在枚举值中找到最高的设置标志

Question

I'm using a enum with flags attribute as a way of tracking status.

An example is the following:

Created = 1
Completed = 2
Dispatched = 4

Without writing anything too rigid (if check this, do that, if check that, do this) i want to be able to find the highest flag which has been set so in this example:

Item.Status = Status.Created | Status.Completed

the mythical method would return 2 - as completed is the flag set with the highest value.

GetMaxSetFlagValue(Item.Status) // returns 2

I've found questions which revolved around the actual enum, just not a value which uses flags. I'm fairly sure this could be achieved with Linq...?

Answer 1

Something like the following should work:

static int GetMaxSetFlagValue<T>(T flags) where T : struct
{
   int value = (int)Convert.ChangeType(flags, typeof(int));
   IEnumerable<int> setValues = Enum.GetValues(flags.GetType()).Cast<int>().Where(f => (f & value) == f);
   return setValues.Any() ? setValues.Max() : 0;
}

The method will fail if T is not an enum type, so a check should preferably be performed in the beginning of the method. Also it will not work for an enum with an underlying type larger than int (ie long ).

Answer 2

This is the extension method I use. It will give you the enum back

var maxStatus = Item.Status.GetFlags().Max();

Output: maxStatus = Completed

public static class EnumExtensions {

    /// <summary>Enumerates get flags in this collection.</summary>
    ///
    /// <param name="value">The value.
    /// </param>
    ///
    /// <returns>An enumerator that allows foreach to be used to process get flags in this collection.</returns>
    public static IEnumerable<T> GetFlags<T> (this T value) where T : struct {
        return GetFlags (value, Enum.GetValues (value.GetType ()).Cast<T> ().ToArray ());
    }

    /// <summary>Enumerates get flags in this collection.</summary>
    ///
    /// <param name="value"> The value.
    /// </param>
    /// <param name="values">The values.
    /// </param>
    ///
    /// <returns>An enumerator that allows foreach to be used to process get flags in this collection.</returns>
    private static IEnumerable<T> GetFlags<T> (T value, T [] values) where T : struct {
        if (!typeof (T).IsEnum) {
            throw new ArgumentException ("Type must be an enum.");
        }
        ulong bits = Convert.ToUInt64 (value);
        var results = new List<T> ();
        for (int i = values.Length - 1; i >= 0; i--) {
            ulong mask = Convert.ToUInt64 (values [i]);
            if (i == 0 && mask == 0L)
                break;
            if ((bits & mask) == mask) {
                results.Add (values [i]);
                bits -= mask;
            }
        }
        if (bits != 0L)
            return Enumerable.Empty<T> ();
        if (Convert.ToUInt64 (value) != 0L)
            return results.Reverse<T> ();
        if (bits == Convert.ToUInt64 (value) && values.Length > 0 && Convert.ToUInt64 (values [0]) == 0L)
            return values.Take (1);
        return Enumerable.Empty<T> ();
    }
}

Answer 3

As you can cast back and forth to uint, you could use:

public uint LowestBit(uint x) 
{
    return ~(x&x-1)&x;
}
public uint HighestBit(uint x)
{
    uint last = x;
    while (x!=0) 
    {
        last=x;
        x&=x-1;
    }
    return last;
}