How to use LINQ to select object with minimum or maximum property value

Question

I have a Person object with a Nullable DateOfBirth property   Is there a way to use LINQ to query a list of Person objects for the one with the earliest smallest DateOfBirth value   Here s what I started with   var firstBornDate   People Min p   gt  p DateOfBirth GetValueOrDefault DateTime MaxValue      Null DateOfBirth values are set to DateTime MaxValue in order to rule them out of the Min consideration  assuming at least one has a specified DOB    But all that does for me is to set firstBornDate to a DateTime value   What I d like to get is the Person object that matches that   Do I need to write a second query like so   var firstBorn   People Single p  gt   p DateOfBirth    DateTime MaxValue     firstBornDate     Or is there a leaner way of doing it

User · Answer

Solution with no extra packages   var min   lst OrderBy i   gt  i StartDate  FirstOrDefault    var max   lst OrderBy i   gt  i StartDate  LastOrDefault      also you can wrap it into extension   public static class LinqExtensions       public static T MinBy lt T  TProp gt  this IEnumerable lt T gt  source  Func lt T  TProp gt  propSelector                return source OrderBy propSelector  FirstOrDefault               public static T MaxBy lt T  TProp gt  this IEnumerable lt T gt  source  Func lt T  TProp gt  propSelector                return source OrderBy propSelector  LastOrDefault              and in this case   var min   lst MinBy i   gt  i StartDate   var max   lst MaxBy i   gt  i StartDate       By the way     O n 2  is not the best solution  Paul Betts gave fatster solution than my  But my is still LINQ solution and it s more simple and more short than other solutions here

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People Aggregate  curMin  x    gt   curMin    null     x DateOfBirth    DateTime MaxValue   lt      curMin DateOfBirth   x   curMin

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People OrderBy p   gt  p DateOfBirth GetValueOrDefault DateTime MaxValue   First     Would do the trick

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Try the following idea   var firstBornDate   People GroupBy p   gt  p DateOfBirth  Min g   gt  g Key  FirstOrDefault

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A way via extension function on IEnumerable that returns both the object and the minimum found  It takes a Func that can do any operation on the object in the collection  public static  double min  T obj  tMin lt T gt  this IEnumerable lt T gt  ienum               Func lt T  double gt  aFunc                        var okNull   default T               if  okNull    null                  throw new ApplicationException  quot object passed to Min not nullable quot                  double aMin  T okObj  best    double MaxValue  okNull               foreach  T obj in ienum                                double q   aFunc obj                   if  q  lt  best aMin                      best    q  obj                             return  best              Example where object is an Airport and we want to find closest Airport to a given  latitude  longitude   Airport has a dist lat  lon  function   double okDist  Airport best  greatestPort   airPorts tMin x   gt  x dist okLat  okLon

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The following is the more generic solution  It essentially does the same thing  in O N  order  but on any IEnumberable types and can mixed with types whose property selectors could return null   public static class LinqExtensions       public static T MinBy lt T gt  this IEnumerable lt T gt  source  Func lt T  IComparable gt  selector                if  source    null                        throw new ArgumentNullException nameof source                      if  selector    null                        throw new ArgumentNullException nameof selector                      return source Aggregate  min  cur    gt                        if  min    null                                return cur                            var minComparer   selector min               if  minComparer    null                                return cur                            var curComparer   selector cur               if  curComparer    null                                return min                            return minComparer CompareTo curComparer   gt  0   cur   min                        Tests   var nullableInts   new int     5  null  1  4  0  3  null  1   Assert AreEqual 0  nullableInts MinBy i   gt  i     should pass

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public class Foo       public int bar      public int stuff      void Main         List lt Foo gt  fooList   new List lt Foo gt         new Foo   bar 1 stuff 2       new Foo   bar 3 stuff 4       new Foo   bar 2 stuff 3         Foo result   fooList Aggregate  u v    gt  u bar  lt  v bar   u  v       result Dump

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I was looking for something similar myself  preferably without using a library or sorting the entire list  My solution ended up similar to the question itself  just simplified a bit    var firstBorn   People FirstOrDefault p   gt  p DateOfBirth    People Min p2   gt  p2 DateOfBirth

