Unit Testing with xUnit

xUnit is the most popular unit testing framework for .NET, favoured by the .NET team itself. Tests are plain classes with no base class requirement. A method decorated with `[Fact]` is a single test case; `[Theory]` combined with `[InlineData]` or `[MemberData]` creates parameterised tests that run with multiple input sets. The Arrange-Act-Assert (AAA) pattern is the standard structure for a unit test: set up prerequisites, invoke the code under test, then verify the outcome. xUnit's `Assert` class provides assertions for equality, nullability, exceptions, collections, and type checking. `IDisposable` and `IAsyncLifetime` interfaces handle per-test setup and teardown. Shared context (e.g. a database container) is handled through class fixtures (`IClassFixture`) and collection fixtures (`ICollectionFixture`), keeping expensive initialisation to a minimum.

Example

// xUnit unit tests with Fact, Theory, and Arrange-Act-Assert

using Xunit;

// Class under test
public static class Calculator
{
    public static int  Add(int a, int b)      => a + b;
    public static int  Subtract(int a, int b) => a - b;
    public static double Divide(double a, double b)
    {
        if (b == 0) throw new DivideByZeroException();
        return a / b;
    }
    public static IEnumerable<int> Range(int start, int count)
        => Enumerable.Range(start, count);
}

public class CalculatorTests
{
    // [Fact] — single test case
    [Fact]
    public void Add_TwoPositiveNumbers_ReturnsSum()
    {
        // Arrange
        int a = 3, b = 4;

        // Act
        int result = Calculator.Add(a, b);

        // Assert
        Assert.Equal(7, result);
    }

    // [Theory] — parameterised test
    [Theory]
    [InlineData(10,  2,   5.0)]
    [InlineData(9,   3,   3.0)]
    [InlineData(-6,  2,  -3.0)]
    public void Divide_ValidInputs_ReturnsCorrectQuotient(double a, double b, double expected)
    {
        double result = Calculator.Divide(a, b);
        Assert.Equal(expected, result, precision: 10);
    }

    [Fact]
    public void Divide_ByZero_ThrowsDivideByZeroException()
    {
        Assert.Throws<DivideByZeroException>(() => Calculator.Divide(5, 0));
    }

    [Fact]
    public void Range_ValidInput_ReturnsCorrectSequence()
    {
        IEnumerable<int> result = Calculator.Range(1, 5);
        Assert.Equal(new[] { 1, 2, 3, 4, 5 }, result);
    }

    [Fact]
    public void Subtract_ResultIsNegative_ReturnsNegativeValue()
    {
        int result = Calculator.Subtract(3, 10);
        Assert.True(result < 0);
        Assert.Equal(-7, result);
    }
}

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