[c#] How can I get LINQ to return the object which has the max value for a given property?

If I have a class that looks like:

public class Item
{
    public int ClientID { get; set; }
    public int ID { get; set; }
}

And a collection of those items...

List<Item> items = getItems();

How can I use LINQ to return the single "Item" object which has the highest ID?

If I do something like:

items.Select(i => i.ID).Max(); 

I'll only get the highest ID, when what I actually want returned is the Item object itself which has the highest ID? I want it to return a single "Item" object, not an int.

You could use a captured variable.

Item result = items.FirstOrDefault();
items.ForEach(x =>
{
  if(result.ID < x.ID)
    result = x;
});

Use MaxBy from the morelinq project:

items.MaxBy(i => i.ID);

.OrderByDescending(i=>i.id).First(1)

Regarding the performance concern, it is very likely that this method is theoretically slower than a linear approach. However, in reality, most of the time we are not dealing with the data set that is big enough to make any difference.

If performance is a main concern, Seattle Leonard's answer should give you linear time complexity. Alternatively, you may also consider to start with a different data structure that returns the max value item at constant time.

try this:

var maxid = from i in items
            group i by i.clientid int g
            select new { id = g.Max(i=>i.ID }

In case you don't want to use MoreLINQ and want to get linear time, you can also use Aggregate:

var maxItem = 
  items.Aggregate(
    new { Max = Int32.MinValue, Item = (Item)null },
    (state, el) => (el.ID > state.Max) 
      ? new { Max = el.ID, Item = el } : state).Item;

This remembers the current maximal element (Item) and the current maximal value (Item) in an anonymous type. Then you just pick the Item property. This is indeed a bit ugly and you could wrap it into MaxBy extension method to get the same thing as with MoreLINQ:

public static T MaxBy(this IEnumerable<T> items, Func<T, int> f) {
  return items.Aggregate(
    new { Max = Int32.MinValue, Item = default(T) },
    (state, el) => {
      var current = f(el.ID);
      if (current > state.Max) 
        return new { Max = current, Item = el };
      else 
        return state; 
    }).Item;
}

Or you can write your own extension method:

static partial class Extensions
{
    public static T WhereMax<T, U>(this IEnumerable<T> items, Func<T, U> selector)
    {
        if (!items.Any())
        {
            throw new InvalidOperationException("Empty input sequence");
        }

        var comparer = Comparer<U>.Default;
        T   maxItem  = items.First();
        U   maxValue = selector(maxItem);

        foreach (T item in items.Skip(1))
        {
            // Get the value of the item and compare it to the current max.
            U value = selector(item);
            if (comparer.Compare(value, maxValue) > 0)
            {
                maxValue = value;
                maxItem  = item;
            }
        }

        return maxItem;
    }
}

int max = items.Max(i => i.ID);
var item = items.First(x => x.ID == max);

This assumes there are elements in the items collection of course.

In LINQ you can solve it the following way:

Item itemMax = (from i in items
     let maxId = items.Max(m => m.ID)
     where i.ID == maxId
     select i).FirstOrDefault();

This will loop through only once.

Item biggest = items.Aggregate((i1,i2) => i1.ID > i2.ID ? i1 : i2);

Thanks Nick - Here's the proof

class Program
{
    static void Main(string[] args)
    {
        IEnumerable<Item> items1 = new List<Item>()
        {
            new Item(){ ClientID = 1, ID = 1},
            new Item(){ ClientID = 2, ID = 2},
            new Item(){ ClientID = 3, ID = 3},
            new Item(){ ClientID = 4, ID = 4},
        };
        Item biggest1 = items1.Aggregate((i1, i2) => i1.ID > i2.ID ? i1 : i2);

        Console.WriteLine(biggest1.ID);
        Console.ReadKey();
    }


}

public class Item
{
    public int ClientID { get; set; }
    public int ID { get; set; }
}  

Rearrange the list and get the same result

This is an extension method derived from @Seattle Leonard 's answer:

 public static T GetMax<T,U>(this IEnumerable<T> data, Func<T,U> f) where U:IComparable
 {
     return data.Aggregate((i1, i2) => f(i1).CompareTo(f(i2))>0 ? i1 : i2);
 }