Each enum type has a corresponding integral type called the underlying type of the enum type. This underlying type must be able to represent all the enumerator values defined in the enumeration. An enum declaration may explicitly declare an underlying type of byte, sbyte, short, ushort, int, uint, long or ulong. Note that char cannot be used as an underlying type. An enum declaration that does not explicitly declare an underlying type has an underlying type of int.
enum Color: long
declares an enum with an underlying type of long. A developer might choose to use an underlying type of long, as in the example, to enable the use of values that are in the range of long but not in the range of int, or to preserve this option for the future.
An enum-declaration may optionally include a sequence of enum modifiers:
It is a compile-time error for the same modifier to appear multiple times in an enum declaration.
The modifiers of an enum declaration have the same meaning as those of a class declaration (§10.1.1). Note, however, that the abstract and sealed modifiers are not permitted in an enum declaration. Enums cannot be abstract and do not permit derivation.
The body of an enum type declaration defines zero or more enum members, which are the named constants of the enum type. No two enum members can have the same name.
Each enum member has an associated constant value. The type of this value is the underlying type for the containing enum. The constant value for each enum member must be in the range of the underlying type for the enum. The example
enum Color: uint
Red = -1,
Green = -2,
Blue = -3
results in a compile-time error because the constant values -1, -2, and –3 are not in the range of the underlying integral type uint.
Multiple enum members may share the same associated value. The example
Max = Blue
shows an enum in which two enum members—Blue and Max—have the same associated value.
The associated value of an enum member is assigned either implicitly or explicitly. If the declaration of the enum member has a constant-expression initializer, the value of that constant expression, implicitly converted to the underlying type of the enum, is the associated value of the enum member. If the declaration of the enum member has no initializer, its associated value is set implicitly, as follows:
If the enum member is the first enum member declared in the enum type, its associated value is zero.
Otherwise, the associated value of the enum member is obtained by increasing the associated value of the textually preceding enum member by one. This increased value must be within the range of values that can be represented by the underlying type, otherwise a compile-time error occurs.
prints out the enum member names and their associated values. The output is:
Red = 0
Green = 10
Blue = 11
for the following reasons:
the enum member Red is automatically assigned the value zero (since it has no initializer and is the first enum member);
the enum member Green is explicitly given the value 10;
and the enum member Blue is automatically assigned the value one greater than the member that textually precedes it.
The associated value of an enum member may not, directly or indirectly, use the value of its own associated enum member. Other than this circularity restriction, enum member initializers may freely refer to other enum member initializers, regardless of their textual position. Within an enum member initializer, values of other enum members are always treated as having the type of their underlying type, so that casts are not necessary when referring to other enum members.
results in a compile-time error because the declarations of A and B are circular. A depends on B explicitly, and B depends on A implicitly.
Enum members are named and scoped in a manner exactly analogous to fields within classes. The scope of an enum member is the body of its containing enum type. Within that scope, enum members can be referred to by their simple name. From all other code, the name of an enum member must be qualified with the name of its enum type. Enum members do not have any declared accessibility—an enum member is accessible if its containing enum type is accessible.