How to perform .Max() on a property of all objects in a collection and return the object with maximum value [duplicate]

Answer 1

This would require a sort (O(n log n)) but is very simple and flexible. Another advantage is being able to use it with LINQ to SQL:

var maxObject = list.OrderByDescending(item => item.Height).First();

Note that this has the advantage of enumerating the list sequence just once. While it might not matter if list is a List<T> that doesn't change in the meantime, it could matter for arbitrary IEnumerable<T> objects. Nothing guarantees that the sequence doesn't change in different enumerations so methods that are doing it multiple times can be dangerous (and inefficient, depending on the nature of the sequence). However, it's still a less than ideal solution for large sequences. I suggest writing your own MaxObject extension manually if you have a large set of items to be able to do it in one pass without sorting and other stuff whatsoever (O(n)):

static class EnumerableExtensions {
    public static T MaxObject<T,U>(this IEnumerable<T> source, Func<T,U> selector)
      where U : IComparable<U> {
       if (source == null) throw new ArgumentNullException("source");
       bool first = true;
       T maxObj = default(T);
       U maxKey = default(U);
       foreach (var item in source) {
           if (first) {
                maxObj = item;
                maxKey = selector(maxObj);
                first = false;
           } else {
                U currentKey = selector(item);
                if (currentKey.CompareTo(maxKey) > 0) {
                    maxKey = currentKey;
                    maxObj = item;
                }
           }
       }
       if (first) throw new InvalidOperationException("Sequence is empty.");
       return maxObj;
    }
}

and use it with:

var maxObject = list.MaxObject(item => item.Height);

Answer 2

In NHibernate (with NHibernate.Linq) you could do it as follows:

return session.Query<T>()
              .Single(a => a.Filter == filter &&
                           a.Id == session.Query<T>()
                                          .Where(a2 => a2.Filter == filter)
                                          .Max(a2 => a2.Id));

Which will generate SQL like follows:

select *
from TableName foo
where foo.Filter = 'Filter On String'
and foo.Id = (select cast(max(bar.RowVersion) as INT)
              from TableName bar
              where bar.Name = 'Filter On String')

Which seems pretty efficient to me.

Answer 3

Doing an ordering and then selecting the first item is wasting a lot of time ordering the items after the first one. You don't care about the order of those.

Instead you can use the aggregate function to select the best item based on what you're looking for.

var maxHeight = dimensions
    .Aggregate((agg, next) => 
        next.Height > agg.Height ? next : agg);

var maxHeightAndWidth = dimensions
    .Aggregate((agg, next) => 
        next.Height >= agg.Height && next.Width >= agg.Width ? next: agg);