How can I get LINQ to return the object which has the max value for a given property

Question

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 lt Item gt  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   gt  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

User · Answer

In LINQ you can solve it the following way   Item itemMax    from i in items      let maxId   items Max m   gt  m ID       where i ID    maxId      select i  FirstOrDefault

User · Answer

This will loop through only once   Item biggest   items Aggregate  i1 i2    gt  i1 ID  gt  i2 ID   i1   i2     Thanks Nick - Here s the proof  class Program       static void Main string   args                IEnumerable lt Item gt  items1   new List lt Item gt                          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    gt  i1 ID  gt  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

User · Answer

Use MaxBy from the morelinq project   items MaxBy i   gt  i ID

User · Answer

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    gt   el ID  gt  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 lt T gt  items  Func lt T  int gt  f      return items Aggregate      new   Max   Int32 MinValue  Item   default T          state  el    gt          var current   f el ID         if  current  gt  state Max           return new   Max   current  Item   el          else          return state          Item

User · Answer

Or you can write your own extension method   static partial class Extensions       public static T WhereMax lt T  U gt  this IEnumerable lt T gt  items  Func lt T  U gt  selector                if   items Any                          throw new InvalidOperationException  Empty input sequence                       var comparer   Comparer lt U gt  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   gt  0                                maxValue   value                  maxItem    item                                   return maxItem

User · Answer

OrderByDescending i  gt 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

User · Answer

You could use a captured variable   Item result   items FirstOrDefault    items ForEach x   gt      if result ID  lt  x ID      result   x

User · Answer

This is an extension method derived from  Seattle Leonard  s answer    public static T GetMax lt T U gt  this IEnumerable lt T gt  data  Func lt T U gt  f  where U IComparable         return data Aggregate  i1  i2    gt  f i1  CompareTo f i2   gt 0   i1   i2

User · Answer

try this    var maxid   from i in items             group i by i clientid int g             select new   id   g Max i  gt i ID

User · Answer

int max   items Max i   gt  i ID   var item   items First x   gt  x ID    max     This assumes there are elements in the items collection of course

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

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