Bitwise Enum No Flags

ID

csharp.bitwise_enum_no_flags

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

CSharp

Tags

enum, reliability

Description

Reports |, & and ^ bitwise expressions whose operands resolve to an enum that does not carry the [Flags] attribute. Bitwise composition only makes sense for an enum specifically designed as a bit set; on a plain enum the result is a value that does not correspond to any declared member and that ToString() cannot round-trip.

Rationale

The [Flags] attribute documents the intent ("this enum is a bit set") and changes the behaviour of ToString() and the HasFlag machinery. Composing values of a plain enum with | or & silently produces out-of-range numeric values that defeat exhaustive switches and serialize as a meaningless integer.

public enum Color { Red = 1, Green = 2, Blue = 4 }

[Flags]
public enum Permission { Read = 1, Write = 2, Execute = 4 }

public class Sample
{
    public Color Mix()
    {
        return Color.Red | Color.Green;              // FLAW — Color lacks [Flags]
    }

    public bool HasAny(Color a, Color b)
    {
        return (a & b) != 0;                         // FLAW — same enum without [Flags]
    }

    public Permission FullAccess()
    {
        return Permission.Read | Permission.Write;   // OK — [Flags] enum
    }
}

Remediation

Either add [Flags] to the enum and renumber its members as powers of two, or stop composing values of an enum that is not meant to be a bit set. If the bitwise composition is part of a larger expression involving non-enum integers, cast the enum to int to make the integer arithmetic explicit.

References