NUnit

Ported from JUnit, NUnit is one of the oldest and most feature-rich frameworks for .NET. Ideally suited for complex test setups and legacy migrations.

When to Use

• Legacy/Enterprise: Large existing codebases often use NUnit.
• Complex Lifecycle: If you really need [OneTimeSetUp], [SetUp], [TearDown] hooks which xUnit discourages.
• Parallelism: Strong parallel execution support ([Parallelizable]).

Quick Start

using NUnit.Framework;

[TestFixture]
public class Tests
{
    [SetUp]
    public void Setup()
    {
    }

    [Test]
    public void Test1()
    {
        Assert.Pass();
    }

    [TestCase(1, 2, 3)]
    [TestCase(2, 2, 4)]
    public void TestAdd(int a, int b, int expected)
    {
       Assert.AreEqual(expected, a + b);
    }
}

Core Concepts

Constraints Model (Assert.That)

NUnit offers a powerful fluent assertion style. Assert.That(result, Is.EqualTo(4)); Assert.That(list, Has.Exactly(1).EqualTo("foo"));

Attributes

• [Test]: Marks a method as a test.
• [TestCase]: Parameterized test input.
• [Category]: Grouping for filtering (dotnet test --filter Category=Integration).

Best Practices (2025)

Do:

• Use the Constraint Model (Assert.That): It enables better error messages than Assert.AreEqual.
• Parallel execution: Enable it in AssemblyInfo.cs to speed up lengthy suites. [assembly: Parallelizable(ParallelScope.Fixtures)].

Don't:

• Don't share state in static fields: NUnit reuses the same test class instance for all tests (unlike xUnit), so dirty static state can leak between tests.

References

• NUnit Documentation