Flags- and non-flags-variant of an enum

Question

Let's say I need both an enum both in a flags and in a non-flags variant.

• Option 1: I could duplicate everything:

  enum Color { Red, Blue, Green }
  
  [Flags]
  enum Colors {
      None = 0,
      Red = 1,
      Blue = 2,
      Green = 4
  }
  
  // use cases
  Color currentColor;
  Colors supportedColors;
  

• Option 2: I could just use the Flags variant for everything:

  Colors currentColor; // ugly, since neither "None" nor "Red | Blue" should be valid
  

I don't like either of these: In Option 1, Color.Red, and Colors.Red are completely unrelated, which might require binding code. In addition, I'd have to keep the two enums synchronized. The drawback of Option 2 is obvious. What I'd actually like is something like

enum Colors = Flag set of Color;

Is there a more elgant solution to this requirement?

Answer 1

I would simply use the [Flags] version for everything, and simply ensure in a few places that it is only a single value. You need to do that either way, because even without [Flags] the following is valid:

var flags = (Color)47; // why not

So you need to check that the Color is one you were expecting anyway. The [Flags] will only help serialization/parsing.

Answer 2

Recently I had same problem. Solved by option 2, with simple check:

bool IsColorValid(Color color)
{
    return (color != 0 && (color & (color - 1)) == 0);
}

Answer 3

Definitely do not make two enums called Color and Colors (option 1). This would make your code very confusing.

Unless I'm missing something, I don't think option 2 is so ugly. In general, it's reasonable to initialize a system parameter to a default value "None" prior to being set at runtime.

Answer 4

The only possible drawback of option 2 is running out of bits. If that's your problem, a flags enum is not suitible for you at all. Instead, make supported colors a HashSet<Color>