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CA1027: Mark enums with FlagsAttribute

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Property Value
Rule ID CA1027
Title Mark enums with FlagsAttribute
Category Design
Fix is breaking or non-breaking Non-breaking
Enabled by default in .NET 9 No

Cause

The values of an 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.

By default, this rule only looks at externally visible enumerations, but this is configurable.

Rule description

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.

How to fix violations

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

When to suppress warnings

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

Suppress a warning

If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

#pragma warning disable CA1027
// The code that's violating the rule is on this line.
#pragma warning restore CA1027

To disable the rule for a file, folder, or project, set its severity to none in the configuration file.

[*.{cs,vb}]
dotnet_diagnostic.CA1027.severity = none

For more information, see How to suppress code analysis warnings.

Configure code to analyze

Use the following option to configure which parts of your codebase to run this rule on.

You can configure this option for just this rule, for all rules it applies to, or for all rules in this category (Design) that it applies to. For more information, see Code quality rule configuration options.

Include specific API surfaces

You can configure which parts of your codebase to run this rule on, based on their accessibility. For example, to specify that the rule should run only against the non-public API surface, add the following key-value pair to an .editorconfig file in your project:

dotnet_code_quality.CAXXXX.api_surface = private, internal

Example

In the following example, DaysEnumNeedsFlags is an enumeration that meets the requirements for using FlagsAttribute but doesn't 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.

// 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.
[FlagsAttribute]
public enum ColorEnumShouldNotHaveFlag
{
    None = 0,
    Red = 1,
    Orange = 3,
    Yellow = 4
}

See also