Flags and << operation on enums? C#

Question

Ok so I am new to C#, and for the life of me I cannot comprehend what exactly the below code (from a legacy project) is supposed to do:

[Flags]
public enum EAccountStatus
{
    None = 0,
    FreeServiceApproved = 1 << 0,
    GovernmentAccount = 1 << 1,
    PrivateOrganisationAccount = 1 << 2,
    All = 8
}

What exactly does the << operator do here on the enums? Why do we need this?

Please check pmg's answer on : http://stackoverflow.com/questions/3999922/why-use-the-bitwise-shift-operator-for-values-in-a-c-enum-definition — manish, Jun 21 '16 at 06:02
All should be defined as FreeServiceApproved | GovernmentAccount | PrivateOrganisationAccount = 7, not 8 :( — João Lourenço, May 23 '19 at 10:04

score 27 · Accepted Answer · edited May 23 '17 at 12:22

27

Behind the scenes, the enumeration is actually an int.
<< is the Bitwise Left Shift Operator
An equivalent way of writing this code is :

[Flags]
public enum EAccountStatus
{
    None = 0,
    FreeServiceApproved = 1,
    GovernmentAccount = 2,
    PrivateOrganisationAccount = 4,
    All = 8
}

Please note, that this enumeration has the Flag attribute

As stated in the msdn:

Use the FlagsAttribute custom attribute for an enumeration only if a bitwise operation (AND, OR, EXCLUSIVE OR) is to be performed on a numeric value.

This way, if you want to have multiple options set you can use:

var combined =  EAccountStatus.FreeServiceApproved  | EAccountStatus.GovernmentAccount

which is equivalent to:

  00000001  // =1 - FreeServiceApproved 
| 00000010  // =2 - GovernmentAccount 
 ---------
  00000011  //= 3 - FreeServiceApproved  and  GovernmentAccount

this SO thread has a rather good explanation about the flags attribute

edited May 23 '17 at 12:22

Community

1
1

answered Jun 21 '16 at 05:59

Avi Turner

10,234
7
48
75

Thanks Avi, that was beautifully explained :) – Joel Min Jun 21 '16 at 06:09
:) I'm glad I could be of help. – Avi Turner Jun 21 '16 at 06:11
2

There shouldn't be an ALL ("all flags") option on FLAGS. The combination of all the flags here would be 7, not 8, which results in 2 values meaning ALL. A solution to this is to binary OR all options, but a bug will almost certainly appear when another enum is added and a dev doesn't update the ALL enum. Another problem is if the value 8 is used when storing data, then the flag enum EAccountStatus is effectively sealed meaning no more options can be added without modifying stored data. Besides reporting, it's rare that you will need to check for ALL. – Dan Jan 11 '21 at 06:50
1

@Dan Agreed. The answer was simply using the example used in the original question. – Avi Turner Jan 12 '21 at 05:20
@AviTurner no worries, it wasn't really at you. Was leaving it here for people that don't know better – Dan Jan 13 '21 at 17:00
1

I read a handful of bitwise shift articles and watched a video, explaining how the bits get shifted.. all the while missing the context of why we'd do this in the first place. This answer was exactly what I needed to read. – DontFretBrett Jul 02 '21 at 21:53

Failed Scientist · Answer 2 · 2016-06-21T06:00:20.817

2

<< is doing simply what does i.e. Shift left operation.

As far as why in an enum is concerned, its just a way of evaluating the expression as enums allow expressions (and evaluate them on compile time)

edited Jun 21 '16 at 06:00

answered Jun 21 '16 at 05:56

Failed Scientist

1,977
3
29
48

Thanks, but why? Why a shift left operation on an enum? – Joel Min Jun 21 '16 at 05:58
@JoelMin because it is a easy way to calculate 1, 2, 4. When using `[Flags]` you want only a single bit set. When you do `1 << 0` only the 0th bit will be set, when you do `1 << 2` only the 2nd bit will be set, and so on. – Scott Chamberlain Jun 21 '16 at 06:00
@ScottChamberlain explained it so well. Hope you understood it now! – Failed Scientist Jun 21 '16 at 06:01
Thanks for the prompt answer!! +1 – Joel Min Jun 21 '16 at 06:04
@ScottChamberlain Interesting. Is there any runtime advantage, or would you expect the compiler to clean it up anyhow? – Bernhard Jun 21 '16 at 06:05
@Bernhard it would be a compile time constant, it is the same way the compiler will turn `int val = 1+1;` to `int val = 2;` when compiled – Scott Chamberlain Jun 21 '16 at 13:15

Flags and << operation on enums? C#

2 Answers2