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Code Analyzer for C#

There is a newer version: 10.2.0.105762

Why is this an issue?
Enumerations are commonly used to identify
distinct elements from a set of values.
However, they can also serve as bit flags, enabling bitwise operations to combine multiple
elements within a single value.
// Saturday = 0b00100000, Sunday = 0b01000000, weekend = 0b01100000
var weekend = Days.Saturday | Days.Sunday;  // Combining elements

When enumerations are used as bit flags, it is considered good practice to
annotate the enum type with the FlagsAttribute:
enum Permissions
{
  None = 0,
  Read = 1,
  Write = 2,
  Execute = 4
}

// ...

var x = Permissions.Read | Permissions.Write;  // Noncompliant: enum is not annotated with [Flags]

The FlagsAttribute explicitly marks an enumeration as bit flags, making it clear that it uses bit fields and is intended to be used as
flags.
[Flags]
enum Permissions
{
  None = 0,
  Read = 1,
  Write = 2,
  Execute = 4
}

// ...

var x = Permissions.Read | Permissions.Write;  // Compliant: enum is annotated with [Flags]

Additionally, adding the FlagsAttribute to the enumeration enable a better string representation when using the Enum.ToString method.
Resources
Documentation

   Enumeration in C#
    
       Enumeration
      types as bit flags 
    
  
   FlagsAttribute class 
   Enum.ToString method 
   Bit field - Wikipedia