When is it preferable to apply unit tests during development?

Unit tests are preferably used early in the development process, immediately after writing small parts of code (functions, methods, classes). This helps quickly identify errors and ensure that individual components work correctly. Unit tests are also useful during refactoring and adding new features to ensure that changes do not break existing logic.

Example in C#:

[TestMethod]
public void Add_TwoNumbers_ReturnsSum()
{
    var calculator = new Calculator();
    int result = calculator.Add(2, 3);
    Assert.AreEqual(5, result);
}

This test checks the addition method and can be run automatically whenever the code is changed.