xUnit.net

xUnit.net is the modern, open-source unit testing tool for .NET (C#, F#, VB). It is chosen by Microsoft/dotnet for their own repositories.

When to Use

• .NET Core / .NET 5+: The default template choice.
• Modern Practices: Enforces good habits (Isolation, Constructor Injection for fixtures).

Quick Start

using Xunit;

public class CalculatorTests
{
    [Fact]
    public void PassingTest()
    {
        Assert.Equal(4, Add(2, 2));
    }

    [Theory]
    [InlineData(3)]
    [InlineData(5)]
    [InlineData(6)]
    public void MyTheory(int value)
    {
        Assert.True(IsOdd(value));
    }
}

Core Concepts

Facts vs Theories

• [Fact]: A test that is always true. Invariant.
• [Theory]: A test that is true for a particular set of data (Parameterized).

Fixtures (IClassFixture)

xUnit creates a new instance of the Test Class for every test method (High isolation). To share context (like a DB connection), implement IClassFixture<T>.

Assert

Assert.Equal, Assert.Throws, Assert.Collection.

Best Practices (2025)

Do:

• Use xunit.runner.visualstudio: To run tests in VS/VS Code.
• Use FluentAssertions: xUnit’s asserts are okay, but value.Should().Be(4) (FluentAssertions) is much more readable.
• Constructor Injection: Use the constructor for Setup, and Dispose() (IDisposable) for Teardown. xUnit abolished [SetUp] and [TearDown] attributes to force cleaner design.

Don’t:

• Don’t use Console.WriteLine: Use ITestOutputHelper injected in the constructor to log outputs.

References

• xUnit Documentation