Coding interviews
Reveal answer and reasoning
A timed take-home has a deadline policy that should be testable without waiting thirty real minutes. The sample injects the actual BCL TimeProvider abstraction and substitutes a small owned clock for GetUtcNow. It defines an exclusive deadline: a submission is allowed strictly before the deadline and rejected exactly at it. This is a timestamp decision, not a timer, scheduler or asynchronous cancellation implementation.
Distinguish an absolute submission deadline from a timer that actively cancels work. This policy checks a captured UTC instant whenever Allowed is called. It does not schedule callbacks or interrupt a running calculation, so deterministic timestamp control is sufficient for the current tests without claiming to simulate an asynchronous runtime.
Creating the window and checking it use O(1) time and state. Capturing one deadline avoids repeatedly extending the interval on every query. UTC timestamps simplify this fixture, but a real elapsed-duration budget may require monotonic timestamps and a clear clock-change policy. TimeProvider offers both date/time and timestamp capabilities, but overriding GetUtcNow here does not alter every capability. An elapsed-duration timeout may prefer monotonic timestamp measurement; a published calendar deadline may need an absolute instant.
Select the measurement corresponding to the business requirement before choosing the test double.
What a strong answer covers
- A timed take-home has a deadline policy that should be testable without waiting thirty real minutes.
- The strict comparison makes the equality boundary observable.
- Creating the window and checking it use O(1) time and state.
Common pitfall
A manual GetUtcNow override does not automatically control TimeProvider timers or Task.Delay. Client-supplied timestamps are also not trustworthy authorization evidence. The code neither persists deadlines across restarts nor arbitrates concurrent submissions arriving at a server boundary.
Follow-up questions
- Test zero duration, a negative duration and one tick before the deadline. Hint: equality belongs to the rejected side because the comparison is strictly less than. Avoid adding sleeps to obtain these boundary cases.
- How should an absolute deadline behave when the authoritative wall clock moves backward?
Self-review
Choose what to revisit. These marks are your own assessment and are separate from lesson completion.