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Code Analyzer for C#
There is a newer version: 10.2.0.105762
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
}

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

How to fix it
Code examples
Noncompliant code example
[Flags]
enum FruitType
{
    Void = 0,        // Non-Compliant
    Banana = 1,
    Orange = 2,
    Strawberry = 4
}

Compliant solution
[Flags]
enum FruitType
{
    None = 0,        // Compliant
    Banana = 1,
    Orange = 2,
    Strawberry = 4
}

Resources
Documentation

   FlagsAttribute 
   Bit field 
   Bitwise and shift operators
  (C# reference) 
   Enum.HasFlag(Enum) Method 

Articles & blog posts

   Designing Flags Enumerations