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NOTE  I include this answer for completeness since the OP didn t mention what the data source is and we shouldn t make any assumptions   This query gives the correct answer  but could be slower since it might have to sort all the items in People  depending on what data structure People is   var oldest   People OrderBy p   gt  p DateOfBirth    DateTime MaxValue  First      UPDATE  Actually I shouldn t call this solution  naive   but the user does need to know what he is querying against  This solution s  slowness  depends on the underlying data  If this is a array or List lt T gt   then LINQ to Objects has no choice but to sort the entire collection first before selecting the first item  In this case it will be slower than the other solution suggested  However  if this is a LINQ to SQL table and DateOfBirth is an indexed column  then SQL Server will use the index instead of sorting all the rows  Other custom IEnumerable lt T gt  implementations could also make use of indexes  see i4o  Indexed LINQ  or the object database db4o  and make this solution faster than Aggregate   or MaxBy   MinBy   which need to iterate the whole collection once  In fact  LINQ to Objects could have  in theory  made special cases in OrderBy   for sorted collections like SortedList lt T gt   but it doesn t  as far as I know

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So you are asking for ArgMin or ArgMax  C  doesn t have a built-in API for those  I ve been looking for a clean and efficient  O n  in time  way to do this  And I think I found one  The general form of this pattern is  var min   data Select x   gt   key x   x   Min   Item2                                                                  the sorting key                   take the associated original item                                 Min by key     Specially  using the example in original question  For C  7 0 and above that supports value tuple  var youngest   people Select p   gt   p DateOfBirth  p   Min   Item2   For C  version before 7 0  anonymous type can be used instead  var youngest   people Select p   gt  new  age   p DateOfBirth  ppl   p   Min   ppl   They work because both value tuple and anonymous type have sensible default comparers  for  x1  y1  and  x2  y2   it first compares x1 vs x2  then y1 vs y2  That s why the built-in  Min can be used on those types  And since both anonymous type and value tuple are value types  they should be both very efficient  NOTE In my above ArgMin implementations I assumed DateOfBirth to take type DateTime for simplicity and clarity  The original question asks to exclude those entries with null DateOfBirth field   Null DateOfBirth values are set to DateTime MaxValue in order to rule them out of the Min consideration  assuming at least one has a specified DOB    It can be achieved with a pre-filtering people Where p   gt  p DateOfBirth HasValue   So it s immaterial to the question of implementing ArgMin or ArgMax  NOTE 2 The above approach has a caveat that when there are two instances that have the same min value  then the Min   implementation will try to compare the instances as a tie-breaker  However  if the class of the instances does not implement IComparable  then a runtime error will be thrown   At least one object must implement IComparable  Luckily  this can still be fixed rather cleanly  The idea is to associate a distanct  quot ID quot  with each entry that serves as the unambiguous tie-breaker  We can use an incremental ID for each entry  Still using the people age as example  var youngest   Enumerable Range 0  int MaxValue                  Zip people   idx  ppl    gt   ppl DateOfBirth  idx  ppl   Min   Item3

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Perfectly simple use of aggregate  equivalent to fold in other languages    var firstBorn   People Aggregate  min  x    gt  x DateOfBirth  lt  min DateOfBirth   x   min     The only downside is that the property is accessed twice per sequence element  which might be expensive  That s hard to fix

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Unfortunately there isn t a built-in method to do this  but it s easy enough to implement for yourself  Here are the guts of it  public static TSource MinBy lt TSource  TKey gt  this IEnumerable lt TSource gt  source      Func lt TSource  TKey gt  selector        return source MinBy selector  null      public static TSource MinBy lt TSource  TKey gt  this IEnumerable lt TSource gt  source      Func lt TSource  TKey gt  selector  IComparer lt TKey gt  comparer        if  source    null  throw new ArgumentNullException  quot source quot        if  selector    null  throw new ArgumentNullException  quot selector quot        comparer     Comparer lt TKey gt  Default       using  var sourceIterator   source GetEnumerator                  if   sourceIterator MoveNext                          throw new InvalidOperationException  quot Sequence contains no elements quot                      var min   sourceIterator Current          var minKey   selector min           while  sourceIterator MoveNext                          var candidate   sourceIterator Current              var candidateProjected   selector candidate               if  comparer Compare candidateProjected  minKey   lt  0                                min   candidate                  minKey   candidateProjected                                  return min           Example usage  var firstBorn   People MinBy p   gt  p DateOfBirth    DateTime MaxValue    Note that this will throw an exception if the sequence is empty  and will return the first element with the minimal value if there s more than one  Alternatively  you can use the implementation we ve got in MoreLINQ  in MinBy cs   There s a corresponding MaxBy  of course   Install via package manager console   PM gt  Install-Package morelinq

