Chapter 22: Jenkins Language Projects — Java to Python

22.5 .NET — start to end

By Ravindra Bagale · Last updated: 25 October 2026

Why: the problem

Think of the ICICI Bank branch software team. Many Indian banks and insurance companies, like ICICI and LIC vendors, write their backend in C# on .NET. Earlier .NET ran only on Windows, so teams thought Jenkins on Linux could not build it. Today .NET 8 runs on Linux, and the dotnet command does everything: restore libraries, build, test and publish. Publish makes a folder with the app dll and everything it needs. In this lesson you build the badge rule as a small .NET web API with an xUnit test project, run it on Linux, and push it so a Jenkins agent with the .NET SDK, or the official SDK Docker image, does the same.

What we will build

You will create a solution with a web API project and an xUnit test project, run dotnet test and dotnet publish, write a Jenkinsfile that uses the Microsoft SDK image as the agent, publish TRX test results, and smoke test the published app.

What: every term in plain English

• .NET SDK — The tools to build .NET apps: the dotnet command.
• Solution (.sln) — A file that groups projects, like an app and its tests.
• Project (.csproj) — One app or library and its settings.
• NuGet — The .NET library store, like Maven Central.
• dotnet restore — Downloads NuGet packages.
• xUnit — A common .NET test framework.
• dotnet publish — Makes a ready-to-run folder.
• TRX — The .NET test result file format.

How: step by step

1. Make the solution

   mkdir -p ~/devops-lab/ch22/badge-dotnet && cd ~/devops-lab/ch22/badge-dotnet && dotnet new sln -n Badge
   

   What it does: Groups projects.

   Output

   The template "Solution File" was created successfully.
   

2. Web API project

   dotnet new web -n Badge.Api -o src/Badge.Api && dotnet sln add src/Badge.Api
   

   What it does: Minimal API.

   Output

   Project `src/Badge.Api/Badge.Api.csproj` added to the solution.
   

3. Test project

   dotnet new xunit -n Badge.Tests -o tests/Badge.Tests && dotnet sln add tests/Badge.Tests && dotnet add tests/Badge.Tests reference src/Badge.Api
   

   What it does: Tests can see the app code.

   Output

   Reference `..\..\src\Badge.Api\Badge.Api.csproj` added to the project.
   

4. Write the code

   Replace Program.cs and add BadgeRules.cs and BadgeTests.cs from Example 1
   

   What it does: Our rule.

   Output

   (no output)
   

5. Test

   dotnet test
   

   What it does: Restore, build, test.

   Output

   Passed!  - Failed: 0, Passed: 3, Skipped: 0, Total: 3
   

6. Publish

   dotnet publish src/Badge.Api -c Release -o out
   

   What it does: Ready folder.

   Output

   Badge.Api -> /home/ravi/devops-lab/ch22/badge-dotnet/out/
   

7. Run

   dotnet out/Badge.Api.dll --urls http://localhost:5080 & sleep 3; curl -s "localhost:5080/badge?points=60"
   

   What it does: It answers.

   Output

   {"points":60,"level":"silver"}
   

Example 1: The C# code and tests

Stack. .NET 8, C#, xUnit

Goal. The C# code and tests.

src/Badge.Api/BadgeRules.cs

namespace Badge.Api;

public static class BadgeRules
{
    public static string Level(int points) =>
        points >= 100 ? "gold" : points >= 50 ? "silver" : "bronze";
}

src/Badge.Api/Program.cs

using Badge.Api;

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

app.MapGet("/health", () => Results.Ok(new { status = "UP" }));
app.MapGet("/badge", (int points) => new { points, level = BadgeRules.Level(points) });

app.Run();

tests/Badge.Tests/BadgeTests.cs

using Badge.Api;
using Xunit;

public class BadgeTests
{
    [Theory]
    [InlineData(100, "gold")]
    [InlineData(50, "silver")]
    [InlineData(0, "bronze")]
    public void Level_is_right(int points, string expected) =>
        Assert.Equal(expected, BadgeRules.Level(points));
}

What each line does

• static class BadgeRules — Pure rule, easy to test.
• MapGet("/badge") — GET /badge returns JSON.
• /health — For smoke tests and load balancers.
• [Theory] + [InlineData] — One test, three inputs.
• Assert.Equal — Fails if values differ.

Green output

Passed!  - Failed:     0, Passed:     3, Skipped:     0, Total:     3, Duration: 12 ms - Badge.Tests.dll (net8.0)

Expected result. Three test cases pass on Linux.

Example 2: Jenkinsfile with the SDK image as agent

Stack. Jenkins, Docker, .NET

Goal. Jenkinsfile with the SDK image as agent.

Jenkinsfile

pipeline {
  agent {
    docker {
      image 'mcr.microsoft.com/dotnet/sdk:8.0'
      args '-v $HOME/.nuget:/root/.nuget -e DOTNET_CLI_HOME=/tmp -e HOME=/tmp'
    }
  }
  options { timestamps() }
  environment { DOTNET_CLI_TELEMETRY_OPTOUT = '1'; DOTNET_NOLOGO = '1' }
  stages {
    stage('Restore') { steps { sh 'dotnet restore' } }
    stage('Build')   { steps { sh 'dotnet build -c Release --no-restore' } }
    stage('Test')    { steps { sh 'dotnet test -c Release --no-build --logger "trx;LogFileName=results.trx" --results-directory TestResults' } }
    stage('Publish') { steps { sh 'dotnet publish src/Badge.Api -c Release --no-restore -o out' } }
  }
  post {
    always  { mstest testResultsFile: 'TestResults/*.trx' }
    success { archiveArtifacts 'out/**' }
  }
}

What each line does

• sdk:8.0 — Official image; no .NET install on the agent.
• .nuget mount — Cache packages.
• HOME=/tmp — The container user can write its cache.
• --no-restore / --no-build — Do not repeat earlier work.
• --logger trx — Write test results.
• mstest — Plugin step that reads TRX files.

