== 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,csharp]
----
[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:

[source,csharp]
----
[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 https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/bitwise-and-shift-operators[bitwise operators] and https://learn.microsoft.com/en-us/dotnet/api/system.enum.hasflag[Enum.HasFlag]:

[source,csharp]
----
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 flags 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,csharp]
----
[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
----

== How to fix it

=== Code examples

==== Noncompliant code example

[source,csharp,diff-id=1,diff-type=noncompliant]
----
[Flags]
enum FruitType
{
    Void = 0,        // Non-Compliant
    Banana = 1,
    Orange = 2,
    Strawberry = 4
}
----

==== Compliant solution

[source,csharp,diff-id=1,diff-type=compliant]
----
[Flags]
enum FruitType
{
    None = 0,        // Compliant
    Banana = 1,
    Orange = 2,
    Strawberry = 4
}
----

== 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/csharp/language-reference/operators/bitwise-and-shift-operators[Bitwise and shift operators (C# reference)]
* 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[]