Explain the principle of operation and purpose of modular tests in the software development process.

Modular tests are automated tests that verify the functionality of small, individual parts of code (modules), such as functions or methods.

Purpose:

• Ensure the correctness of each part of the program independently of others.
• Quickly identify errors early in development.
• Facilitate refactoring by providing confidence that changes do not break existing functionality.

Working principle:

• The test is isolated from other parts of the system.
• It checks a specific scenario or behavior.
• Usually executed automatically during project build.

Example in C# using NUnit:

[Test]
public void Sum_TwoNumbers_ReturnsCorrectResult()
{
    var calculator = new Calculator();
    var result = calculator.Sum(2, 3);
    Assert.AreEqual(5, result);
}