Green output

[Pipeline] stage (Test)
Passed!  - Failed: 0, Passed: 3, Skipped: 0, Total: 3
[Pipeline] stage (Publish)
Badge.Api -> /var/jenkins_home/workspace/badge-dotnet-pipeline/out/
Archiving artifacts
Finished: SUCCESS

Expected result. Green build with test results and a downloadable publish folder.

Example 3: Smoke test the published app in CI

Stack. Jenkins, .NET

Goal. Smoke test the published app in CI.

stage

stage('Smoke') {
  steps {
    sh 'nohup dotnet out/Badge.Api.dll --urls http://localhost:15080 > app.log 2>&1 & echo $! > app.pid'
    sh 'for i in $(seq 1 20); do curl -fs localhost:15080/health && break; sleep 1; done'
    sh 'curl -fs "localhost:15080/badge?points=120" | grep gold'
    sh 'kill $(cat app.pid)'
  }
}

What each line does

• --urls — Choose the port.
• wait loop — App needs a second or two.
• grep gold — Fail if the answer is wrong.

Green output

{"status":"UP"}
{"points":120,"level":"gold"}

Expected result. The publish folder really runs.

Example 4: A small runtime image for production

Stack. Docker, .NET

Goal. A small runtime image for production.

Dockerfile

FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
WORKDIR /src
COPY . .
RUN dotnet publish src/Badge.Api -c Release -o /out

FROM mcr.microsoft.com/dotnet/aspnet:8.0
WORKDIR /app
COPY --from=build /out .
USER app
ENV ASPNETCORE_URLS=http://+:8080
EXPOSE 8080
ENTRYPOINT ["dotnet", "Badge.Api.dll"]

What each line does

• AS build — First stage has the SDK.
• aspnet:8.0 — Runtime only, much smaller.
• USER app — Non-root user that the image already has.
• ASPNETCORE_URLS — Listen on 8080 inside the container.

Green output

docker build -t badge-dotnet:1 .
docker images badge-dotnet
badge-dotnet   1   9a1b...   222MB

Expected result. SDK image is about 850 MB; the final image is about 220 MB.

Verify: did it work?

1. Tests pass

   dotnet test --nologo -v q
   

   What it does: All green.

   Output

   Passed!  - Failed: 0, Passed: 3
   

2. Publish folder

   ls out | grep Badge.Api.dll
   

   What it does: App dll.

   Output

   Badge.Api.dll
   

3. TRX exists

   ls TestResults
   

   What it does: For Jenkins.

   Output

   results.trx
   

When it is red 1

dotnet: command not found

Cause. No SDK on the agent.

Fix, step by step

1. Use the mcr.microsoft.com/dotnet/sdk:8.0 Docker agent.
2. Or install with apt install dotnet-sdk-8.0 on Ubuntu 24.04.

When it is red 2

Access to the path '/.dotnet' is denied

Cause. Container user has no home folder.

Fix, step by step

1. Set HOME=/tmp and DOTNET_CLI_HOME=/tmp in args.

When it is red 3

error NU1301: Unable to load the service index for source https://api.nuget.org/v3/index.json

Cause. No internet or proxy to NuGet.

Fix, step by step

1. Check the proxy.
2. Add a company feed in nuget.config.

When it is red 4

error NETSDK1045: The current .NET SDK does not support targeting .NET 9.0

Cause. Project targets a newer .NET than the SDK image.

Fix, step by step

1. Use sdk:9.0, or set TargetFramework to net8.0.

Cleanup

1. Stop app and clean

   kill %1 2>/dev/null; rm -rf out TestResults
   

   What it does: Free the port.

   Output

   (no output)
   

Practice tasks

Homework: try at home

Recap

• .NET 8 runs on Linux.
• dotnet does restore, build, test, publish.
• Use the SDK image as agent.
• Publish TRX results.
• Ship the aspnet runtime image.

FAQ

Do I need Windows agents for .NET?

Not for .NET 6 and newer. Old .NET Framework 4.x still needs Windows.

Where do NuGet packages come from?

nuget.org or a company feed in nuget.config.

What is Release vs Debug?

Release is optimised for production.

Can I use MSBuild directly?

Yes, but dotnet build calls it for you.

How do I store connection strings?

Environment variables from Jenkins credentials or a secret manager.

Is Visual Studio needed on the agent?

No. Only the SDK.

Interview questions

1. How do you build .NET in Linux CI?

   Use the dotnet SDK, restore, build, test with a logger, publish.

2. Difference between sdk and aspnet images?

   sdk builds; aspnet only runs.

3. How do you show .NET tests in Jenkins?

   TRX logger plus the mstest or xunit plugin.

4. What does NETSDK1045 mean?

   SDK older than the target framework.

5. How do you cache NuGet in CI?

   Mount or cache ~/.nuget/packages.

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