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);

Answer 1

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));
}