Fluent Assertions in C# with xUnit – Clean and Readable Unit Tests

Category: Technology
Tags: C#, xUnit, FluentAssertions, Unit Testing, .NET

✅ Introduction

Unit tests are crucial for maintaining healthy and predictable codebases. Traditional assertions like Assert.Equal() in xUnit get the job done, but they can often be less readable, especially when your test logic grows.

This is where Fluent Assertions comes in — it makes your unit test statements more expressive, human-readable, and developer-friendly.

💡 What Is Fluent Assertions?

Fluent Assertions is a popular .NET library that allows you to write unit test assertions in a more natural, fluent syntax. This improves both readability and maintainability of your test code.

📦 NuGet Package: FluentAssertions
🌐 GitHub: https://github.com/fluentassertions/fluentassertions

🔧 Setting Up Fluent Assertions in xUnit

Step 1: Install via NuGet

dotnet add package FluentAssertions

Or through the NuGet Package Manager in Visual Studio.

✍️ Example: Traditional vs Fluent Assertions

Let’s walk through a simple unit test for a method that adds two numbers.

🧪 Method to Test: Calculator.Add()

public class Calculator
{
    public int Add(int a, int b)
    {
        return a + b;
    }
}

🔍 xUnit Test Without Fluent Assertions

public class CalculatorTests
{
    [Fact]
    public void Add_ShouldReturnCorrectSum()
    {
        var calculator = new Calculator();

        var result = calculator.Add(2, 3);

        Assert.Equal(5, result); // Not very descriptive
    }
}

🌟 xUnit Test With Fluent Assertions

using FluentAssertions;

public class CalculatorTests
{
    [Fact]
    public void Add_ShouldReturnCorrectSum()
    {
        var calculator = new Calculator();

        var result = calculator.Add(2, 3);

        result.Should().Be(5); // More readable & expressive
    }
}

🧠 This reads like: “result should be 5” — just like we speak.

✅ Why Use Fluent Assertions?

Benefit	Description
🧠 Readability	result.Should().Be(5) is easier to understand than Assert.Equal(5, result)
🛠 Clear Failure Messages	Fluent Assertions gives better failure output when tests fail
🔁 Consistency	Uniform syntax across test types: objects, strings, exceptions, collections
🚀 Productivity Boost	Helps you write & debug faster

📌 Bonus: Asserting Exceptions

[Fact]
public void Divide_ByZero_ShouldThrowException()
{
    Action act = () => { var result = 10 / 0; };

    act.Should().Throw<DivideByZeroException>();
}

That’s much clearer than wrapping try/catch or using Assert.Throws().

📦 Output When Failing Test

Here’s what you’ll see if your test fails:

Expected result to be 6, but found 5.

Instead of a generic:

Assert.Equal() Failure
Expected: 6
Actual:   5

🧪 Summary

Traditional xUnit	Fluent Assertions Equivalent
Assert.Equal(5, result)	result.Should().Be(5)
Assert.True(condition)	condition.Should().BeTrue()
Assert.NotNull(obj)	obj.Should().NotBeNull()
Assert.Throws<>()	act.Should().Throw<>()

🧭 Final Thoughts

Fluent Assertions is an excellent addition to your unit testing toolbox. It makes your test code easier to read, easier to write, and easier to maintain. For any serious .NET project, it's a must-have library.

🔗 Useful Links

• Official FluentAssertions GitHub Repo