What does the [Flags] Enum Attribute mean in C#?

Question

From time to time I see an enum like the following:

[Flags]
public enum Options 
{
    None    = 0,
    Option1 = 1,
    Option2 = 2,
    Option3 = 4,
    Op         


        

Answer 1

Combining answers https://stackoverflow.com/a/8462/1037948 (declaration via bit-shifting) and https://stackoverflow.com/a/9117/1037948 (using combinations in declaration) you can bit-shift previous values rather than using numbers. Not necessarily recommending it, but just pointing out you can.

Rather than:

[Flags]
public enum Options : byte
{
    None    = 0,
    One     = 1 << 0,   // 1
    Two     = 1 << 1,   // 2
    Three   = 1 << 2,   // 4
    Four    = 1 << 3,   // 8

    // combinations
    OneAndTwo = One | Two,
    OneTwoAndThree = One | Two | Three,
}

You can declare

[Flags]
public enum Options : byte
{
    None    = 0,
    One     = 1 << 0,       // 1
    // now that value 1 is available, start shifting from there
    Two     = One << 1,     // 2
    Three   = Two << 1,     // 4
    Four    = Three << 1,   // 8

    // same combinations
    OneAndTwo = One | Two,
    OneTwoAndThree = One | Two | Three,
}

---

Confirming with LinqPad:

foreach(var e in Enum.GetValues(typeof(Options))) {
    string.Format("{0} = {1}", e.ToString(), (byte)e).Dump();
}

Results in:

None = 0
One = 1
Two = 2
OneAndTwo = 3
Three = 4
OneTwoAndThree = 7
Four = 8