How do you test classes that use Singleton or static instances?

Short answer: Testing singletons/statics is tricky due to global state. Best practices include: Refactor to use interfaces and DI instead of static/singletons. Wrap static calls behind interfaces so you can mock them. Use specialized mocking tools (e.g., Microsoft Fakes) if refactoring isn't possible. Avoid static state to improve testability.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.