The Enum.HasFlag(Enum) method returns the result of the following Boolean expression.

thisInstance And flag = flag 

If the underlying value of flag is zero, the method returns true. If this behavior is not desirable, you can use the Enum.Equals(object) method to test for equality with zero and call Enum.HasFlag(Enum) only if the underlying value of flag is non-zero, as the following example illustrates.

code reference: System.Enum.HasFlag#1

The Enum.HasFlag(Enum) method is designed to be used with enumeration types that are marked with the FlagsAttribute attribute and can be used to determine whether multiple bit fields are set. For enumeration types that are not marked with the FlagsAttribute attribute, call either the Enum.Equals(object) method or the Enum.CompareTo(object) method.