rspec/rules/S2346/vbnet/rule.adoc

== Why is this an issue?

An enumeration can be decorated with the https://learn.microsoft.com/en-us/dotnet/api/system.flagsattribute[FlagsAttribute] to indicate that it can be used as a https://en.wikipedia.org/wiki/Bit_field[bit field]: a set of flags, that can be independently set and reset.

For example, the following definition of the day of the week:

[source,vbnet]
----
<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:

[source,vbnet]
----
<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 https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/operators-and-expressions/logical-and-bitwise-operators#bitwise-operations[bitwise operators] and https://learn.microsoft.com/en-us/dotnet/api/system.enum.hasflag[Enum.HasFlag]:

[source,vbnet]
----
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.

[source,vbnet]
----
<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

[source,vbnet,diff-id=1,diff-type=noncompliant]
----
<Flags()>
Enum FruitType
    Void = 0        ' Non-Compliant
    Banana = 1
    Orange = 2
    Strawberry = 4
End Enum
----

==== Compliant solution

[source,vbnet,diff-id=1,diff-type=compliant]
----
<Flags()>
Enum FruitType
    None = 0        ' Compliant
    Banana = 1
    Orange = 2
    Strawberry = 4
End Enum
----

== Resources

=== Documentation

* https://learn.microsoft.com/en-us/dotnet/api/system.flagsattribute[FlagsAttribute]
* https://en.wikipedia.org/wiki/Bit_field[Bit field]
* https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/operators-and-expressions/logical-and-bitwise-operators#bitwise-operations[Logical and Bitwise Operators in Visual Basic]
* https://learn.microsoft.com/en-us/dotnet/api/system.enum.hasflag[Enum.HasFlag(Enum) Method]

=== Articles & blog posts

* https://learn.microsoft.com/en-us/previous-versions/dotnet/netframework-4.0/ms229062(v=vs.100)[Designing Flags Enumerations]

include::../rspecator.adoc[]