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How would you write an extension method to safely add or remove a specific flag from a [Flags] enum value without manual bitwise operators scattered throughout your code? How do you safely validate that an enum value received from external input (like deserialized JSON or a database) is actually a DEFINED member, given that enums don't enforce this by default? How would you use pattern matching (switch expressions) with an enum, including exhaustiveness warnings from the compiler? How do enums interact with generic constraints, and why has it historically been awkward to write generic, reusable flag-manipulation code?
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How would you design an enum-based state machine using pattern matching to enforce valid state transitions at compile time or runtime?

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Answer

Represent states as an enum, and define an explicit transition-validation method (often using a switch expression matching CURRENT state and DESIRED next state as a tuple) that only allows specific, intentional transitions — throwing or returning false for any transition not explicitly permitted, preventing invalid state changes from silently succeeding.
public enum OrderState { Created, Paid, Shipped, Cancelled }

public bool CanTransition(OrderState from, OrderState to) => (from, to) switch {
  (OrderState.Created, OrderState.Paid) => true,
  (OrderState.Created, OrderState.Cancelled) => true,
  (OrderState.Paid, OrderState.Shipped) => true,
  (OrderState.Paid, OrderState.Cancelled) => true,
  _ => false // every other transition is explicitly disallowed
};
Real-world example Enforcing a valid order lifecycle (Created -> Paid -> Shipped) and rejecting invalid transitions like Shipped -> Created.

Common follow-ups: How would you extend this design to also carry data specific to each state, beyond what a plain enum alone can represent?

Records & Pattern Matching
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