Introduction

Unit testing is a necessary evil in software development. It’s the thing you know you should do but often try to avoid—until, of course, your code explodes in production, and you regret not writing enough tests. Enter modern unit testing tools like Stryker, AutoFixture, SharpFuzz, Moq, NSubstitute, JustMock, and TypeMock, each designed to make unit testing easier, faster, and less soul-crushing.

In this article, we’ll take a deep dive into these frameworks, comparing their strengths, weaknesses, and how they can be combined for the ultimate testing setup.

What is White-Box Testing and Automated Test Generation?

Before we jump into the specifics, let’s cover some basics.

White-Box Testing

White-box testing means testing the internal structure of the code, not just the inputs and outputs. It’s like looking under the hood of a car instead of just checking if the wheels turn.

Automated Test Generation

Wouldn’t it be great if your tests could write themselves? That’s what tools like Pex, IntelliTest, and Stryker try to do—they analyze your code and generate test cases automatically.

Framework Comparison Table

Code Examples for Each Tool

Stryker.NET – Mutation Testing

Mutation testing works by introducing bugs into your code and seeing if your unit tests catch them.

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// Simple example of a test Stryker might check
public int Add(int a, int b) {
    return a + b; // What if we change this to a - b?
}

Stryker will mutate the code and check if your tests detect the change.

AutoFixture – Random Test Data Generation

AutoFixture removes the hassle of manually creating test data.

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var fixture = new Fixture();
var fakePerson = fixture.Create<Person>();

Now you don’t need to manually create test objects!

SharpFuzz – Fuzz Testing

SharpFuzz throws random inputs at your code to see if it breaks.

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public void ProcessInput(string input) {
    if (input.Contains("crash")) throw new Exception("Boom!");
}

SharpFuzz would eventually find "crash" and expose the bug.

Moq – Classic Mocking

Moq is the most popular mocking library.

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var mock = new Mock<IUserService>();
mock.Setup(s => s.GetUser()).Returns(new User { Name = "John" });

NSubstitute – Simpler Alternative to Moq

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var service = Substitute.For<IUserService>();
service.GetUser().Returns(new User { Name = "John" });

JustMock & TypeMock – Mocking Statics

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Mock.SetupStatic(typeof(DateTime), Behavior.Strict);
DateTime.Now.Returns(new DateTime(2025, 1, 1));

Only JustMock and TypeMock allow mocking statics.

How These Tools Relate to Pex and Moles

Back in the day, Pex and Moles were Microsoft’s answer to automated testing and mocking. They have since evolved into IntelliTest and the Fakes Framework.

• Pex → IntelliTest
• Moles → Microsoft Fakes

Some of the ideas from Pex (auto-test generation) and Moles (mocking statics) are still used in modern tools like JustMock and TypeMock.

Pros and Cons of Each Tool

Key Ideas

• Stryker is great for finding weak tests.
• AutoFixture removes the pain of creating test data.
• SharpFuzz is awesome for security and stability testing.
• Moq and NSubstitute make dependency mocking easy.
• JustMock and TypeMock are the only ones that can mock static methods.
• Modern tools borrow ideas from Pex and Moles.

References

1. Stryker.NET Official Docs
2. AutoFixture GitHub
3. SharpFuzz Repository
4. Moq Official Docs
5. NSubstitute Docs
6. JustMock by Telerik
7. TypeMock Official Site