Bitwise operations in Visual Basic .NET

Question

Hello guys,

I'm re-writing some old application in VB.NET to C# and ASP.NET 3.5. Everything is going OK but I have this problem - and, as the database will not be changed, I must find a solution to it.

The old app saves the list o desired days (from Sunday to Saturday) in a byte. This is the way it do it:

If chkDaily.Checked Then
            daysBitmask = 127    ' This is for a full week
        Else
            For i = 0 To chklstWeekly.Items.Count - 1
                If chklstWeekly.Items(i).Selected Then
                    ' List the selected items
                    daysBitmask += 2 ^ CInt(chklstWeekly.Items(i).Value)
                    daysBitmask = daysBitmask
                    Message2.InnerText = daysBitmask
                End If
            Next
        End If

and I converted it to

        if (rdbDaysList.SelectedValue == DAILY)
            daysBitmask = 127;
        //This is for a full week
        else
            foreach (var d in ckbDaysList.Items)
            {
                ListItem day = d as ListItem;
                if (day.Selected)
                {
                    daysBitmask += Convert.ToByte(Math.Pow(2, Convert.ToDouble(day.Value)));
                }
            }

This works. My biggest problem is to convert from the byte to the C# code. The original VB.NET is right above:

            If (dr("DesireDays").ToString() = "127") Then
                chkDaily.Checked = True
                chkWeekly.Checked = False
                chklstWeekly.Enabled = False
            Else
                chkWeekly.Checked = True
                chkDaily.Checked = False
                chklstWeekly.Enabled = True

                Dim AD As Integer = dr("DesireDays").ToString()
                Dim i As Integer

                For i = 0 To chklstWeekly.Items.Count - 1
                    If AD And 2 ^ i Then
                        chklstWeekly.Items(i).Selected = True

                    End If
                Next i

Any help will be appreciated.

Answer 1

You don't want to use ToDouble, as you want an int (that's what CInt gives you).

In addition, for doing powers of two, bitshifting is usually a little clearer than doing exponents (though choose whatever works for you).

As an aside, it's also usually safer to use a bitwise OR (the vertical | bar) rather than addition.

ListItem day = d as ListItem;

if (day.Selected)
{
    daysBitmask |= 1 << Convert.ToInt32(day.Value);
}

To answer the actual question, the reverse will work:

ListItem day = d as ListItem;

if ((daysBitmask & (1 << Convert.ToInt32(day.Value))) > 0)
{
    day.Selected = true;
}