Enum is defined does not work as expected

Question

I have the following enum that I need to validate based on client selections

    [Flags]
    enum Colour
    {
        Black = 1,
        Blue = 2,
        Green = 4,
        Yellow = 8
    }



var isValid = Enum.IsDefined(typeof(Colour), 5);

Why is returning false if 5 is a valid value (Colour.Black | Colour.Green)

Are you expecting the call to generate all possible combinations? It's only checking those that are actually defined. — Enigmativity, Sep 12 '17 at 07:09
5 is not a defined value in your enum. It will only be true if you have a entry with the value 5. — Filnor, Sep 12 '17 at 07:10
5 is not defined.... how can the compiler be aware of fuse combinations that generate the value 5 — ΦXocę 웃 Пepeúpa ツ, Sep 12 '17 at 07:11
I have a dozen of enums so I need to find a generic solution — codejunkie, Sep 12 '17 at 07:13
@codejunkie - then what is the actual problem you are facing? Regarding the dozen enums — Gilad Green, Sep 12 '17 at 07:14
@rene - `5` is also `2 + 3`. So should all combinations be tested? Or just "powers of two"? What if someone actually defines `3`? What would the sensible thing for the developers be to put as default behaviour? — Corak, Sep 12 '17 at 07:15
There isn't necessarily a generic solution since I've encountered some enums which represent a "hybrid" approach - where some values are meant to be selected between exclusively and other values are intended to be combined via or. Without understanding the semantics of that specific enum, you cannot know whether a value is well defined. (There's one somewhere in the BCL, can't find it at the moment) — Damien_The_Unbeliever, Sep 12 '17 at 07:16
I assumed it was clear how the Flags attribute works but yes, it is powers of 2 in this case. @Corak — rene, Sep 12 '17 at 07:17
So there is no way of checking whether the client input is valid? — codejunkie, Sep 12 '17 at 07:21
@codejunkie the client enter a number? weird... but in any case, in the specific enum shown, you can check if it's between 1-15 — Ofir Winegarten, Sep 12 '17 at 07:22
I want to avoid corrupt input that might cause problem in server. Because it is a web app someone can try to cause problems — codejunkie, Sep 12 '17 at 07:26
@rene -- it is. But `5` still isn't a *defined* value. If someone just wants an `IsValid` method, the easiest could be: `return value < (maxEnumValue * 2);`, because every combination could be valid. -- but again, that wouldn't be what I (and seemingly the developers) would expect from a method called `IsDefined`. — Corak, Sep 12 '17 at 07:27
I think the duplicate offers all the info the OP needs @Corak — rene, Sep 12 '17 at 07:31

Danil Eroshenko · Accepted Answer · 2017-09-12T07:53:50.050

2

Because it is proper result. "If enumType is an enumeration that is defined by using the FlagsAttribute attribute, the method returns false if multiple bit fields in value are set but value does not correspond to a composite enumeration value, or if value is a string concatenation of the names of multiple bit flags." See MSDN for details about how IsDefined works.

UPD: Solution for any enum:

static class EnumExtensions
{
    public static bool IsSuitable(Type enumType, int value)
    {
        if (!enumType.IsEnum)
        {
            throw new ArgumentException(nameof(enumType));
        }

        var entities = Enum.GetValues(enumType);
        int composite = 0;
        foreach (var entity in entities)
        {
            composite |= (int)entity;
        }

        return (composite | value) == composite;
    }
}

It gives this result:

var suit = EnumExtensions.IsSuitable(typeof(Colour), 5); // true
var suit2 = EnumExtensions.IsSuitable(typeof(Colour), 333); //false

edited Sep 12 '17 at 07:53

answered Sep 12 '17 at 07:14

Danil Eroshenko

470
2
10

```private readonly int composite = Colour.Black | Colour.Blue | Colour.Green | Colour.Yellow;``` This does not compile – codejunkie Sep 12 '17 at 07:37
To ("save typing" and) make it "future proof" if contents of `Colour` change: `private readonly Colour composite = Enum.GetValues(typeof(Colour)).OfType().Aggregate((Colour)0, (c, v) => c | v);` – Corak Sep 12 '17 at 07:39
Thanks but now the IsColour(int value) does not compile – codejunkie Sep 12 '17 at 07:43
A moment. A generic solution is in approach. – Danil Eroshenko Sep 12 '17 at 07:45

score 1 · Answer 2 · answered Sep 12 '17 at 07:16

1

The return value of Enum.IsDefined() function is depend on the enum definition. you are actually checked if '5' is in the set of values from the enum you defined.

answered Sep 12 '17 at 07:16

Michael Gabbay

413
1
4
20

Leon Barkan · Answer 3 · 2017-09-12T07:28:12.080

0

    [Flags]
    enum Colour
    {
        Black = 1,  // 0001
        Blue = 2,   // 0010
        Green = 4,  // 0100
        Yellow = 8  // 1000
    }

    static void Main(string[] args)
    {
        var isValid = Enum.IsDefined(typeof(Colour), 5); // 5 is 0101
    }

Enum.IsDefined: Returns an indication whether a constant with a specified value exists in a specified enumeration.

in your example you have values 1,2,4,8 why you thinking that value 5 should return true ??

    [Flags]
    enum Colour
    {
        Black = 1,
        Blue = 785,
        Green = 4,
        Yellow = 666
    }

    static void Main(string[] args)
    {
        var isValid = Enum.IsDefined(typeof(Colour), 666); // true
        var isValid2 = Enum.IsDefined(typeof(Colour), 785); // true
        var isValid3 = Enum.IsDefined(typeof(Colour), 5); // false
    }

edited Sep 12 '17 at 07:28

answered Sep 12 '17 at 07:19

Leon Barkan

2,676
2
19
43

2

What is the difference between this and the code I posted? – codejunkie Sep 12 '17 at 07:20
in your enum you doesn't have value that representing 5 in binary represantation (add Red = 5) and your validation will be true – Leon Barkan Sep 12 '17 at 07:22
you looking for represintation 0101 in your example – Leon Barkan Sep 12 '17 at 07:23
sorry still dont get you – codejunkie Sep 12 '17 at 07:25
when you asking for 5 what you thinking that the function do ? – Leon Barkan Sep 12 '17 at 07:32
anyway it seems that you dont understand what I want to achieve thanks for the help – codejunkie Sep 12 '17 at 07:37
better way to ask a quastion is write what you tring to achieve and your code that you already tried (i thinking that your wrong in logic in that case) – Leon Barkan Sep 12 '17 at 07:40
when you asking for 5 it will not do 1 + 4 = 5 it's will look if you have 5 in the enum – Leon Barkan Sep 12 '17 at 07:41
I already wrote what I am trying to achieve, – codejunkie Sep 12 '17 at 07:44

Enum is defined does not work as expected

3 Answers3