023 9:17 AM 6144 DotNet.Docker.dll
-a--- 9/22/2023 9:17 AM 157696 DotNet.Docker.exe
-a--- 9/22/2023 9:17 AM 11688 DotNet.Docker.pdb
-a--- 9/22/2023 9:17 AM 353 DotNet.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 the directory containing the .csproj and open it in a text editor. This tutorial uses the ASP.NET Core runtime image (which contains the .NET runtime image) and corresponds with the .NET console application.
FROM mcr.microsoft.com/dotnet/sdk:8.0@sha256:35792ea4ad1db051981f62b313f1be3b46b1f45cadbaa3c288cd0d3056eefb83 AS build-env
WORKDIR /App
# Copy everything
COPY . ./
# Restore as distinct layers
RUN dotnet restore
# Build and publish a release
RUN dotnet publish -c Release -o out
# Build runtime image
FROM mcr.microsoft.com/dotnet/aspnet:8.0@sha256:6c4df091e4e531bb93bdbfe7e7f0998e7ced344f54426b7e874116a3dc3233ff
WORKDIR /App
COPY --from=build-env /App/out .
ENTRYPOINT ["dotnet", "DotNet.Docker.dll"]
Note
The ASP.NET Core runtime image is used intentionally here, although the mcr.microsoft.com/dotnet/runtime:8.0 image could have been used.
Tip
This Dockerfile uses multi-stage builds, which optimizes the final size of the image by layering the build and leaving only required artifacts. For more information, see Docker Docs: multi-stage builds.
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 segment is the container repository, whereas the sdk or aspnet segment is the container image name. The image is tagged with 8.0, which is used for versioning. Thus, mcr.microsoft.com/dotnet/aspnet:8.0 is the .NET 8.0 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 8.0 SDK, and the base image referred to in the Dockerfile is tagged with 8.0.
Important
When using Windows-based container images, you need to specify the image tag beyond simply 8.0, for example, mcr.microsoft.com/dotnet/aspnet:8.0-nanoserver-1809 instead of mcr.microsoft.com/dotnet/aspnet:8.0. Select an image name based on whether you're using Nano Server or Windows Server Core and which version of that OS. You can find a full list of all supported tags on .NET's Docker Hub page.
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
├── DotNet.Docker.csproj
├── Program.cs
├──📂 bin
│ └──📂 Release
│ └──📂 net8.0
│ └──📂 publish
│ ├── DotNet.Docker.deps.json
│ ├── DotNet.Docker.exe
│ ├── DotNet.Docker.dll
│ ├── DotNet.Docker.pdb
│ └── DotNet.Docker.runtimeconfig.json
└──📁 obj
└──...
The ENTRYPOINT instruction sets dotnet as the host for the DotNet.Docker.dll. However, it's possible to instead define the ENTRYPOINT as the app executable itself, relying on the OS as the app host:
ENTRYPOINT ["./DotNet.Docker"]
This causes the app to be executed directly, without dotnet, and instead relies on the app host and the underlying OS. For more information on deploying cross-platform binaries, see Produce a cross-platform binary.
To build the container, 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 sets the build context of the image. The -f switch is the path to the 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 2f15637dc1f6 10 minutes ago 217MB
The counter-image repository is the name of the image. The latest tag is the tag that is used to identify the image. The 2f15637dc1f6 is the image ID. The 10 minutes ago is the time the image was created. The 217MB is the size of the image. The final steps of the Dockerfile are to create a container from the image and run the app, copy the published app to the container, and define the entry point.
FROM mcr.microsoft.com/dotnet/aspnet:8.0
WORKDIR /App
COPY --from=build-env /App/out .
ENTRYPOINT ["dotnet", "DotNet.Docker.dll"]
The FROM command specifies the base image and tag to use. The WORKDIR command changes the current directory inside of the container to App.
The COPY command tells Docker to copy the specified source directory to a destination folder. In this example, the publish contents in the build-env layer were output into the folder named App/out, so it's the source to copy from. All of the published contents in the App/out directory are copied into current working directory (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.
Tip
Before .NET 8, containers configured to run as read-only may fail with Failed to create CoreCLR, HRESULT: 0x8007000E. To address this issue, specify a DOTNET_EnableDiagnostics environment variable as 0 (just before the ENTRYPOINT step):
ENV DOTNET_EnableDiagnostics=0
For more information on various .NET environment variables, see .NET environment variables.
Note
.NET 6 standardizes on the prefix DOTNET_ instead of COMPlus_ for environment variables that configure .NET run-time behavior. However, the COMPlus_ prefix will continue to work. If you're using a previous version of the .NET runtime, you should still use the COMPlus_ prefix for environment variables.
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
This docker create command creates 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:
d0be06126f7db6dd1cee369d911262a353c9b7fb4829a0c11b4b2eb7b2d429cf
To see a list of all containers, use the docker ps -a command:
docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
d0be06126f7d counter-image "dotnet DotNet.Docke…" 12 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
cf01364df453 counter-image "dotnet DotNet.Docke…" 53 seconds ago Up 10 seconds core-counter
Similarly, the docker stop command stops 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 ends the process in the container unless otherwise specified, which would stop the container. The --sig-proxy=false parameter ensures that Ctrl+C won't 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 this article, you don't want containers hanging around that don't do anything. 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 DotNet.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 app. To instruct the .NET app to count only to three, pass in 3.
docker run -it --rm counter-image 3
Counter: 1
Counter: 2
Counter: 3
With docker run -it, the Ctrl+C command stops the process that's 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
List all containers
docker ps -aStop containers that are running by their name.
docker stop core-counterDelete the container
docker rm core-counter
Next, delete any images that you no longer want on your machine. Delete the image created by your Dockerfile and then delete the .NET 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/aspnet:8.0
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.
