Question

Edit: It seems most people misunderstood my question.

I know how enum works, and I know binary. I’m wondering why the enums with the [Flags] attribute is designed the way it is.

Original post:

This might be a duplicate, but I didn’t find any other posts, so here goes.

I bet there has been some good rationale behind it, I just find it a bit bug prone.

[Flag]
public enum Flagged
{
  One, // 0
  Two, // 1
  Three, // 2
  Four, // 3
}

Flagged f; // Defaults to Flagged.One = 0
f = Flagged.Four;
(f & Flagged.One) != 0; // Sure.. One defaults to 0
(f & Flagged.Two) != 0; // 3 & 1 == 1
(f & Flagged.Three) != 0; // 3 & 2 == 2

Wouldn’t it have made more sense if it did something like this?

[Flag]
public enum Flagged
{
  One = 1 << 0, // 1
  Two = 1 << 1, // 2
  Three = 1 << 2, // 4
  Four = 1 << 3, // 8
}

Flagged f; // Defaults to 0
f = Flagged.Four;
(f & Flagged.One) != 0; // 8 & 1  == 0
(f & Flagged.Two) != 0; // 8 & 2 == 0
(f & Flagged.Three) != 0; // 8 & 4 == 0
(f & Flagged.Four) != 0; // 8 & 8 == 8

Of course.. I’m not quite sure how it should handle custom flags like this

[Flag]
public enum Flagged
{
   One, // 1
   Two, // 2
   LessThanThree = One | Two,
   Three, // 4? start from Two?
   LessThanFour = Three | LessThanThree,
   Three, // 8? start from Three?
}

The spec gives some guidelines

Define enumeration constants in powers of two, that is, 1, 2, 4, 8, and so on. This means the individual flags in combined enumeration constants do not overlap.

But this should perhaps be done automatically as I bet you would never want my first example to occur. Please enlighten me 🙂

Answer 1

The attribute is [Flags] not [Flag] and there’s nothing magical about it. The only thing it seems to affect is the ToString method. When [Flags] is specified, the values come out comma delimited. It’s up to you to specify the values to make it valid to be used in a bit field.