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How do you declare an enum in C#, and what underlying type do its members have by default? How do you convert between an enum value and its underlying numeric value, and between an enum and its string name? How do you create a 'flags' enum that supports combining multiple values using bitwise operations? 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 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? 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 does the [Flags] attribute change the output of ToString() on a combined flags enum value?

Intermediate

Answer

Without [Flags], calling ToString() on a value that doesn't exactly match any single named member just returns the raw numeric value as a string; WITH [Flags], the runtime recognizes the value as a combination and formats it as a comma-separated list of the individual flag names that make it up.
[Flags]
public enum Permissions { None = 0, Read = 1, Write = 2, Delete = 4 }
var combined = Permissions.Read | Permissions.Write;
Console.WriteLine(combined.ToString()); // "Read, Write" -- thanks to [Flags]

// Without [Flags], the same combined value would just print "3"
Real-world example Displaying a human-readable summary of a user's combined permissions in a log message or admin dashboard.

Common follow-ups: How would you use HasFlag() as an alternative, more readable way to check for a specific flag's presence?

String Handling & StringBuilder
All 10 Enums & Flags questions