C# Foundations·C# from first file to first API

Testing with xUnit

xUnit turns expectations into repeatable, runnable checks with dotnet test.

Beginner 14 min read Lesson 5 of 300 Challenge included

What you will be able to do

Structure each test as arrange, act, assert.
Use [Fact] for one case and [Theory] with InlineData for many.
Replace slow or external dependencies with test doubles.
Run dotnet test and keep every test green.

In plain English

xUnit is a .NET test framework where each [Fact] marks a single test and [Theory] runs one test across many sets of data. Arrange sets up the inputs, act calls the code, and assert checks the outcome. A test double is a stand-in that replaces a real dependency so tests stay fast and isolated. Run dotnet test from the command line to execute the whole suite.

Step by step

1Create a test project and add the xUnit package.
2Write a [Fact] that asserts a sum.
3Convert it to a [Theory] with InlineData rows.
4Assert that invalid input throws.
5Run dotnet test and read the result.

CalculatorTests.csC#

using Xunit;

public class Calculator
{
    public int Add(int a, int b) => a + b;

    public int Divide(int a, int b)
    {
        if (b == 0) throw new ArgumentException("Cannot divide by zero");
        return a / b;
    }
}

public class CalculatorTests
{
    private readonly Calculator calculator = new Calculator();

    [Fact]
    public void Add_ReturnsSum()
    {
        var result = calculator.Add(2, 3);
        Assert.Equal(5, result);
    }

    [Theory]
    [InlineData(6, 3, 2)]
    [InlineData(9, 3, 3)]
    public void Divide_ReturnsQuotient(int a, int b, int expected)
    {
        Assert.Equal(expected, calculator.Divide(a, b));
    }

    [Fact]
    public void Divide_ByZero_Throws()
    {
        Assert.Throws<ArgumentException>(() => calculator.Divide(1, 0));
    }
}

Common mistakes

A test with no assert passes but verifies nothing.
Tests that touch a database or the network are slow and flaky.
Overusing [Theory] hides which case failed when the name is unclear.
Sharing mutable state between tests breaks isolation and ordering.

Your turn

Write a parameterised test

Easy

Write a [Theory] with two InlineData rows that verifies Add returns the expected sum.

CSHARP · starter

public class CalculatorTests
{
    [Fact]
    public void Add_ReturnsSum()
    {
        var calculator = new Calculator();
        Assert.Equal(5, calculator.Add(2, 3));
    }
}

Check your understanding before moving on.

Knowledge checkQuestion 1

What does [Fact] mark in xUnit?

Select an answer to continue

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