As your application grows, you'll likely introduce external dependencies. Assuredly, the developers of these systems, applications, and libraries have thoroughly tested their offerings. You should focus your attention on testing your application, not on testing third-party code. Your application should be robust enough to handle edge cases, and you'll want to account for expected and unexpected behavior. You'll want to abstract away the details of the third-party code and test for expected (and unexpected) results.

So, what is third-party code? Anything you didn't write. That includes the .NET Framework itself. One way to achieve the abstraction of third-party code is with the use of test doubles.