CA1027: Mark enums with FlagsAttribute


For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

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The values of a public enumeration are powers of two or are combinations of other values that are defined in the enumeration, and the System.FlagsAttribute attribute is not present. To reduce false positives, this rule does not report a violation for enumerations that have contiguous values.

An enumeration is a value type that defines a set of related named constants. Apply FlagsAttribute to an enumeration when its named constants can be meaningfully combined. For example, consider an enumeration of the days of the week in an application that keeps track of which day's resources are available. If the availability of each resource is encoded by using the enumeration that has FlagsAttribute present, any combination of days can be represented. Without the attribute, only one day of the week can be represented.

For fields that store combinable enumerations, the individual enumeration values are treated as groups of bits in the field. Therefore, such fields are sometimes referred to as bit fields. To combine enumeration values for storage in a bit field, use the Boolean conditional operators. To test a bit field to determine whether a specific enumeration value is present, use the Boolean logical operators. For a bit field to store and retrieve combined enumeration values correctly, each value that is defined in the enumeration must be a power of two. Unless this is so, the Boolean logical operators will not be able to extract the individual enumeration values that are stored in the field.

To fix a violation of this rule, add FlagsAttribute to the enumeration.

Suppress a warning from this rule if you do not want the enumeration values to be combinable.

In the following example, DaysEnumNeedsFlags is an enumeration that meets the requirements for using FlagsAttribute, but does not have it. The ColorEnumShouldNotHaveFlag enumeration does not have values that are powers of two, but incorrectly specifies FlagsAttribute. This violates rule CA2217: Do not mark enums with FlagsAttribute.

using System;

namespace DesignLibrary
// Violates rule: MarkEnumsWithFlags.

   public enum DaysEnumNeedsFlags 
      None        = 0,
      Monday      = 1,
      Tuesday     = 2,
      Wednesday   = 4,
      Thursday    = 8,
      Friday      = 16,
      All         = Monday| Tuesday | Wednesday | Thursday | Friday
   // Violates rule: DoNotMarkEnumsWithFlags.
   public enum ColorEnumShouldNotHaveFlag 
      None        = 0,
      Red         = 1,
      Orange      = 3,
      Yellow      = 4

CA2217: Do not mark enums with FlagsAttribute