Question

Let’s say I have an enum like this:

[Flags]
public enum NotificationMethodType {
    Email = 1,
    Fax = 2,
    Sms = 4
}

And let’s say I have a variable defined as:

NotificationMethodType types = (NotificationMethodType.Email | NotificationMethodType.Fax)

How do I figure out all of the NotificationMethodType values that are not defined in the “types” variable? In other words:

NotificationMethodType notAssigned = NotificationMethodType <that are not> types

Answer 1

If the list of types never changes, you can do this:

NotificationMethodType allTypes = NotificationMethodType.Email |
    NotificationMethodType.Fax |
    NotificationMethodType.Sms;

NotificationMethodType notAssigned = allTypes & ~types;

The ~ creates an inverse value, by inverting all the bits.

A typical way to define such enums to at least keep the definition of “allTypes” local to the enum would be to include two new names into the enum:

[Flags]
public enum NotificationMethodType {
    None = 0,
    Email = 1,
    Fax = 2,
    Sms = 4,
    All = Email | Fax | Sms
}

Note: If you go the route of adding the All value to the enum, note that if types was empty, you would not get an enum that would print as “Email, Fax, Sms”, but rather as “All”.

If you don’t want to manually maintain the list of allTypes, you can do it using the Enum.GetValues method:

NotificationMethodType allTypes = 0;
foreach (NotificationMethodType type in Enum.GetValues(typeof(NotificationMethodType)))
    allTypes |= type;

or you can do the same with LINQ:

NotificationMethodType allTypes = 
    Enum.GetValues(typeof(NotificationMethodType))
    .Cast<NotificationMethodType>()
    .Aggregate ((current, value) => current | value);

This builds the allType value by OR’ing together all the individual values of the enum.