An answer that may not exist in C#
C# interview exercise on Exceptions & Disposal, difficulty 1 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write Average(nums) that returns the mean, or null for an empty list. LINQ's Average() throws InvalidOperationException on an empty sequence; decide before it happens.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.