What is Test Driven Development (TDD) and how do you use it in .NET Core?

Test Driven Development (TDD) is a software development process in which tests are written first, before writing the actual code. The idea is to define what the code should do by writing tests that will fail because the code doesn't exist yet, and then write the code to pass those tests. The process is repeated, with additional tests and code, until the desired functionality is achieved.

In .NET Core, TDD can be implemented using a testing framework like xUnit or NUnit. The process typically involves the following steps:

1. Write a test: Write a failing test for a specific behavior or feature.
2. Run the test: Verify that the test fails as expected.
3. Write the code: Write the minimum amount of code necessary to make the test pass.
4. Run the test again: Verify that the test passes.
5. Refactor the code: Clean up the code, ensuring that all tests continue to pass.
6. Repeat: Repeat the process with additional tests until the desired functionality is achieved.

By using TDD, developers can ensure that their code is thoroughly tested, and that new features or changes to existing features do not break existing functionality. It also helps to improve code quality and maintainability by encouraging modular, testable code.