Kubernetes ❤️ dotnet run app.cs

.NET has had several scripting capabilities over the years, but never one ‘built in’. Until now that is! With .NET 10, C# move closer to things like Python, JavaScript or PowerShell. No (visible) csproj or dlls.

Lets try it!

echo "Console.WriteLine(Environment.Version);" > app.cs
dotnet run app.cs

It’s like C# but with a sprinkle of Python!

You can even skip the file and pipe directly from stdin

echo 'Console.WriteLine(Environment.Version);' | dotnet run -

Or like this

dotnet run - <<EOF
Console.WriteLine("first");
Console.WriteLine("second");
EOF

Want to build a quick small cli tool? Start with a ‘shebang’ #!/usr/bin/dotnet run and you can execute the .cs file directly. On Linux that is, on Windows it is ignored.

To use a different SDK such as ASP.Net, just add #:sdk Microsoft.NET.Sdk.Web

And to reference nuget packages you can use the #:package directive like below

#:package Figgle.Fonts@0.6.5
Console.WriteLine(Figgle.Fonts.FiggleFonts.Slant.Render("soapfault.com"));

Use cases?

I’m sure I’ll use this both in CI/CD pipeline scenarios but also in Kubernetes.

Kubernetes

In Kubernetes I have often leveraged the power of dotnet’s SDK image for debug and troubleshooting. One way was to mount both a ‘.csproj’ and a ‘.cs’ from a configmap and then do a ‘dotnet run’ in the entrypoint.

We can now simplify that even more by just piping a ‘.cs’ file in the entrypoint.

Lets deploy a job

---
apiVersion: batch/v1
kind: Job
metadata:
  name: net10-inline-job
spec:
  template:
    spec:
      restartPolicy: Never
      containers:
        - name: app
          image: mcr.microsoft.com/dotnet/sdk:10.0
          command: ["/bin/bash", "-c"]
          args:
            - |
              dotnet run - <<EOF
              #:package Figgle.Fonts@0.6.5
              Console.WriteLine(Figgle.Fonts.FiggleFonts.Slant.Render(Environment.MachineName));
              EOF

If we apply the manifest and look in the log

We can take this one step further and deploy a ASP.Net minimal api.

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: net10-inline
spec:
  replicas: 1
  selector:
    matchLabels:
      app: net10-inline
  template:
    metadata:
      labels:
        app: net10-inline
    spec:
      containers:
        - name: app
          image: mcr.microsoft.com/dotnet/sdk:10.0
          command: ["/bin/bash", "-c"]
          args:
            - |
              dotnet run - <<EOF
              #:sdk Microsoft.NET.Sdk.Web
              #:property PublishTrimmed=false

              var builder = WebApplication.CreateBuilder(args);
              var app = builder.Build();

              app.MapGet("/troubleshoot", () =>
                new { Server = Environment.MachineName });

              app.Run();
              EOF

          ports:
            - containerPort: 8080

We now have the API running, lets call it.

All in all this is a really useful feature!