Random value from Flags enum

Question

Say I have a function that accepts an enum decorated with the Flags attribute. If the value of the enum is a combination of more than one of the enum elements how can I extract one of those elements at random? I have the following but it seems there must be a better way.

[Flags]
enum Colours
{
    Blue = 1,
    Red = 2,
    Green = 4
}

public static void Main()
{
    var options = Colours.Blue | Colours.Red | Colours.Green;
    var opts = options.ToString().Split(',');
    var rand = new Random();
    var selected = opts[rand.Next(opts.Length)].Trim();
    var myEnum = Enum.Parse(typeof(Colours), selected);
    Console.WriteLine(myEnum);
    Console.ReadLine();
}

Answer 1

You can call Enum.GetValues to get an array of the enum's defined values, like this:

var rand = new Random();

Colors[] allValues = (Colors[])Enum.GetValues(typeof(Colors));
Colors value = allValues[rand.Next(allValues.Length)];

Answer 2

var options = Colours.Blue | Colours.Green;

var matching = Enum.GetValues(typeof(Colours))
                   .Cast<Colours>()
                   .Where(c => (options & c) == c)    // or use HasFlag in .NET4
                   .ToArray();

var myEnum = matching[new Random().Next(matching.Length)];

Answer 3

If you don't mind a little casting, and your enum is of underlying int type, the following will work and is fast.

var rand = new Random();
const int mask = (int)(Colours.Blue | Colours.Red | Colours.Green);
return (Colours)(mask & (rand.Next(mask) + 1));

If you only want a single flag to be set, you could do the following:

var rand = new Random();
return (Colours)(0x1 << (rand.Next(3)));

Answer 4

If I understand correctly, the question is about returning a random enum value from a flags enum value, not returning a random member from a flags enum.

    [Flags]
    private enum Shot
    {
        Whisky = 1,
        Absynthe = 2,
        Pochin = 4,
        BrainEraser = Whisky | Absynthe | Pochin
    }

    [Test]
    public void Test()
    {
        Shot myCocktail = Shot.Absynthe | Shot.Whisky;

        Shot randomShotInCocktail = GetRandomShotFromCocktail(myCocktail);
    }

    private static Shot GetRandomShotFromCocktail(Shot cocktail)
    {
        Random random = new Random();

        Shot[] cocktailShots = Enum.GetValues(typeof(Shot)).
           Cast<Shot>().
           Where(x => cocktail.HasFlag(x)).ToArray();

        Shot randomShot = cocktailShots[random.Next(0, cocktailShots.Length)];

        return randomShot;
    }

Edit

And obviously you should check that the enum is a valid value, eg:

 Shot myCocktail = (Shot)666;

Edit

Simplified

Answer 5

In my case - I have enums with missing members eg 0x01, 0x02, 0x08 , and large ulong enum with 0x200000000 maximal value.

This code works for all of my cases:

/// <summary>
///     Gets <see cref="System.Random"/> instance.
/// </summary>
public static Random Random { get; } = new Random(Guid.NewGuid().GetHashCode());

/// <summary>
///     Gets random combination of Flags Enum.
/// </summary>
/// <typeparam name="T">Enum type.</typeparam>
/// <returns>Random Flags Enum combination.</returns>
public static T GetRandomFlagsEnumValue<T>()
    where T : Enum
{
    var allValues = (T[])Enum.GetValues(typeof(T));

    ulong numberValue = allValues.OrderBy(x => Random.Next())
        .Take(GetRandomInteger(1, allValues.Length - 1))
        .Select(e => Convert.ToUInt64(e, CultureInfo.InvariantCulture))
        .Aggregate((a, c) => a + c);

    return (T)Enum.ToObject(typeof(T), numberValue);
}