For stackoverflow question at http://stackoverflow.com/questions/17242522/linq-not-trivial-group-by To run use linqpad and set it to C# program

test.cs

void Main()
{
  Person p1 = new Person();
  p1.Groups = new List<Group>();
  p1.Groups.Add(new Group { GroupType = GroupTypesEnum.a });
  p1.Groups.Add(new Group { GroupType = GroupTypesEnum.b });
  Message m1 = new Message();
  m1.Recipient = p1;
 
  Person p2 = new Person();
  p2.Groups = new List<Group>();
  p2.Groups.Add(new Group { GroupType = GroupTypesEnum.b });
  p2.Groups.Add(new Group { GroupType = GroupTypesEnum.c });
  Message m2 = new Message();
  m2.Recipient = p2;
 
  Person p3 = new Person();
  p3.Groups = new List<Group>();
  p3.Groups.Add(new Group { GroupType = GroupTypesEnum.b });
  Message m3 = new Message();
  m3.Recipient = p3;
  
  List<Message> messages = new List<Message>();
  messages.Add(m1);
  messages.Add(m2);
  messages.Add(m3);
 
  var result = messages
    .SelectMany(m => m.Recipient.Groups)
    .GroupBy(t => t.GroupType)
    .Select(g => new { gType = g.Key, count = g.Count()});
    
    result.Dump();
}
 
enum GroupTypesEnum { a, b, c};
 
// Define other methods and classes here
class Message {
   public Person Recipient;
   
}
 
class Person {
   public List<Group> Groups;
   
}
 
class Group {
   public GroupTypesEnum GroupType;
}