An enumeration (enum) is a value type that inherits directly from System..::.Enum and that supplies alternate names for the values of an underlying primitive type. An enumeration type has a name, an underlying type that must be one of the built-in signed or unsigned integer types (such as Byte, Int32, or UInt64), and a set of fields. The fields are static literal fields, each of which represents a constant. The same value can be assigned to multiple fields. When this occurs, you must mark one of the values as the primary enumeration value for reflection and string conversion.

You can assign a value of the underlying type to an enumeration and vice versa (no cast is required by the runtime). You can create an instance of an enumeration and call the methods of System..::.Enum, as well as any methods defined on the enumeration's underlying type. However, some languages might not let you pass an enumeration as a parameter when an instance of the underlying type is required (or vice versa).

The following additional restrictions apply to enumerations:

The FlagsAttribute attribute denotes a special kind of enumeration called a bit field. The runtime itself does not distinguish between traditional enumerations and bit fields, but your language might do so. When this distinction is made, bitwise operators can be used on bit fields, but not on enumerations, to generate unnamed values. Enumerations are generally used for lists of unique elements, such as days of the week, country or region names, and so on. Bit fields are generally used for lists of qualities or quantities that might occur in combination, such as Red And Big And Fast.