Prioritizing unit testing is a best practice in .NET Core and C# development. Unit testing is a software testing method where individual units or components of the software are tested in isolation from the rest of the system. This means that each unit is tested independently to ensure that it works as expected. By prioritizing unit testing, developers can catch bugs early in the development process, which can save time and money in the long run.

Unit testing is useful for several reasons. First, it helps to ensure that the code works as expected. By testing each unit in isolation, developers can catch bugs early in the development process, which can save time and money in the long run. Second, unit testing helps to ensure that changes to the code do not break existing functionality. By running tests after each change, developers can catch any issues that arise and fix them before they become bigger problems. Finally, unit testing helps to improve code quality. By writing tests for each unit, developers are forced to think about how the code should work and how it should be used, which can lead to better code overall.

Here is an example of a unit test in C# using the NUnit testing framework:

[TestFixture]
public class CalculatorTests
{
    [Test]
    public void Add_TwoNumbers_ReturnsCorrectResult()
    {
        // Arrange
        var calculator = new Calculator();

        // Act
        var result = calculator.Add(2, 3);

        // Assert
        Assert.AreEqual(5, result);
    }
}

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

In this example, we have a simple calculator class with an Add method that takes two integers and returns their sum. We also have a unit test that tests the Add method to ensure that it returns the correct result. The test creates a new instance of the calculator class, calls the Add method with two numbers, and then checks that the result is correct using the Assert.AreEqual method. By running this test, we can ensure that the Add method works as expected and that changes to the code do not break existing functionality.