Remove privilege enum flags the right way in C#

Question

I have an enum type for user privileges that looks like this:

[Flags]
public enum UserPrivileges : byte
{
    None = 0,                                     // 0000 0000
    View = 1 << 0,                                // 0000 0001
    Import = 1 << 1,                              // 0000 0010
    Export = 1 << 2,                              // 0000 0100
    Supervisor = View | Import | Export | 1 << 3, // 0000 1111
    Admin = Supervisor | 1 << 4                   // 0001 1111
}

These values are bound to CheckBoxes in the GUI with a value converter. (I wanted to do this as generic as possible because there are also different privileges [eg EmployeePrivileges])

public class ByteFlagsEnumValueConverter : IValueConverter
{
    private byte _targetValue;

    public object Convert(object value, Type targetType,
                          object parameter, CultureInfo culture)
    {
        var mask = (byte)parameter;
        _targetValue = (byte)value;
        return ((mask | _targetValue) == _targetValue);
    }

    public object ConvertBack(object value, Type targetType,
                              object parameter, CultureInfo culture)
    {
        var mask = (byte)parameter;

        if ((bool)value)
        {
            _targetValue |= mask;
        }
        else
        {
            // Get next superflag for mask (e.g. 0110 -> 1111)
            var b = mask;
            b--;
            b |= (byte)(b >> 1);
            b |= (byte)(b >> 2);
            b |= (byte)(b >> 4);
            b |= (byte)(b >> 8);

            // if you remove a superflag (e.g. 1111) also remove
            // everything higher than this flag
            if (mask == b || mask == 1)
                _targetValue &= (byte)(mask >> 1);
            else
                // ????????
        }

        return Enum.Parse(targetType, _targetValue.ToString());
    }
}

This works really fine for displaying and adding privileges to the user in the GUI. Also it works for removing Superflags like Supervisor (all flags >= Supervisor get removed, the other flags don't change).

The problem is when I uncheck Import for example, I want to remove all Superflags (Supervisor, Admin) but would like to keep the other flags (View, Export).

0001 1111 // Admin
0000 0010 // Import
---------
0000 0101 // View | Export

But I haven't come up with an good idea how to accomplish this. Anyboy who has a good solution for this?

Answer 1

If I understand what you want , this should do the job

    byte tmp = 1 << 3 | 1 << 4;
    byte removeSuperFlagMask = (byte) (~tmp);

    byte notSuperflagsMask = 1 << 3 | 1 << 2 | 1 << 1;
    if ((valueToRemove & notSuperflagsMask) != 0)
    {
        newValue = (byte)(removeSuperFlagMask & currentValue & ~valueToRemove);
    }

Answer 2

If I understood correctly you want to remove Supervisor and Admin so:

UserPrivileges newPrivileges = (UserPrivileges)(((byte)currentPrivileges) & 7;

It will perform like this:

0001 1111 // Admin
0000 0111 // 7 flag
---------
0000 0111 // Remain only where 7 bit was set.

Another example

0000 0011 // View | Import
0000 0111 // 7 flag
---------
0000 0011 // Remain only where 7 bit was set.

By remain I mean that where 7 flag is set it will persist the value in the result (beein 0 or 1 ). And where the 7 flag is 0 it will kill the value to 0 .

Answer 3

Mfz brought me on the right way but, it wasn't generic enough for me, so I came up with another solution:

    public object ConvertBack(object value, Type targetType,
                              object parameter, CultureInfo culture)
    {
        var mask = (byte)parameter;

        if ((bool)value)
        {
            _targetValue |= mask;
        }
        else
        {
            if (IsSuperFlag(mask) && mask != 1)
                _targetValue &= (byte)(mask >> 1);
            else
            {
                // Get all flags from enum type that are no SuperFlags
                var flags = Enum.GetValues(targetType).Cast<Enum>();
                flags = flags.Where(x => !IsSuperFlag(System.Convert.ToByte(x)));

                long nonSuperFlags = 0;

                foreach (var flag in flags)
                {
                    nonSuperFlags |= System.Convert.ToByte(flag);
                }

                // Now only remove everything except the nonSuperFlags
                // and then remove the mask
                _targetValue &= (byte)(~(_targetValue ^ nonSuperFlags | mask));

            }
        }

        return Enum.Parse(targetType, _targetValue.ToString());
    }

    private bool IsSuperFlag(byte flag)
    {
        var b = flag;
        b--;
        b |= (byte)(b >> 1);
        b |= (byte)(b >> 2);
        b |= (byte)(b >> 4);
        b |= (byte)(b >> 8);

        return b == flag;
    }

I simply got all non superflags for the enum type and them removed only the superflags and afterwards removed the flag.