Question

I have a list of objects with field UserID, Property:

I would like to order the list by most shared property value. So if every user has Property= “Popular”, that should come up first. If everyone but one user has Property=”Second” that should come up second in list…
even if its only used once for each user.

I would do distinct() on each possible Property and but that doesnt seem efficient with many possible Property.

Answer 1

You can use a grouping on Property, order the groups by the number of counts in each group and then flatten the list again using SelectMany():

var items = myList.GroupBy(x => x.Property)
                  .OrderByDescending(g => g.Count())
                  .SelectMany(g => g);
                  .ToList();

From your question its not quite clear to me whether you want duplicates to show up or not and if you are at all interested in the UserID. If not, you can just select the keys of the groups to give you a List<string> of unique Property values in the desired order:

var props = myList.GroupBy(x => x.Property)
                  .OrderByDescending(g => g.Count())
                  .Select(g => g.Key);
                  .ToList();

Edit:

It seems like this would be more what you are actually are looking for – groups are are ordered by the number of unique users that have a given property.

var props = myList.GroupBy(x => x.Property)
                  .OrderByDescending(g => g.Select(x=> x.UserID)
                                           .Distinct()
                                           .Count())
                  .Select(g => g.Key);
                  .ToList();