Using Where on Enum Values

Question

I have defined an enum with the [Flag] attribute.

Given an integer value, I would like to print all the enum values for the bits set in the integer value.

Here's what I have so far.

string s = string.Join(", ", Enum.GetValues(typeof(MyEnumType)).OfType<Enum>()
                                 .Where(x => (MyIntValue & (int)x) != 0));

But the typecast to int in the last line gives me the following error.

Cannot convert type 'System.Enum' to 'int'

I wish Microsoft would replace all the Framework code that returns Array in favor of types that support LINQ.

But is there an easy way to do this?

casting your int value to the Enum and calling ToString() should do exactly what you want. See http://stackoverflow.com/questions/8447/what-does-the-flags-enum-attribute-mean-in-c?rq=1 — devio, May 12 '16 at 19:48
"I wish Microsoft would replace all the Framework code that returns Array in favor of types that support LINQ." - and what should they do about all of the legacy code that was written before Linq? Just let all that code developed over the years suddenly break? — D Stanley, May 12 '16 at 19:49
BTW, Arrays support Linq just fine as they implement `IEnumerable` as well as `IEnumerable`. — D Stanley, May 12 '16 at 19:51
@DStanley Uh, Microsoft updates the Framework all the time. They can do it so it doesn't break code. Create a new class to do the work. There are endless ways. Why would you assume I am asking Microsoft to break all the existing code? — Jonathan Wood, May 12 '16 at 20:06
@JonathanWood Sorry, I thought you were talking about arrays like `int[]`. I stand corrected. — D Stanley, May 12 '16 at 20:10

Yacoub Massad · Answer 1 · 2016-05-12T19:46:30.127

4

Casting to MyEnumType instead of the generic Enum should fix the problem:

string s = string.Join(
    ", ",
    Enum.GetValues(typeof(MyEnumType))
        .OfType<MyEnumType>()
        .Where(x => (MyIntValue & (int)x) != 0));

You should really be using Cast instead of OfType since you are sure that the items are of type MyEnumType.

edited May 12 '16 at 19:46

answered May 12 '16 at 19:42

Yacoub Massad

27,509
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Yeah, you're right. This code is still too messy and executes too slow. Microsoft needs to update the Framework. Thanks. – Jonathan Wood May 12 '16 at 19:45
@JonathanWood Which part executes "too slow" and how do you propose that Microsoft fix it? – D Stanley May 12 '16 at 19:52
I read `OfType()` can impact performance. What if `Enum.GetValues()` returned `List`, where `T` was my enum type? That would be so much more natural, particularly when using LINQ. The problem is that these routines were written before generics. That's why I think they should be rewritten. – Jonathan Wood May 12 '16 at 20:05
1

I think `Cast` should be slightly faster because it doesn't filter (and filtering isn't necessary). But what is slow? Does the enum type have so many values that this becomes noticeably slow? – Gert Arnold May 12 '16 at 20:23

Ivan Stoev · Accepted Answer · 2016-05-12T21:49:11.777

Too much LINQ way of thinking, so sometimes we forget where we come from:) Starting from the very first .NET version, Enum.GetValues(typeof(MyEnum)) actually returns MyEnum[], so what you need is neither OfType, nor Cast, but simple C# cast:

string s = string.Join(", ", ((MyEnum[])Enum.GetValues(typeof(MyEnumType)))
    .Where(x => (MyIntValue & (int)x) != 0));

Since that typeof combined with the casting to the same type is quite annoying, from a very long time I have this little helper in my pocket:

public static class EnumInfo
{
    public static T[] GetValues<T>() where T : struct // I (and not only) wish I could say where T : enum
    {
        return (T[])Enum.GetValues(typeof(T));
    }
}

which allows me to use much more concise

string s = string.Join(", ", EnumInfo.GetValues<MyEnumType>()
    .Where(x => (MyIntValue & (int)x) != 0));

Shouldn't your `GetValues()` method return something other than `T`? For example, `T[]`? — Jonathan Wood, May 12 '16 at 20:56
@JonathanWood Absolutely, thank you (you know how it is when you write the code directly in the browser:) — Ivan Stoev, May 12 '16 at 21:50

score 1 · Answer 3 · answered May 12 '16 at 20:18

You can write your own GetValues method that uses generics to get what you need:

public static class EnumUtil
{
    public static IEnumerable<T> GetValues<T>()
    {
        foreach(T enumVal in Enum.GetValues(typeof(T)))
        {
            yield return enumVal;
        }
    }
}

Which is then used like so:

var s = string.Join(", ", EnumUtil.GetValues<TestEnum>()
    .Where(x => (myIntValue & (int)x) != 0));

Using Where on Enum Values

3 Answers3