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How do you create a 'flags' enum that supports combining multiple values using bitwise operations? How does the [Flags] attribute change the output of ToString() on a combined flags enum value? What is the performance consideration when using Enum.HasFlag() versus a direct bitwise AND check? 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 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? 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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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?

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Answer

Since C# enums are just named integers, a value like (Permissions)999 compiles and runs fine even though it doesn't correspond to any defined member (or valid flag combination) — use Enum.IsDefined() for simple enums, or a manual bitwise check against a computed 'all valid flags' mask for [Flags] enums, to validate untrusted input before trusting it.
public enum Status { Active = 0, Inactive = 1 }
int rawValue = 99; // e.g., from untrusted deserialized JSON
var status = (Status)rawValue; // compiles and runs, but is INVALID
bool isValid = Enum.IsDefined(typeof(Status), status); // false -- correctly caught
Real-world example Validating a status or category field parsed from external API input before it's trusted and used in business logic.

Common follow-ups: Why doesn't Enum.IsDefined() work correctly out of the box for validating combined [Flags] enum values?

Exception Handling
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