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Hello world - .NET Core

A simple web app written in C# using .NET Core 3.1 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.

Do the following steps to create the sample code and then deploy the app to your cluster. You can also download a working copy of the sample, by running the following commands:

git clone -b "release-0.26" knative-docs
cd knative-docs/docs/serving/samples/hello-world/helloworld-csharp

Before you begin

  • A Kubernetes cluster with Knative installed and DNS configured. 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 3.1.

Recreating the sample code

  1. First, make sure you have .NET Core SDK 3.1 installed:
dotnet --version
  1. From the console, create a new empty web project using the dotnet command:
dotnet new web -o helloworld-csharp
  1. Update the CreateHostBuilder definition in Program.cs by adding .UseUrls() to define the serving port:
public static IHostBuilder CreateHostBuilder(string[] args)
    string port = Environment.GetEnvironmentVariable("PORT") ?? "8080";
    string url = String.Concat("", port);

   return Host.CreateDefaultBuilder(args)
       .ConfigureWebHostDefaults(webBuilder =>
  1. Update the app.UseEndpoints(...) statement in Startup.cs to read and return the TARGET environment variable:
app.UseEndpoints(endpoints =>
   endpoints.MapGet("/", async context =>
       var target = Environment.GetEnvironmentVariable("TARGET") ?? "World";
       await context.Response.WriteAsync($"Hello {target}!\n");
  1. In your project directory, create a file named Dockerfile and copy the following code block into it. For detailed instructions on dockerizing an ASP.NET Core app, see Docker images for ASP.NET Core.
# Use Microsoft's official build .NET image.
FROM AS build

# Install production dependencies.
# Copy csproj and restore as distinct layers.
COPY *.csproj ./
RUN dotnet restore

# Copy local code to the container image.
COPY . ./

# Build a release artifact.
RUN dotnet publish -c Release -o out

# Use Microsoft's official runtime .NET image.
FROM AS runtime
COPY --from=build /app/out ./

# Run the web service on container startup.
ENTRYPOINT ["dotnet", "helloworld-csharp.dll"]
  1. Create a .dockerignore file to ensure that any files related to a local build do not affect the container that you build for deployment.
  1. 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
        - image:{username}/helloworld-csharp
            - 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
  1. 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
  1. Now that your service is created, Knative will perform the following steps:

  2. Create a new immutable revision for this version of the app.

  3. Network programming to create a route, ingress, service, and load balance for your app.
  4. Automatically scale your pods up and down (including to zero active pods).

  5. To find the URL for your service, use

kubectl get ksvc helloworld-csharp,URL:.status.url
NAME                URL
  1. Now you can make a request to your app and see the result. Replace the following URL with the URL returned in the previous command.
Hello C# Sample v1!

Removing the sample app deployment

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

kubectl delete --filename service.yaml
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