GroupBy using [Flag] enum grouping by single enum

Question

Let's say I have

[Flags]
public enum MyEnum
{
   ValueZero = 1,
   ValueOne = 2,
   ValueTwo = 4
}

public class MyClass
{
   public string Property { get; set; }
   public MyEnum EnumValue { get; set; }
}

I'd want to be able to use GroupBy on a List<MyClass> to group by enums without considering that it is a flag enum.

When I use GroupBy (example at the end of the question), the groupings are made with the aggregated enums like this

//Grouping             Values
ValueZero | ValueOne : myClass1, myClass2
ValueOne  | ValueTwo : myClass3, myClass4

I'm looking to get the following (using the GroupBy method because it is more performant than using Where 3 times)

//Grouping  Values
ValueZero : myClass1, myClass2
ValueOne  : myClass1, myClass2, myClass3, myClass4
ValueTwo  : myClass4

I thought using this would work :

var list = new List<MyClass>();
var groupedList = list.GroupBy(c => c.EnumValue);

FYI, you shouldn't use a zero valued item in a Flags enumeration - EXCEPT if it is used to represent none. — Mike, Aug 13 '14 at 18:17
AFAIK `GroupBy` cannot group a single item into two groups. Which is what you're looking for, GroupBy won't help I guess. — Sriram Sakthivel, Aug 13 '14 at 18:20
I believe it makes sense, so I don't have the loop through my list to place each object in the list I'd want it to be? — IEatBagels, Aug 13 '14 at 18:21
Check this [thread](http://stackoverflow.com/questions/6806684/can-i-add-same-object-to-multiple-groups-in-linq) — Sriram Sakthivel, Aug 13 '14 at 18:34

score 2 · Accepted Answer · edited May 23 '17 at 11:49

Start by splitting each enum value into individual flags (like shown here), then flatten the results and group.

It should look like this:

var groupedList = list
    .Select(c => new { Flags=GetFlags(c.EnumValue), Item=c }) // split flags
    .SelectMany(c => c.Flags.Select(x => new { Flag=x, Item=c.Item })) // flatten
    .GroupBy(c => c.Flag, i => i.Item); // group

where GetFlags is a method which gets individual enum flags, like the method in the linked answer:

static IEnumerable<MyEnum> GetFlags(MyEnum input)
{
    return Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>()
       .Where(f => input.HasFlag(f));
}

apros · Answer 2 · 2014-08-13T19:38:38.997

If I guess your intention than in code it has an implementation like this:

var list = new List<MyClass>();
list.Add(new MyClass() {
   EnumValue = MyEnum.ValueZero | MyEnum.ValueOne, Property = "myClass1"
});
list.Add(new MyClass() {
   EnumValue = MyEnum.ValueZero | MyEnum.ValueOne, Property = "myClass2"
});
list.Add(new MyClass() {
   EnumValue = MyEnum.ValueOne , Property = "myClass3"
});
list.Add(new MyClass() {
   EnumValue = MyEnum.ValueOne | MyEnum.ValueTwo, Property = "myClass4"
});

var enumValues = Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>().ToArray();
var grouped = list.SelectMany(
      o => enumValues
         .Where(flag => o.EnumValue.HasFlag(flag))
         .Select(v => new { o, v })
   )
   .GroupBy(t => t.v, t => t.o);

foreach (IGrouping<MyEnum, MyClass> group in grouped)
{                
   Console.WriteLine(group.Key.ToString() + ": " + string.Join(", ", group.Select(x=>x.Property)));
}

GroupBy using [Flag] enum grouping by single enum

2 Answers2