Question

I have a method that converts a bitmask to a list of days using an Enum. I am trying to do the reverse, but am having trouble. I have DaysOfWeekToEnum working, but not DaysOfWeekFromEnum. Below is what I am trying to do. Can anyone help?

public static short DaysOfWeekFromEnum(IEnumerable<DaysInWeekIds> daysOfWeek)
{
    short mask;

    foreach (var item in daysOfWeek)
    {
        mask &= item; // ????
    }

    return mask
}

public static IEnumerable<DaysInWeekIds> DaysOfWeekToEnum(short mask)
{
    var values = new List<DaysInWeekIds>();

    foreach (short enumValue in Enum.GetValues(typeof(DaysInWeekIds)))
    {
        if (mask & enumValue == enumValue)
        {
            values.Add((DaysInWeekIds) Enum.ToObject(typeof(DaysInWeekIds),
                enumValue));
        }
    }

    return values;
}

/// <summary> The days in week ids. </summary>
public enum DaysInWeekIds : short
{
    M = 1,
    Tu = 2,
    W = 4,
    Th = 8,
    F = 16,
    Sa = 32,
    Su = 64
}

Answer 1

It should be |=, not &=, to set the additional bits.

You could also join string representations together, and pass the result to Enum.Parse method. This is not as efficient, but the code will look shorter.

var res = (DaysInWeekIds)Enum.Parse(
    typeof(DaysInWeekIds)
,   string.Join(",", daysOfWeek.Select(d => d.ToString()).ToArray())
);

This returns 25 for {M, Th, F} (link to ideone).