Testing in C#

After this lesson, you will be able to: Write unit tests in C# with xUnit, mock dependencies with Moq, and assert fluently with FluentAssertions.

Testing in C# is mature: xUnit is the modern default, Moq + NSubstitute handle mocking, FluentAssertions reads like English. Real projects always have these.

Prerequisites:Unity intro

xUnit basics

Most popular .NET test framework. Run with `dotnet test`.

tsx

// Create test project: dotnet new xunit -o myapp.Tests
// Add reference: dotnet add reference ../myapp/myapp.csproj

using Xunit;

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

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

    [Fact]
    public void Divide_ByZero_Throws() {
        var calc = new Calculator();
        Assert.Throws<DivideByZeroException>(() => calc.Divide(1, 0));
    }
}

Mocking with Moq

Isolate the unit under test by faking its dependencies.

tsx

using Moq;
using Xunit;

public interface IEmailService {
    Task SendAsync(string to, string subject);
}

public class UserService {
    private readonly IEmailService _email;
    public UserService(IEmailService email) { _email = email; }

    public async Task RegisterAsync(string email) {
        // ... save user
        await _email.SendAsync(email, "Welcome!");
    }
}

public class UserServiceTests {
    [Fact]
    public async Task RegisterAsync_SendsWelcomeEmail() {
        var mockEmail = new Mock<IEmailService>();
        var service = new UserService(mockEmail.Object);

        await service.RegisterAsync("[email protected]");

        mockEmail.Verify(e =>
            e.SendAsync("[email protected]", "Welcome!"),
            Times.Once);
    }
}

FluentAssertions

Reads like English; better error messages.

html

using FluentAssertions;

// Plain Assert
Assert.Equal(5, result);

// FluentAssertions
result.Should().Be(5);
result.Should().BeGreaterThan(0).And.BeLessThan(10);

var users = new[] { "Alex", "Sam" };
users.Should().HaveCount(2);
users.Should().Contain("Alex");
users.Should().BeInAscendingOrder();

Action act = () => calc.Divide(1, 0);
act.Should().Throw<DivideByZeroException>();

// On error:
//   Plain:   Assert.Equal() Failure
//   Fluent:  Expected result to be 5, but found 4.

Common mistakes

Mocking everything → tests pass but don't actually test the system. Asserting on private fields via reflection (extremely brittle). Single-test files with 50 tests (split by behavior). Naming tests `Test1`, `Test2` (use `Method_Scenario_ExpectedResult`).

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