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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? 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 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 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?

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Answer

Define generic (or type-specific) extension methods that encapsulate the bitwise OR (for adding) and AND-with-complement (for removing) operations, giving calling code a clean, readable API instead of repeating raw bitwise expressions everywhere.
public static class PermissionsExtensions {
  public static Permissions AddFlag(this Permissions value, Permissions flag) => value | flag;
  public static Permissions RemoveFlag(this Permissions value, Permissions flag) => value & ~flag;
}

var perms = Permissions.Read;
perms = perms.AddFlag(Permissions.Write);      // Read, Write
perms = perms.RemoveFlag(Permissions.Read);    // Write
Real-world example Providing a clean, discoverable API for manipulating a permissions or feature-flags enum throughout a large codebase.

Common follow-ups: Could this same pattern be made generic to work with ANY [Flags] enum type, not just Permissions specifically?

Extension Methods
All 10 Enums & Flags questions