How Does the C# [Flags] Attribute Enable Bitwise Operations in Enums?

Leveraging the C# [Flags] Attribute for Bitwise Enum Operations

The [Flags] attribute in C# empowers enumerations to function as flag sets, supporting bitwise combinations.

[Flags] Attribute Functionality:

This attribute enables:

• Bitwise Combinations: Multiple enum values can be combined using bitwise operators (like OR |) to represent several options within a single value.
• Enhanced String Representation: The string representation of a flagged enum displays all active flags, neatly separated by commas.

Illustrative Example:

[Flags]
public enum Options
{
    None = 0,
    OptionA = 1,
    OptionB = 2,
    OptionC = 4,
    OptionD = 8
}

Here, Options acts as a flag set. Combining options is achieved using the bitwise OR operator:

var combinedOptions = Options.OptionB | Options.OptionC; 

combinedOptions will hold the value 6 (2 4), signifying both OptionB and OptionC are active.

Important Considerations:

• Powers of Two: For seamless bitwise operations, enum values must be powers of two (1, 2, 4, 8, etc.).
• None Value: The 0 value signifies no flags are set. Direct use in bitwise AND operations is not recommended.

Identifying Individual Flags:

The HasFlag() method (available from .NET 4 onwards) efficiently checks if a specific flag is present:

if (combinedOptions.HasFlag(Options.OptionB))
{
    // OptionB is set
}

Internal Mechanism:

The [Flags] attribute facilitates the use of enum values as bit flags. Their binary representations underly the bitwise operations and the improved string output.

Further Reading:

• Microsoft Docs: Flags Attribute
• Microsoft Docs: Designing Flags Enums

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