An enum should generally have a zero value. If the enum is not decorated with the Flags attribute, it should have a member ...

An enum should generally have a zero value. If the enum is not decorated with the Flags attribute, it should have a member with a value of zero that represents the empty state. Optionally, this value is named 'None'. For a Flags-attributed enum, a zero-valued member is optional and, if it exists, should always be named 'None'. This value should indicate that no values have been set in the enum. Using a zero-valued member for other purposes is contrary to the use of the Flags attribute in that the bitwise AND and OR operators are useless with the member.