Ask HN: How to test if a candidate knows a programming language in an interview?

I am about to start interviewing candidates for a position that involves expanding an in-house analytics platform written by a developer who is leaving the company. The code is well-documented and under source control, the build process is very straightforward, deployments are not complex, and there is an extensive data dictionary that documents all aspects of the database. Everything has been written and documented solely by the developer who is leaving, and this developer will be one member of the team who is interviewing potential candidates.

Everything is written in C# and the .NET Framework, and my team and I are struggling with the question of how to assess a candidate's understanding of the language and framework. While someone could list vast industry experience on their resume, this isn't a guarantee that s/he can understand and expand upon the code that is already written. I am looking for something a little more in-depth than a typical FizzBuzz test, but also want a more interactive interview than simply quizzing the candidate on the language features most-used in the codebase. One of my colleagues had an idea to provide the candidate with some example code and have a discussion about that, including how that code would fit into a larger application and what aspects of the code the candidate would change, but writing a fresh piece of code is time consuming, and we don't want to spend a lot of time explaining non-programming concepts if the candidates don't have a solid knowledge of our domain (finance).

Does anyone have any suggestions?