Question

Initially I had the following:

[Flags]
public enum QueryFlag
{
    None = 0x00,
    CustomerID = 0x01,
    SalesPerson = 0x02,
    OrderDate = 0x04
}

As check boxes are checked/unchecked, I would add/remove flags from:

QueryFlag qflag;

My idea – when the user clicks the Search button, I would iterate the actual flags set in qflag to modify a .Where clause in my LINQ to Sql. However, Enum.GetValues(qflag.GetType()) returns all the values of the QueryFlag itself. Not helpful.

My solution:

class myForm : Form
{
    List<QueryFlag> qflag = new List<QueryFlag>();

    private void chkOrderDate_CheckedChanged(object sender, EventArgs e)
    {
        if (chkOrderDate.Checked && !qflags.Contains(QueryFlag.OrderDate))
            qflags.Add(QueryFlag.OrderDate);
        else
            qflags.Remove(QueryFlag.OrderDate);
    }

    private void cmdSearch_Click(object sender, EventArgs e)
    {
        if (qflags.Count == 0)
        {
            rtfLog.AppendText("\nNo search criteria selected.");
            return;
        }

        foreach (QueryFlag flag in qflag)
        {
            rtfLog.AppendText(string.Format("\nSearching {0}", flag.ToString()));

            // add switch for flag value
        }
    }
}

public enum QueryFlag
{
    CustomerID,
    SalesPerson,
    OrderDate
}

I have 3 check boxes and this works without any issues to this point. But I am wondering if there is a better way to perform this iteration.

Answer 1

The way you had it originally was correct; you just got messed up by the Enum.GetValues method. This method returns every defined value for a particular enum type, yes. So what you do with this is check each defined value against your particular enum value to see whether the defined value is set within your value.

That is, you should do this:

private void chkOrderDate_CheckedChanged(object sender, EventArgs e)
{
    if (chkOrderDate.Checked)
    {
        qFlag |= QueryFlag.OrderDate;
    }
    else
    {
        qFlag &= (~QueryFlag.OrderDate);
    }
}

…and likewise for your other CheckBoxes. Then when you want to enumerate what flags you have set:

static IEnumerable<QueryFlag> GetFlags(QueryFlag flags)
{
    foreach (QueryFlag flag in Enum.GetValues(typeof(QueryFlag)))
    {
        // Presumably you don't want to include None.
        if (flag == QueryFlag.None)
        {
            continue;
        }

        if ((flags & flag) == flag)
        {
            yield return flag;
        }
    }
}

In fact, you could even abstract the above into a handy extension method for all enum types:

public static class FlagsHelper
{
    // This is not exactly perfect, as it allows you to call GetFlags on any
    // struct type, which will throw an exception at runtime if the type isn't
    // an enum.
    public static IEnumerable<TEnum> GetFlags<TEnum>(this TEnum flags)
        where TEnum : struct
    {
        // Unfortunately this boxing/unboxing is the easiest way
        // to do this due to C#'s lack of a where T : enum constraint
        // (there are ways around this, but they involve some
        // frustrating code).
        int flagsValue = (int)(object)flags;

        foreach (int flag in Enum.GetValues(typeof(TEnum)))
        {
            if ((flagsValue & flag) == flag)
            {
                // Once again: an unfortunate boxing/unboxing
                // due to the lack of a where T : enum constraint.
                yield return (TEnum)(object)flag;
            }
        }
    }
}

So your cmdSearch_Click handler could look like this:

private void cmdSearch_Click(object sender, EventArgs e)
{
    if (qFlag == QueryFlags.None)
    {
        rtfLog.AppendText("\nNo search criteria selected.");
        return;
    }

    foreach (QueryFlag flag in qFlag.GetFlags())
    {
        rtfLog.AppendText(string.Format("\nSearching {0}", flag.ToString()));

        // add switch for flag value
    }
}