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I have a list of object that has two properties, let's say a and b. a is an Enum like:

[Flags] enum MyEnum { first = 1, second = 2, third = 4, fourth = 8 };

b is an unsigned integer, which is a mask (combination of the MyEnum flags).

now i need to sum every object by their a - meaning an object can be summed twice if obj.a = first | third and i can't seem to do a groupBy to them. is there a different way to sum them?

I'm sorry i don't share my code, but i can't. i can tell you i did it using foreach in just some if - else blocks but i think i should learn how to do it in Linq

EDIT: I think i wasn't clear. i want to sum the object by the Enum, meaning if i have:

obj1.a = first, obj1.b = 5 obj2.a = first | second, obj2.b = 3

then the output will be

first sum = 8 second sum = 3
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  • could you provide the expected result? your question is not really clear to me Commented Aug 2, 2013 at 9:45
  • Your problem is still not clear. What if we have obj3 = first | second | third; What are the sums? Can they be first sum = 15, second sum = 8, third sum = 3, fourth sum = 7, fifth sum = 8, sixth sum = 14? Commented Aug 2, 2013 at 9:59
  • @KingKing exactly. i sum by the enum values, so if an object has in the enum first|second|third and 3 in the value field then 3 will be added to the sum of first, second and third Commented Aug 2, 2013 at 10:03

2 Answers 2

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3

Given a MyEnum and a MyClass

[Flags] enum MyEnum { first = 1, second = 2, third = 4, forth = 8 } class MyClass { public MyEnum MyEnum; public uint Value; }

And some values

var mcs = new[] { new MyClass { MyEnum = MyEnum.first | MyEnum.third, Value = 10 }, new MyClass { MyEnum = MyEnum.second, Value = 20 }, new MyClass { MyEnum = MyEnum.first, Value = 100 }, };

This LINQ expression will return for each value of the enum the sum of the values. 

var ret = from p in Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>() select new { MyEnum = p, Sum = mcs.Where(q => q.MyEnum.HasFlag(p)).Sum(q => q.Value) };

Note that it will return a "row" even for MyEnum.fourth with value 0.

The expression starts with the values of the enum (Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>()) and then for each value it sums the values of the mcs that have the same MyEnum (mcs.Where(q => q.MyEnum.HasFlag(p)).Sum(q => q.Value))

If you want to exclude the values of the enum that aren't used:

var ret = from p in Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>() let temp = mcs.Where(q => q.MyEnum.HasFlag(p)).Select(q => q.Value).ToArray() where temp.Length > 0 select new { MyEnum = p, Sum = temp.Sum(q => q) };

The expression starts with the values of the enum (Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>()) and then for each value it "saves" the values of the mcs that have the same MyEnum (let temp = mcs.Where(q => q.MyEnum.HasFlag(p)).Select(q => q.Value).ToArray()) in temp, skips the temp that are empty (where temp.Length > 0) and sum the remaining temp (select new { MyEnum = p, Sum = temp.Sum(q => q) }). Note that if you use an uint you have to use temp.Sum(q => q), but with an int you can use temp.Sum() (or you can use temp.Sum(q => q)).

Another way is through a double from and a group by

var ret = from p in Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>() from q in mcs where q.MyEnum.HasFlag(p) group q.Value by p into r select new { MyEnum = r.Key, Sum = r.Sum(p => p) };

It's probably equivalent to using the SelectMany as suggested by ChaseMedallion (a double from is converted by the compiler to a SelectMany)

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1 Comment

wow, really thank you. very explained and good for future use too. excellent answer
3

Try using HasFlag and GetValues to extract all the flags for a particular enum value:

var objs = new[] { new { a = MyEnum.first | MyEnum.second }, new { a = MyEnum.first }... }; var enumValues = Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>().ToArray(); // first, use SelectMany to create a sequence with one instance of each object for // each flag value in it's a var grouped = objs.SelectMany( o => enumValues.Where(flag => o.a.HasFlag(flag)) .Select(v => new { o, flag }) ) // then group by the extracted flag value // the element selector (t => t.o) gets us back to an IGrouping of o's (dropping the extra flag property .GroupBy(t => t.flag, t => t.o); // finally, we can get sums by a as follows: var sumsByA = grouped.ToDictionary(g => g.Key, g => g.Sum(o => o.b));
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thank you for your answer, really good and explained, will try is ASAP

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