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Enumeration types (C# reference)

• Article
• 09/29/2022
• 3 minutes to read

In this article

An enumeration type (or enum type) is a value type defined by a set of named constants of the underlying integral numeric type. To define an enumeration type, use the enum keyword and specify the names of enum members:

enum Season
{
    Spring,
    Summer,
    Autumn,
    Winter
}

By default, the associated constant values of enum members are of type int; they start with zero and increase by one following the definition text order. You can explicitly specify any other integral numeric type as an underlying type of an enumeration type. You can also explicitly specify the associated constant values, as the following example shows:

enum ErrorCode : ushort
{
    None = 0,
    Unknown = 1,
    ConnectionLost = 100,
    OutlierReading = 200
}

You cannot define a method inside the definition of an enumeration type. To add functionality to an enumeration type, create an extension method.

The default value of an enumeration type E is the value produced by expression (E)0, even if zero doesn't have the corresponding enum member.

You use an enumeration type to represent a choice from a set of mutually exclusive values or a combination of choices. To represent a combination of choices, define an enumeration type as bit flags.

Enumeration types as bit flags

If you want an enumeration type to represent a combination of choices, define enum members for those choices such that an individual choice is a bit field. That is, the associated values of those enum members should be the powers of two. Then, you can use the bitwise logical operators | or & to combine choices or intersect combinations of choices, respectively. To indicate that an enumeration type declares bit fields, apply the Flags attribute to it. As the following example shows, you can also include some typical combinations in the definition of an enumeration type.

[Flags]
public enum Days
{
    None      = 0b_0000_0000,  // 0
    Monday    = 0b_0000_0001,  // 1
    Tuesday   = 0b_0000_0010,  // 2
    Wednesday = 0b_0000_0100,  // 4
    Thursday  = 0b_0000_1000,  // 8
    Friday    = 0b_0001_0000,  // 16
    Saturday  = 0b_0010_0000,  // 32
    Sunday    = 0b_0100_0000,  // 64
    Weekend   = Saturday | Sunday
}

public class FlagsEnumExample
{
    public static void Main()
    {
        Days meetingDays = Days.Monday | Days.Wednesday | Days.Friday;
        Console.WriteLine(meetingDays);
        // Output:
        // Monday, Wednesday, Friday

        Days workingFromHomeDays = Days.Thursday | Days.Friday;
        Console.WriteLine($"Join a meeting by phone on {meetingDays & workingFromHomeDays}");
        // Output:
        // Join a meeting by phone on Friday

        bool isMeetingOnTuesday = (meetingDays & Days.Tuesday) == Days.Tuesday;
        Console.WriteLine($"Is there a meeting on Tuesday: {isMeetingOnTuesday}");
        // Output:
        // Is there a meeting on Tuesday: False

        var a = (Days)37;
        Console.WriteLine(a);
        // Output:
        // Monday, Wednesday, Saturday
    }
}

For more information and examples, see the System.FlagsAttribute API reference page and the Non-exclusive members and the Flags attribute section of the System.Enum API reference page.

The System.Enum type and enum constraint

The System.Enum type is the abstract base class of all enumeration types. It provides a number of methods to get information about an enumeration type and its values. For more information and examples, see the System.Enum API reference page.

You can use System.Enum in a base class constraint (that is known as the enum constraint) to specify that a type parameter is an enumeration type. Any enumeration type also satisfies the struct constraint, which is used to specify that a type parameter is a non-nullable value type.

Conversions

For any enumeration type, there exist explicit conversions between the enumeration type and its underlying integral type. If you cast an enum value to its underlying type, the result is the associated integral value of an enum member.

public enum Season
{
    Spring,
    Summer,
    Autumn,
    Winter
}

public class EnumConversionExample
{
    public static void Main()
    {
        Season a = Season.Autumn;
        Console.WriteLine($"Integral value of {a} is {(int)a}");  // output: Integral value of Autumn is 2

        var b = (Season)1;
        Console.WriteLine(b);  // output: Summer

        var c = (Season)4;
        Console.WriteLine(c);  // output: 4
    }
}

Use the Enum.IsDefined method to determine whether an enumeration type contains an enum member with the certain associated value.

For any enumeration type, there exist boxing and unboxing conversions to and from the System.Enum type, respectively.

C# language specification

For more information, see the following sections of the C# language specification:

• Enums
• Enum values and operations
• Enumeration logical operators
• Enumeration comparison operators
• Explicit enumeration conversions
• Implicit enumeration conversions

See also

• C# reference
• Enumeration format strings
• Design guidelines - Enum design
• Design guidelines - Enum naming conventions
• switch expression
• switch statement

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