Unit testing C# with NUnit and .NET Core

This tutorial takes you through an interactive experience building a sample solution step-by-step to learn unit testing concepts. If you prefer to follow the tutorial using a pre-built solution, view or download the sample code before you begin. For download instructions, see Samples and Tutorials.

Prerequisites

Creating the source project

Open a shell window. Create a directory called unit-testing-using-nunit to hold the solution. Inside this new directory, run the following command to create a new solution file for the class library and the test project:

dotnet new sln

Next, create a PrimeService directory. The following outline shows the directory and file structure so far:

/unit-testing-using-nunit
    unit-testing-using-nunit.sln
    /PrimeService

Make PrimeService the current directory and run the following command to create the source project:

dotnet new classlib

Rename Class1.cs to PrimeService.cs. You create a failing implementation of the PrimeService class:

using System;

namespace Prime.Services
{
    public class PrimeService
    {
        public bool IsPrime(int candidate)
        {
            throw new NotImplementedException("Please create a test first.");
        }
    }
}

Change the directory back to the unit-testing-using-nunit directory. Run the following command to add the class library project to the solution:

dotnet sln add PrimeService/PrimeService.csproj

Creating the test project

Next, create the PrimeService.Tests directory. The following outline shows the directory structure:

/unit-testing-using-nunit
    unit-testing-using-nunit.sln
    /PrimeService
        Source Files
        PrimeService.csproj
    /PrimeService.Tests

Make the PrimeService.Tests directory the current directory and create a new project using the following command:

dotnet new nunit

The dotnet new command creates a test project that uses NUnit as the test library. The generated template configures the test runner in the PrimeService.Tests.csproj file:

<ItemGroup>
  <PackageReference Include="nunit" Version="3.10.1" />
  <PackageReference Include="NUnit3TestAdapter" Version="3.10.0" />
  <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.8.0" />
</ItemGroup>

The test project requires other packages to create and run unit tests. dotnet new in the previous step added the Microsoft test SDK, the NUnit test framework, and the NUnit test adapter. Now, add the PrimeService class library as another dependency to the project. Use the dotnet add reference command:

dotnet add reference ../PrimeService/PrimeService.csproj

You can see the entire file in the samples repository on GitHub.

The following outline shows the final solution layout:

/unit-testing-using-nunit
    unit-testing-using-nunit.sln
    /PrimeService
        Source Files
        PrimeService.csproj
    /PrimeService.Tests
        Test Source Files
        PrimeService.Tests.csproj

Execute the following command in the unit-testing-using-nunit directory:

dotnet sln add ./PrimeService.Tests/PrimeService.Tests.csproj

Creating the first test

You write one failing test, make it pass, then repeat the process. In the PrimeService.Tests directory, rename the UnitTest1.cs file to PrimeService_IsPrimeShould.cs and replace its entire contents with the following code:

using NUnit.Framework;
using Prime.Services;

namespace Prime.UnitTests.Services
{
    [TestFixture]
    public class PrimeService_IsPrimeShould
    {
        [Test]
        public void IsPrime_InputIs1_ReturnFalse()
        {
            PrimeService primeService = CreatePrimeService();
            var result = primeService.IsPrime(1);

            Assert.IsFalse(result, "1 should not be prime");
        }
        
        /*
        More tests
        */
        
        private PrimeService CreatePrimeService()
        {
             return new PrimeService();
        }
    }
}

The [TestFixture] attribute denotes a class that contains unit tests. The [Test] attribute indicates a method is a test method.

Save this file and execute dotnet test to build the tests and the class library and then run the tests. The NUnit test runner contains the program entry point to run your tests. dotnet test starts the test runner using the unit test project you've created.

Your test fails. You haven't created the implementation yet. Make this test pass by writing the simplest code in the PrimeService class that works:

public bool IsPrime(int candidate)
{
    if (candidate == 1)
    {
        return false;
    }
    throw new NotImplementedException("Please create a test first.");
}

In the unit-testing-using-nunit directory, run dotnet test again. The dotnet test command runs a build for the PrimeService project and then for the PrimeService.Tests project. After building both projects, it runs this single test. It passes.

Adding more features

Now that you've made one test pass, it's time to write more. There are a few other simple cases for prime numbers: 0, -1. You could add new tests with the [Test] attribute, but that quickly becomes tedious. There are other NUnit attributes that enable you to write a suite of similar tests. A [TestCase] attribute is used to create a suite of tests that execute the same code but have different input arguments. You can use the [TestCase] attribute to specify values for those inputs.

Instead of creating new tests, apply this attribute to create a single data driven test. The data driven test is a method that tests several values less than two, which is the lowest prime number:

[TestCase(-1)]
[TestCase(0)]
[TestCase(1)]
public void IsPrime_ValuesLessThan2_ReturnFalse(int value)
{
    var result = _primeService.IsPrime(value);

    Assert.IsFalse(result, $"{value} should not be prime");
}

Run dotnet test, and two of these tests fail. To make all of the tests pass, change the if clause at the beginning of the Main method in the PrimeService.cs file:

if (candidate < 2)

Continue to iterate by adding more tests, more theories, and more code in the main library. You have the finished version of the tests and the complete implementation of the library.

You've built a small library and a set of unit tests for that library. You've structured the solution so that adding new packages and tests is part of the normal workflow. You've concentrated most of your time and effort on solving the goals of the application.