C# with EF enum, lookup-table and the Value to pass it?

Question

I'm taking over a C# project and I have come across a concept that I'm unfamiliar with.

The project is using EF to create look-up tables from enums. The UI is accepting a multi-select input but the receiving model accepts a enum type, not a list/array or something else that would suggest plural.

The enum seems to have some recursive relationship

public enum Options
{
    None = 0,

    [Display(Name = @"On-site Security")]
    OnSiteSecurity = 1 << 0,

    [Display(Name = @"Gated Entry")]
    GatedEntry = OnSiteSecurity << 1,

    [Display(Name = @"Gated Entry - Video")]
    GatedEntryVideo = GatedEntry << 1,

    [Display(Name = @"Closed-Circuit TV")]
    CCTV = GatedEntryVideo << 1, ...

the look-up table has a Value column that with value that grow exponentially, 0,1,2,4,8,16,32,64,128,256,512

and finally the UI has a multi-select input where the value is the same number sequence as the look-up table. There is a sanitation function acting on the value like this (knockout.js)

self.Value = ko.computed({
    read: function () {
        var value = 0;
        $.each(self.Selected(), function (idxitem, item) {
            value |= item;
        });
        return value;
    },
    write: function (value) {

        $.each(self.Available(), function (idxitem, item) {
            if ((value & item.Value) > 0) {
                item.IsSelected(true);
            }
        });

        self.Normalize(value);
    },
    owner: self
});

I do not understand how this is supposed to accept plural selections.

Answer 1

It sounds like its implementing flags . This allows you to have a single Enum value that can represent several different values

Here's a short example

class Program
{
    static void Main(string[] args)
    {
        FlaggedEnum fruitbowl = FlaggedEnum.Apples | FlaggedEnum.Oranges | FlaggedEnum.Pears;

        Console.WriteLine(fruitbowl);

        Console.ReadLine();
    }
}

[Flags]
enum FlaggedEnum
{
    None = 0,
    Apples = 1,
    Pears = 2,
    Oranges = 4,
    Pineapples = 8
}

When you run this you get the output "Apples, Oranges, Pears". Its integer value would be 7 (4 + 2 + 1).

The reason for powers of two is due to their binary equivalents, 1 = 1, 2 = 10 4 = 100, 8 = 1000... So this gives 7 the value of 111.

I hope this puts you on the right path at least.