ASP.NET Core in a container

In this guide you will learn how to:

• Create aDockerfile file describing a simple .NET Core service container.
• Build, run, and verify the functionality of the service.
• Debug the service running as a container.

Prerequisites

• Docker and the VS Code Container Tools extension must be installed as described on theoverview.
• For .NET development, install.NET SDK.
• MicrosoftC# for Visual Studio Code extension.

Create a .NET Web API project

1. Create a folder for the project.

2. Open developer command prompt in the project folder and initialize the project:

   dotnet new webapi --no-https

Add Docker files to the project

1. Open the project folder in VS Code.

2. Wait for the C# extension to prompt you to add required assets for build and debug, and chooseYes. You can also open the Command Palette (⇧⌘P (Windows, LinuxCtrl+Shift+P)) and use the.NET: Generate Assets for Build and Debug command.

3. Open Command Palette (⇧⌘P (Windows, LinuxCtrl+Shift+P)) and useContainers: Add Docker Files to Workspace... command:

   

4. Use.NET: ASP.NET Core when prompted for application platform.

5. ChooseWindows orLinux when prompted to choose the operating system.

   Windows is only applicable if your Docker installation is configured to use Windows containers.

6. You will be asked if you want to add Docker Compose files. We will not use Docker Compose in this tutorial, so both "Yes" and "No" answers are fine.

7. Change the port for application endpoint to5000.

8. Dockerfile and.dockerignore files are added to the workspace.

   The extension will also create a set ofVS Code tasks for building and running the container (in both debug- and release configuration, four tasks in total), and adebugging configuration for launching the container in debug mode.

Build the application

1. Open terminal prompt (⌃` (Windows, LinuxCtrl+`)).

2. Issuedotnet build command to build the application:

   PS C:\source\repos\net> dotnet buildMSBuild version 17.4.0-preview-22470-08+6521b1591 for .NET  Determining projects to restore...  All projects are up-to-date for restore.  net -> C:\source\repos\net\bin\Debug\net7.0\net.dllBuild succeeded.    0 Warning(s)    0 Error(s)Time Elapsed 00:00:08.96

Add an environment variable to the image

You can use the Container Tools extension to author Docker files. The extension provides completions and contextual help. To see these capabilities add an environment variable to your service image by following these steps:

1. Open theDockerfile file.

2. UseENV instruction to add an environment variable to the service container image. The instruction should be placed in thebase stage of theDockerfile (the first stage in the file). Set theASPNETCORE_URLS variable tohttp://*:5000:

   

   Note how the Container Tools extension lists all available Dockerfile instructions and describes the syntax.

   The Container Tools extension uses thebase stage of theDockerfile to create a debug version of the container image for your service. Put theASPNETCORE_URLS environment variable definition in thebase stage to have this variable available in both debug and release versions of the container image.

3. Save theDockerfile file.

Build the image

1. Open the Command Palette (⇧⌘P (Windows, LinuxCtrl+Shift+P)) and issueContainer Images: Build Image... command.

2. Open the Container Explorer and verify that the new image is visible in the Images tree:

   

Test the service container

1. Right-click on the image built in previous step and chooseRun orRun Interactive. The container should start and you should be able to see it in the "Containers" view of the Container Explorer:

   

2. Open the web browser and navigate tohttp://localhost:5000/WeatherForecast. You should see weather data in JSON format, similar to following:

   [    {"date":"2019-11-07T23:31:57.0527092+00:00","temperatureC":4,"temperatureF":39,"summary":"Bracing"},    {"date":"2019-11-08T23:31:57.0539243+00:00","temperatureC":-19,"temperatureF":-2,"summary":"Freezing"},    {"date":"2019-11-09T23:31:57.0539269+00:00","temperatureC":2,"temperatureF":35,"summary":"Freezing"},    {"date":"2019-11-10T23:31:57.0539275+00:00","temperatureC":-4,"temperatureF":25,"summary":"Freezing"},    {"date":"2019-11-11T23:31:57.053928+00:00","temperatureC":9,"temperatureF":48,"summary":"Bracing"} ]

   By default Docker will assign a randomly chosenhost port to a port exposed by a container (thecontainer port). In our application the exposed (container) port is 5000. When you issueRun command for an image, VS Code will try to use the same port number for the host port and container port. This makes it easy to remember which port to use to communicate with the container, but it won't work if the host port is already in use.

   If you cannot see the data from the container in your browser, make sure there are no errors reported by thedocker run command (look at the command output in the terminal window). You can also verify which host port is using by the container by right-clicking the container in the Container Explorer and choosingInspect. This will open a JSON document that describes the container in detail. Search forPortBindings element, for example:

   "PortBindings": {  "5000/tcp": [    {      "HostIp":"",      "HostPort":"5000"    }  ]},

3. When done testing, right-click the container in the Container Explorer and chooseStop.

Debug in container

When Docker files were added to the application, the Container Tools extension also added aVS Code debugger configuration for debugging the service when it is running inside a container. The extension will automatically detect the protocol and port that the service is using and point the browser to the service, but we need to tell it what URL path to use.

1. Set a breakpoint at the beginning of the code for theGet() method of theControllers/WeatherForecastController.cs file.

2. Open.vscode/launch.json file and findContainers: .NET Core Launch debug configuration.

3. AdddockerServerReadyAction toContainers: .NET Core Launch configuration:

   "dockerServerReadyAction": {    "uriFormat":"%s://localhost:%s/WeatherForecast"}

4. Make sure the configuration is selected as active:

   

5. Start debugging (F5).

   • The debug version of the service container builds and starts.
   • The browser opens to request a new weather forecast.
   • The breakpoint in theWeatherForecastController is hit.

You can use specific port on the host by changing the Docker run options used bydocker-run: debug task (defined in.vscode/tasks.json file). For example, if you want to use the same port (5000) to expose the service, thedocker-run: debug task definition would look like this:

 {    "type":"docker-run",    "label":"docker-run: debug",    "dependsOn": [        "docker-build: debug"    ],    "dockerRun": {        "ports": [            {"hostPort":5000,"containerPort":5000 }        ]    },    "netCore": {        "appProject":"${workspaceFolder}/netcorerest.csproj",        "enableDebugging":true    }

Next steps

You're done! Now that your container is ready, you may want to:

• Learn about debugging .NET in a container
• Customize your Docker build and run tasks
• Push your image to a container registry
• Deploy a containerized app to Azure App Service or Azure Container Apps
• Learn about using Docker Compose