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Hello World - .NET Core sample

A simple web app written in C# using .NET Core 2.1 that you can use for testing. It reads in an env variable TARGET and prints "Hello ${TARGET}!". If TARGET is not specified, it will use "World" as the TARGET.


  • A Kubernetes cluster with Knative installed. Follow the installation instructions if you need to create one.
  • Docker installed and running on your local machine, and a Docker Hub account configured (we'll use it for a container registry).
  • You have installed .NET Core SDK 2.1.

Recreating the sample code

While you can clone all of the code from this directory, hello world apps are generally more useful if you build them step-by-step. The following instructions recreate the source files from this folder.

  1. From the console, create a new empty web project using the dotnet command:

    dotnet new web -o helloworld-csharp
  2. Update the CreateWebHostBuilder definition in Program.cs by adding .UseUrls() to define the serving port:

    public static IWebHostBuilder CreateWebHostBuilder(string[] args)
        string port = Environment.GetEnvironmentVariable("PORT") ?? "8080";
        string url = String.Concat("", port);
        return WebHost.CreateDefaultBuilder(args)
  3. Update the app.Run(...) statement in Startup.cs to read and return the TARGET environment variable:

    app.Run(async (context) =>
        var target = Environment.GetEnvironmentVariable("TARGET") ?? "World";
        await context.Response.WriteAsync($"Hello {target}\n");
  4. In your project directory, create a file named Dockerfile and copy the code block below into it. For detailed instructions on dockerizing a .NET core app, see dockerizing a .NET core app.

    # Use Microsoft's official .NET image.
    FROM microsoft/dotnet:2.1-sdk
    # Install production dependencies.
    # Copy csproj and restore as distinct layers.
    WORKDIR /app
    COPY *.csproj .
    RUN dotnet restore
    # Copy local code to the container image.
    COPY . .
    # Build a release artifact.
    RUN dotnet publish -c Release -o out
    # Service must listen to $PORT environment variable.
    # This default value facilitates local development.
    ENV PORT 8080
    # Run the web service on container startup.
    CMD ["dotnet", "out/helloworld-csharp.dll"]
  5. Create a new file, service.yaml and copy the following service definition into the file. Make sure to replace {username} with your Docker Hub username.

    kind: Service
      name: helloworld-csharp
      namespace: default
                  - name: TARGET
                    value: "C# Sample v1"

Building and deploying the sample

Once you have recreated the sample code files (or used the files in the sample folder) you're ready to build and deploy the sample app.

  1. Use Docker to build the sample code into a container. To build and push with Docker Hub, run these commands replacing {username} with your Docker Hub username:

    # Build the container on your local machine
    docker build -t {username}/helloworld-csharp .
    # Push the container to docker registry
    docker push {username}/helloworld-csharp
  2. After the build has completed and the container is pushed to docker hub, you can deploy the app into your cluster. Ensure that the container image value in service.yaml matches the container you built in the previous step. Apply the configuration using kubectl:

    kubectl apply --filename service.yaml
  3. Now that your service is created, Knative will perform the following steps:

    • Create a new immutable revision for this version of the app.
    • Network programming to create a route, ingress, service, and load balance for your app.
    • Automatically scale your pods up and down (including to zero active pods).
  4. To find the IP address for your service, use these commands to get the ingress IP for your cluster. If your cluster is new, it may take sometime for the service to get asssigned an external IP address.

    # In Knative 0.2.x and prior versions, the `knative-ingressgateway` service was used instead of `istio-ingressgateway`.
    # The use of `knative-ingressgateway` is deprecated in Knative v0.3.x.
    # Use `istio-ingressgateway` instead, since `knative-ingressgateway`
    # will be removed in Knative v0.4.
    if kubectl get configmap config-istio -n knative-serving &> /dev/null; then
    kubectl get svc $INGRESSGATEWAY --namespace istio-system
    NAME                     TYPE           CLUSTER-IP     EXTERNAL-IP      PORT(S)                                      AGE
    xxxxxxx-ingressgateway   LoadBalancer   80:32380/TCP,443:32390/TCP,32400:32400/TCP   2d
  5. To find the URL for your service, use

    kubectl get ksvc helloworld-csharp,DOMAIN:.status.domain
    NAME                DOMAIN
  6. Now you can make a request to your app to see the result. Replace {IP_ADDRESS} with the address you see returned in the previous step.

    curl -H "Host:" http://{IP_ADDRESS}
    Hello World!

Removing the sample app deployment

To remove the sample app from your cluster, delete the service record:

kubectl delete --filename service.yaml