TDD in C#: Stop Fearing Tests and Start Writing Code You're Not Ashamed to Show

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Why "tests first" is not masochism, but a superpower

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Imagine: you write code, everything works. You're happy. A month later you return to the project, change one line — and everything breaks. Sound familiar? Welcome to the club of developers who don't write tests. Now imagine another reality: you change the code, run the tests — and within 10 seconds you know nothing is broken. This isn't magic, it's TDD (Test-Driven Development).

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In this article, we'll break down TDD using C# as an example. No boring theory. Only practice, code, and real-world cases. If you think TDD is for "pure architects" or "enterprise monsters" — you'll be surprised. It's a tool that will save your code and your nerves.

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What is TDD and how Red-Green-Refactor works

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TDD is not about testing. It's about design. You don't write tests after the code, but the other way around: first the test, then the minimal code that passes it, and only then refactoring. This cycle is called Red-Green-Refactor:

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Red — write a test that is guaranteed to fail (red).
Green — write the minimal code to make the test pass (green).
Refactor — improve the code without changing its behavior.

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Seems strange? Let's look at an example. Suppose we need a calculator with an addition method. Usually we write the class, then the test. In TDD — it's the opposite.

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Step 1: Red — write a test that will fail

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using NUnit.Framework;\n\n[TestFixture]\npublic class CalculatorTests\n{\n    [Test]\n    public void Add_ShouldReturnSumOfTwoNumbers()\n    {\n        // Arrange\n        var calculator = new Calculator();\n        \n        // Act\n        var result = calculator.Add(2, 3);\n        \n        // Assert\n        Assert.AreEqual(5, result);\n    }\n}

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Run it — the test fails because the Calculator class doesn't exist. Red is normal. Now our task is to make it green.

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Step 2: Green — minimal code to make the test pass

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public class Calculator\n{\n    public int Add(int a, int b)\n    {\n        return a + b; // Minimal implementation\n    }\n}

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The test is green. Hooray. But the code is still ugly. Let's move to step 3.

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Step 3: Refactor — improve without breaking

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At this stage, we can add checks, exception handling, or change the architecture. The main thing is that the tests must remain green.

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public class Calculator\n{\n    public int Add(int a, int b)\n    {\n        if (a < 0 || b < 0)\n            throw new ArgumentException("Only positive numbers allowed");\n        return a + b;\n    }\n}

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Will the test fail? Yes. That means we need to update the test or change the logic. In TDD, refactoring is always a dialogue with the tests.

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