CA1008: Enums should have zero value


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The latest version of this topic can be found at CA1008: Enums should have zero value.

|Breaking Change|Non-breaking - When you are prompted to add a None value to a non-flag enumeration.Breaking - When you are prompted to rename or remove any enumeration values.|

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

The default value of an uninitialized enumeration, just like other value types, is zero. A non-flags−attributed enumeration should define a member that has 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 frequently used member. Note that, by default, if the value of the first enumeration member is not set in the declaration, its value is zero.

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 multiple members that have the value zero occur 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 that has 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 that have a value of zero; this is a breaking change.

Do not suppress 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, that violates the rule.

Imports System

Namespace DesignLibrary

   Public Enum TraceLevel
      Off     = 0
      AnError = 1
      Warning = 2
      Info    = 3
      Verbose = 4
   End Enum

   <Flags> _
   Public Enum TraceOptions
      None         =    0
      CallStack    = &H01
      LogicalStack = &H02
      DateTime     = &H04
      Timestamp    = &H08
   End Enum

   <Flags> _
   Public Enum BadTraceOptions
      CallStack    =    0
      LogicalStack = &H01
      DateTime     = &H02
      Timestamp    = &H04
   End Enum

   Class UseBadTraceOptions

      Shared Sub Main()

         ' Set the flags.
         Dim badOptions As BadTraceOptions = _
            BadTraceOptions.LogicalStack Or BadTraceOptions.Timestamp

         ' Check whether CallStack is set.
         If((badOptions And BadTraceOptions.CallStack) = _
            ' This 'If' statement is always true.
         End If

      End Sub
   End Class

End Namespace

CA2217: Do not mark enums with FlagsAttribute

CA1700: Do not name enum values 'Reserved'

CA1712: Do not prefix enum values with type name

CA1028: Enum storage should be Int32

CA1027: Mark enums with FlagsAttribute