An enumeration can be decorated with the FlagsAttribute to
indicate that it can be used as a bit field: a set of flags, that can be independently set and
reset.
For example, the following definition of the day of the week:
[Flags]
enum Days
{
Monday = 1, // 0b00000001
Tuesday = 2, // 0b00000010
Wednesday = 4, // 0b00000100
Thursday = 8, // 0b00001000
Friday = 16, // 0b00010000
Saturday = 32, // 0b00100000
Sunday = 64 // 0b01000000
}
allows to define special set of days, such as WeekDays and Weekend using the | operator:
[Flags]
enum Days
{
// ...
None = 0, // 0b00000000
Weekdays = Monday | Tuesday | Wednesday | Thursday | Friday, // 0b00011111
Weekend = Saturday | Sunday, // 0b01100000
All = Weekdays | Weekend // 0b01111111
}
These can be used to write more expressive conditions, taking advantage of bitwise operators and Enum.HasFlag:
var someDays = Days.Wednesday | Days.Weekend; // 0b01100100
someDays.HasFlag(Days.Wednesday); // someDays contains Wednesday
var mondayAndWednesday = Days.Monday | Days.Wednesday;
someDays.HasFlag(mondayAndWednesday); // someDays contains Monday and Wednesday
someDays.HasFlag(Days.Monday) || someDays.HasFlag(Days.Wednesday); // someDays contains Monday or Wednesday
someDays & Days.Weekend != Days.None; // someDays overlaps with the weekend
someDays & Days.Weekdays == Days.Weekdays; // someDays is only made of weekdays
Consistent use of None in flag enumerations indicates that all flag values are cleared. The value 0 should not be used to indicate any
other state since there is no way to check that the bit 0 is set.
[Flags]
enum Days
{
Monday = 0, // 0 is used to indicate Monday
Tuesday = 1,
Wednesday = 2,
Thursday = 4,
Friday = 8,
Saturday = 16,
Sunday = 32,
Weekdays = Monday | Tuesday | Wednesday | Thursday | Friday,
Weekend = Saturday | Sunday,
All = Weekdays | Weekend
}
var someDays = Days.Wednesday | Days.Thursday;
someDays & Days.Tuesday == Days.Tuesday // False, because someDays doesn't contains Tuesday
someDays & Days.Monday == Days.Monday // True, even though someDays doesn't contains Monday!
someDays.HasFlag(Days.Monday) // Same issue as above
