How to Use xUnit in C#: Simple Guide with Examples

To use xUnit in C#, install the xunit and xunit.runner.visualstudio NuGet packages, then create test classes with methods marked by the [Fact] attribute. Run tests using Visual Studio Test Explorer or the dotnet test command.

Syntax

In xUnit, you write test methods inside a public class. Each test method is marked with the [Fact] attribute to indicate it is a test. Use assertions like Assert.Equal() to check expected results.

Key parts:

• [Fact]: Marks a method as a test.
• Test method: A public method with no parameters.
• Assertions: Methods to verify test outcomes.

csharp

using Xunit;

public class CalculatorTests
{
    [Fact]
    public void Add_TwoNumbers_ReturnsSum()
    {
        int a = 2;
        int b = 3;
        int result = a + b;
        Assert.Equal(5, result);
    }
}

Example

This example shows a simple test class for a calculator method. It tests that adding two numbers returns the correct sum.

csharp

using Xunit;

public class Calculator
{
    public int Add(int x, int y) => x + y;
}

public class CalculatorTests
{
    [Fact]
    public void Add_ReturnsCorrectSum()
    {
        var calculator = new Calculator();
        int result = calculator.Add(4, 5);
        Assert.Equal(9, result);
    }
}

Output

Test Passed

Common Pitfalls

Common mistakes when using xUnit include:

• Not marking test methods with [Fact], so tests are not discovered.
• Writing test methods with parameters (xUnit requires parameterless [Fact] methods).
• Forgetting to install the xunit.runner.visualstudio package to run tests in Visual Studio.
• Using Assert methods incorrectly, such as mixing expected and actual values.

csharp

using Xunit;

public class WrongTests
{
    // This test will NOT run because it lacks [Fact]
    public void TestWithoutFact()
    {
        Assert.True(true);
    }

    // This test will NOT run because it has parameters
    [Fact]
    public void TestWithParameters(int x)
    {
        Assert.True(x > 0);
    }
}

// Correct usage:
public class CorrectTests
{
    [Fact]
    public void TestRuns()
    {
        Assert.True(true);
    }
}

Quick Reference

• [Fact]: Marks a test method.
• Assert.Equal(expected, actual): Checks if values are equal.
• Assert.True(condition): Checks if condition is true.
• dotnet test: Runs tests from command line.
• Install xunit and xunit.runner.visualstudio NuGet packages to enable testing.

Key Takeaways

Install xUnit and its runner packages to enable testing in C# projects.

Mark test methods with [Fact] and keep them parameterless for discovery.

Use Assert methods to verify expected outcomes in tests.

Run tests using Visual Studio Test Explorer or the dotnet test command.

Avoid common mistakes like missing [Fact] or incorrect Assert usage.