Enum with FlagsAttribute or IEnumerable<Enum>/ISet<T>

Question

Which is the currently-accepted best practice in terms of C# enum usage, a bitwise [Flags] Enum (compared using | and &) or an IEnumerable of enum values? I'm thinking both from a code-conciseness perspective and from a maintenance perspective.

The only downside to using the Flags approach that I can think of is the inability to enumerate the values without doing the reverse enumeration:

var possibleValues = Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>();
var applicableValues = possibleValues.Where(x => (containerClass.EnumProperty & x) == x);

Whereas obviously the IEnumerable is easier to query.

EDIT:

As svick rightly pointed out, the IEnumerable would be perhaps the wrong collection to use, an ISet might be a better example.

• Oded: What would be the purpose of an IEnumerable of enum values? What do you intend to use the Enum for?
• Ed James: To hold a collection of attributes about the container class (the concrete example being attributes about a product, such as "Price Marked" (i.e. has the price printed on the pack)). The options are pre-set, making it a good case for an enum.

Answer 1

You can create a method that encapsulates enumerating the flags. This way, using flags is almost as easy as an IEnumerable<T>.

One reason why I would prefer flags is what those two approaches mean. With flags, you have a set. With IEnumerable<T>, you have a sequence. That means you have another edge case you have to consider: What if the sequence contains some item more than once?

• Ed James: Yes, I'd definitely not repeat the code I posted multiple times (it would work nicely as an extension method). I'm not sure the "more than once" argument is more than an aside, however, as in the situation I described, i.e. dealing with attributes, it would simply not be relevant if the number of matches was 1 or 1000, it would just matter if the attribute was present. Data integrity is another issue ;) Perhaps an ISet<Enum> would suffice to negate your example?
• svick: @EdWoodcock, except that there is no ISet<T> in the .Net library.
• Ed James: See: msdn.microsoft.com/en-us/library/dd412081.aspx . It was added in .net 4 I believe.