C#  

How to Write Tests in C# in 5 Steps & TDD Explained

Whether you’re a beginner or a seasoned developer, writing tests can often feel like a chore. But testing is more than just checking if your code works, it’s about ensuring quality and preventing future bugs.

Developing tests sounds like an easy task at the beginning, after all, I’m just verifying code I wrote or someone else wrote, and the requirements are clear.
As simple as it looks, later, when you start writing tests, you’ll find out that writing meaningful, maintainable, and isolated tests often reveals flaws in the code’s design, tight coupling, unclear responsibilities, or hidden side effects, making testing not just a verification step, but a design challenge in itself.

Here are 5 common steps i follow to develop my tests fast and assure they respond to all needs.

  • Understand the requirement: What should this method /class /component do?
  • Write a failing test: Start with a unit test that expects the correct behavior; it will fail at first.
  • Write the minimum code to pass the test: No need to overthink, just make it pass.
  • Refactor: Clean up code (naming, structure, duplication) without breaking the test.
  • Repeat: Add more tests, more cases, and gradually evolve your design.

What Is the “Golden Rule” of Assertions?

Now ever heard about this Rule? It’s a life changer: Each test should have a single reason to fail.

Simple and practical, don’t overload tests with multiple Assert statements; keep them focused and descriptive.

Don't start by doing this.

Assert.AreEqual("Alice", user.Name);
Assert.AreEqual(30, user.Age);

Even if your test would pass, two assertions ➜ two reasons to fail.

Add a test case foreach, instead start with this in one test.

Assert.AreEqual("Alice", user.Name);

and in another test.

Assert.AreEqual(30, user.Age);

What Is Test-Driven Development (TDD)?

After starting with some basics, going throw how to develop test cases and the golden rule of assertion, we can clear the path for introducing TDD.

Test-Driven Development is a software development practice that flips the usual coding process on its head. Instead of writing code first and tests later, you write tests before you write the implementation.

TDD follows a simple but powerful cycle often referred to as.

Red → Green → Refactor

🔴 Red: Write a Failing Test.

  •  Write a unit test that defines a desired improvement or new function.
  •  At this point, the test will fail because the feature hasn’t been implemented.

🟢 Green:  Write the Minimum Code to Pass the Test.

  •  Write just enough code to make the test pass.
  •  Don’t worry about making it perfect; get it working first.

🟡 Refactor: Clean the Code Without Breaking It.

  • Now that the test passes, clean your code while ensuring it still passes.
  • Improve readability, remove duplication, and apply patterns or principles.

This approach encourages simpler, testable design and a clear scope of what to build.

Forces you to think through the problem before coding. Bugs are caught early.

Many still has not adapted this approach in their test development process entirely, but still more developers are using a hybrid approach: they write tests before for critical or complex components and after for simpler code or when refactoring, which is a good idea.

Example: TDD for a Simple Calculator

Let’s make the idea more easy and implement the process in an example. Let’s say you want to implement a Calculator.Add() method.

Step 1. Write a failing test.

[TestMethod]
public void Add_ReturnsCorrectSum_ForTwoIntegers()
{
    // Arrange & Act
    var result = Calculator.Add(2, 3);
    // Assert
    Assert.AreEqual(5, result);
}

Step 2. Implement just enough to pass the test.

public static class Calculator
{
    public static int Add(int a, int b)
    {
        return a + b;
    }
}

Step 3. Refactor (nothing to change here, it’s simple and clean).

Then you repeat the process for each new feature: subtraction, multiplication, division, etc.

TDD Is Not Just for Unit Tests

TDD is most effective at the unit level, but it can also guide integration and acceptance testing.

However, the main goal of TDD is to build confidence in your code by making sure every part of your system behaves as expected, right from the start.

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