Why is this an issue?
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
End Enum
allows to define special set of days, such as WeekDays and Weekend using the Or operator:
<Flags()>
Enum Days
' ...
None = 0 ' 0b00000000
Weekdays = Monday Or Tuesday Or Wednesday Or Thursday Or Friday ' 0b00011111
Weekend = Saturday Or Sunday ' 0b01100000
All = Weekdays Or Weekend ' 0b01111111
End Enum
These can be used to write more expressive conditions, taking advantage of bitwise operators and Enum.HasFlag:
Dim someDays = Days.Wednesday | Days.Weekend ' 0b01100100
someDays.HasFlag(Days.Wednesday) ' someDays contains Wednesday
Dim mondayAndWednesday = Days.Monday Or Days.Wednesday
someDays.HasFlag(mondayAndWednesday) ' someDays contains Monday and Wednesday
someDays.HasFlag(Days.Monday) OrElse someDays.HasFlag(Days.Wednesday) ' someDays contains Monday or Wednesday
someDays And Days.Weekend <> Days.None ' someDays overlaps with the weekend
someDays And 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 Or Tuesday Or Wednesday Or Thursday Or Friday
Weekend = Saturday Or Sunday
All = Weekdays Or Weekend
End Enum
Dim someDays = Days.Wednesday Or 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
How to fix it
Code examples
Noncompliant code example
<Flags()>
Enum FruitType
Void = 0 ' Non-Compliant
Banana = 1
Orange = 2
Strawberry = 4
End Enum
Compliant solution
<Flags()>
Enum FruitType
None = 0 ' Compliant
Banana = 1
Orange = 2
Strawberry = 4
End Enum