Testing in C# (.NET): Frameworks and Tools

Testing is an integral part of professional development on the .NET platform. High-quality test coverage reduces the number of bugs, simplifies refactoring, and increases trust in the product. The C# ecosystem has many mature frameworks and utilities, each solving specific tasks: from unit testing to integration checks and UI tests. In this article, we will review the main tools, their capabilities, and usage examples.

Main Frameworks for Unit Testing

Unit testing is the foundation of the testing pyramid. In .NET Core / .NET 5+, the de facto standard is xUnit.net, although NUnit and MSTest are also actively used.

xUnit.net

A modern, flexible, and most popular framework. It works on the principle of "facts" and "theories".

Facts — simple tests without parameters.
Theories — parameterized tests with data from attributes [InlineData] or [MemberData].
Does not require the [TestClass] attribute — any public class can contain tests.

using Xunit;

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

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

NUnit

A classic framework with a rich history. It features powerful attributes and support for parameterization.

Uses attributes [Test], [TestCase], [SetUp].
Supports parallel test execution.
Convenient for complex scenarios with test data.

using NUnit.Framework;

[TestFixture]public class CalculatorNUnitTests{    [Test]    public void Subtract_ShouldReturnDifference()    {        var calc = new Calculator();        var result = calc.Subtract(10, 4);        Assert.That(result, Is.EqualTo(6));    }

    [TestCase(5, 3, 2)]    [TestCase(0, 0, 0)]    public void Subtract_WithParameters_ShouldReturnCorrect(int a, int b, int expected)    {        var calc = new Calculator();        Assert.That(calc.Subtract(a, b), Is.EqualTo(expected));    }}

MSTest

A built-in framework from Microsoft. Ideal for simple projects and teams working in the Visual Studio ecosystem.

Attributes: [TestMethod], [TestClass], [DataRow].
Tight integration with Azure DevOps and CI/CD pipelines.

using Microsoft.VisualStudio.TestTools.UnitTesting;

[TestClass]public class CalculatorMSTests{    [TestMethod]    public void Multiply_ShouldReturnProduct()    {        var calc = new Calculator();        var result = calc.Multiply(4, 5);        Assert.AreEqual(20, result);    }

    [DataRow(2, 3, 6)]    [DataRow(0, 100, 0)]    [DataTestMethod]    public void Multiply_WithDataRows_ShouldReturnCorrect(int a, int b, int expected)    {        var calc = new Calculator();        Assert.AreEqual(expected, calc.Multiply(a, b));    }}

Libraries for Mock Objects and Isolation

When unit testing, it is often necessary to isolate the code under test from external dependencies (DB, API, file system). Mock frameworks are used for this purpose.