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IF you want to select object with minimum or maximum property value  another way is to use Implementing IComparable   public struct Money   IComparable lt Money gt       public Money decimal value    this     Value   value       public decimal Value   get  private set       public int CompareTo Money other    return Value CompareTo other Value         Max Implementation will be   var amounts   new List lt Money gt    new Money 20   new Money 10     Money maxAmount   amounts Max      Min Implementation will be   var amounts   new List lt Money gt    new Money 20   new Money 10     Money maxAmount   amounts Min      In this way  you can compare any object and get the Max and Min while returning the object type   Hope This will help someone

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EDIT again    Sorry   Besides missing the nullable I was looking at the wrong function    Min lt  Of  lt  TSource  TResult     IEnumerable lt  Of  lt  TSource      Func lt  Of  lt  TSource  TResult      does return the result type as you said     I would say one possible solution is to implement IComparable and use Min lt  Of  lt  TSource     IEnumerable lt  Of  lt  TSource       which really does return an element from the IEnumerable   Of course  that doesn t help you if you can t modify the element   I find MS s design a bit weird here   Of course  you can always do a for loop if you need to  or use the MoreLINQ implementation Jon Skeet gave

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Another implementation  which could work with nullable selector keys  and for the collection of reference type returns null if no suitable elements found  This could be helpful then processing database results for example     public static class IEnumerableExtensions              lt summary gt          Returns the element with the maximum value of a selector function           lt  summary gt           lt typeparam name  TSource  gt The type of the elements of source  lt  typeparam gt           lt typeparam name  TKey  gt The type of the key returned by keySelector  lt  typeparam gt           lt param name  source  gt An IEnumerable collection values to determine the element with the maximum value of  lt  param gt           lt param name  keySelector  gt A function to extract the key for each element  lt  param gt           lt exception cref  System ArgumentNullException  gt source or keySelector is null  lt  exception gt           lt exception cref  System InvalidOperationException  gt source contains no elements  lt  exception gt           lt returns gt The element in source with the maximum value of a selector function  lt  returns gt      public static TSource MaxBy lt TSource  TKey gt  this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector    gt  MaxOrMinBy source  keySelector  1             lt summary gt          Returns the element with the minimum value of a selector function           lt  summary gt           lt typeparam name  TSource  gt The type of the elements of source  lt  typeparam gt           lt typeparam name  TKey  gt The type of the key returned by keySelector  lt  typeparam gt           lt param name  source  gt An IEnumerable collection values to determine the element with the minimum value of  lt  param gt           lt param name  keySelector  gt A function to extract the key for each element  lt  param gt           lt exception cref  System ArgumentNullException  gt source or keySelector is null  lt  exception gt           lt exception cref  System InvalidOperationException  gt source contains no elements  lt  exception gt           lt returns gt The element in source with the minimum value of a selector function  lt  returns gt      public static TSource MinBy lt TSource  TKey gt  this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector    gt  MaxOrMinBy source  keySelector  -1         private static TSource MaxOrMinBy lt TSource  TKey gt         IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector  int sign              if  source    null  throw new ArgumentNullException nameof source          if  keySelector    null  throw new ArgumentNullException nameof keySelector          Comparer lt TKey gt  comparer   Comparer lt TKey gt  Default        TKey value   default TKey         TSource result   default TSource          bool hasValue   false         foreach  TSource element in source                  TKey x   keySelector element           if  x    null                      if   hasValue                          value   x              result   element              hasValue   true                        else if  sign   comparer Compare x  value   gt  0                          value   x              result   element                                       if   result    null   amp  amp   hasValue          throw new InvalidOperationException  The source sequence is empty           return result               Example   public class A     public int  a    public A int  a    this a   a       var b   a MinBy x   gt  x a   var c   a MaxBy x   gt  x a

[c#] How to use LINQ to select object with minimum or maximum property value

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