XUnit Testing in .NET - MSDN Tutorials

Introduction to XUnit

XUnit is a free, open-source, extensible unit testing tool for the .NET Framework, .NET Core, and Mono. It is the latest unit testing tool from the creator of NUnit and the initiator of the .NET Foundation.

This tutorial will guide you through setting up and using XUnit for effective unit testing in your .NET projects. We'll cover basic test writing, assertions, and some advanced concepts.

Why Unit Testing?

Improves Code Quality: Helps catch bugs early in the development cycle.
Facilitates Refactoring: Provides confidence that changes haven't broken existing functionality.
Enhances Design: Encourages writing loosely coupled, testable code.
Acts as Documentation: Tests demonstrate how code is intended to be used.

Getting Started with XUnit

Step 1: Project Setup

Create a new .NET project (e.g., a Class Library) for your application code and a separate .NET Core Test project for your tests.

You can use the .NET CLI:

dotnet new classlib -n MyApplication
dotnet new xunit -n MyTests
cd MyTests
dotnet add reference ../MyApplication/MyApplication.csproj

Step 2: Writing Your First Test

Let's assume you have a simple class in MyApplication:

// MyApplication/Calculator.cs
            public class Calculator
            {
                public int Add(int a, int b)
                {
                    return a + b;
                }
            }

Now, write a corresponding test in your MyTests project:

// MyTests/CalculatorTests.cs
            using Xunit;
            using MyApplication;

            public class CalculatorTests
            {
                [Fact]
                public void Add_TwoNumbers_ReturnsCorrectSum()
                {
                    // Arrange
                    var calculator = new Calculator();
                    int a = 5;
                    int b = 10;
                    int expected = 15;

                    // Act
                    int actual = calculator.Add(a, b);

                    // Assert
                    Assert.Equal(expected, actual);
                }
            }

The [Fact] attribute marks a method as a test case. The Arrange-Act-Assert (AAA) pattern is a common way to structure tests.

Step 3: Running Tests

Navigate to your test project directory in the terminal and run:

dotnet test

You should see the test run and pass.

Key XUnit Concepts

Assertions

XUnit provides a rich set of assertion methods in the Assert class. Some common ones include:

Assert.Equal(expected, actual)
Assert.True(condition)
Assert.False(condition)
Assert.NotNull(object)
Assert.Throws<TException>(Action) - For testing exceptions.

Example using Assert.Throws:

[Fact]
public void Divide_ByZero_ThrowsDivideByZeroException()
{
    var calculator = new Calculator();
    Assert.Throws<DivideByZeroException>(() => calculator.Divide(10, 0));
}

Collection Fixtures

For scenarios where you need to share setup and teardown logic across multiple tests within the same test class, you can use collection fixtures.

This is particularly useful for database connections or other expensive resources.

Theory and Data Attributes

[Theory] allows you to run the same test multiple times with different inputs. This is often combined with data attributes like [InlineData] and [MemberData].

public class CalculatorTests
{
    [Theory]
    [InlineData(1, 2, 3)]
    [InlineData(-1, 1, 0)]
    [InlineData(5, 5, 10)]
    public void Add_Theory_ReturnsCorrectSum(int a, int b, int expected)
    {
        var calculator = new Calculator();
        Assert.Equal(expected, calculator.Add(a, b));
    }
}

Best Practices

Keep tests small and focused: Each test should verify a single piece of functionality.
Make tests independent: Tests should not rely on the outcome of other tests.
Use descriptive names: Method names should clearly indicate what is being tested and the expected outcome.
Test edge cases: Consider null values, empty strings, zero, and other boundary conditions.
Avoid testing framework internals: Focus on testing your application logic.

Conclusion

XUnit is a powerful and flexible unit testing framework that can significantly improve the reliability and maintainability of your .NET applications. By following the steps and concepts outlined in this tutorial, you'll be well on your way to writing effective unit tests.

For more advanced topics, refer to the official XUnit documentation.