An enumeration without an applied System.FlagsAttribute does not define a member with a value of zero; or an enumeration with an applied FlagsAttribute defines a member with a value of zero but its name is not 'None', or the enumeration defines multiple zero-valued members.

The default value of an un-initialized enumeration, like other value types, is zero. A non-flags attributed enumeration should define a member with the value of zero so that the default value is a valid value of the enumeration. If appropriate, name the member 'None'. Otherwise, assign zero to the most commonly used member. Note that if the value of the first enumeration member is not set in the declaration, its value is zero by default.

If an enumeration that has the FlagsAttribute applied defines a zero-valued member, its name should be 'None' to indicate that no values have been set in the enumeration. Using a zero-valued member for any other purpose is contrary to the use of the FlagsAttribute in that the AND and OR bitwise operators are useless with the member. This implies that only one member should be assigned the value zero. Note that if there are multiple members with the value zero in a flags-attributed enumeration, Enum.ToString() returns incorrect results for members that are not zero.

To fix a violation of this rule for non-flags attributed enumerations define a member with the value of zero; this is a non-breaking change. For flags-attributed enumerations that define a zero-valued member, name this member 'None' and delete any other members with a value of zero; this is a breaking change.

Do not exclude a warning from this rule except for flags-attributed enumerations that have previously shipped.

The following example shows two enumerations that satisfy the rule and an enumeration, BadTraceOptions, which violates the rule.

using System;

namespace DesignLibrary
{
   public enum TraceLevel
   {
      Off     = 0,
      Error   = 1,
      Warning = 2,
      Info    = 3,
      Verbose = 4
   }

   [Flags]
   public enum TraceOptions
   {
      None         =    0,
      CallStack    = 0x01,
      LogicalStack = 0x02,
      DateTime     = 0x04,
      Timestamp    = 0x08,
   }

   [Flags]
   public enum BadTraceOptions
   {
      CallStack    =    0,
      LogicalStack = 0x01,
      DateTime     = 0x02,
      Timestamp    = 0x04,
   }

   class UseBadTraceOptions
   {
      static void Main()
      {
         // Set the flags.
         BadTraceOptions badOptions = 
            BadTraceOptions.LogicalStack | BadTraceOptions.Timestamp;

         // Check whether CallStack is set.
         if((badOptions & BadTraceOptions.CallStack) == 
             BadTraceOptions.CallStack)
         {
            // This 'if' statement is always true.
         }
      }
   }
}

Do not mark enums with FlagsAttribute

Do not name enum values 'Reserved'

Do not prefix enum values with type name

Enum Storage should be Int32

Mark enums with FlagsAttribute

Reference