How to use Flags attribute?

Question

I have some items in database. Each of'em can have many tags, like Browsable, or IsInMenu and so on. A friend of mine suggested to use enums with Flags attribute to create an extensible solution. So, I created a field in DB which takes an integer value, then I created this enum:

 [Flags]
 public enum ItemTags { Browsable = 2, IsInMenu = 4}

Now I'd like to be able to semantically get the list of some items this way:

 public List<Item> GetItems(ItemTags tags)
 {
     /* 
     Code to get data from DB, something like, 
     return repository.GetList(tags);
     */
 }

and in UI, I'd like to call:

 List<Item> items =  GetItems(ItemTags.Browsable | ItemTags.IsInMneu);

But I don't get the desired result. Am I going the right way? By desired result, I mean this: Values stored in database could be one of the 0, 2, 4, 6 values now. 0 means that the item is not in Menu and also not Browsable. 2 Means that item is Browable, but not in Menu. 4 means item is in Menu, but not Browsable. 6 means item is both Browsable and IsInMenu. Now when I call GetItems function, I don't get all the items which are browsable, in menu, or both browsable and in menu.

score 2 · Answer 1 · edited May 23 '17 at 11:48

2

You need to use FlagsAttribute, see this MSDN article, and this usage example, and most importantly this stack overflow answer.

edited May 23 '17 at 11:48

Community

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answered Jul 05 '11 at 11:07

Gleno

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Daniel Hilgarth · Answer 2 · 2011-07-05T11:15:58.040

You are missing the Flags attribute...

Your enum should be declared like this:

[Flags]
public enum ItemTags { Browsable = 2, IsInMenu = 4}

EDIT:
After your update, it looks fine. You should be more precise in what you mean with:

But I don't get the desired result.

The code you showed us looks fine. So either there is a problem elsewhere or the code you really use in your application and the code you showed us here are different.

Caspar Kleijne · Accepted Answer · 2011-07-05T11:18:05.813

1

use the FlagsAttribute Class

Indicates that an enumeration can be treated as a bit field; that is, a set of flags.

[Flags] 
public enum ItemTags 
{ 
  Default =0,
  Browsable = 2, 
  IsInMenu = 4,
  All = 6 // Browsable / IsInMenu
}

More here

note about enums:

an Enum by default has an int underneath, and as do all integers in C# an enum has a default value of 0 when first created. So if 0 is not mapped to an enumeration constant then your enum will be instantiated with an invalid valid

edited Jul 05 '11 at 11:18

answered Jul 05 '11 at 11:12

Caspar Kleijne

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This solution is fine. But what if my tags expand. I mean, in that case, I should write all the combinations, which is really hard. As I'd said in question, I want to be able to say `GetAllItems(ItemTags.IsInMenu | ItemTags.Browsable)`, not `GetAllItems(ItemTags.All)`. Am I right? – Saeed Neamati Jul 05 '11 at 11:24
1

I think you would do a ItemTags.All, since the flagg attribute allows in this case no other enumvalue with a value 6 that represents something different than ItemTags.IsInMenu | ItemTags.Browsable, so both options are the same. – Caspar Kleijne Jul 05 '11 at 11:45
Surely these Flag values are incorrect and should be powers of 2? – JKennedy Feb 25 '16 at 12:20

How to use Flags attribute?

3 Answers3