Distinct with lambda

Question

Right  so I have an enumerable and wish to get distinct values from it  Using System Linq  there s  of course  an extension method called Distinct  In the simple case  it can be used with no parameters  like  var distinctValues   myStringList Distinct     Well and good  but if I have an enumerable of objects for which I need to specify equality  the only available overload is  var distinctValues   myCustomerList Distinct someEqualityComparer    The equality comparer argument must be an instance of IEqualityComparer lt T gt   I can do this  of course  but it s somewhat verbose and  well  cludgy  What I would have expected is an overload that would take a lambda  say a Func lt T  T  bool gt   var distinctValues   myCustomerList Distinct  c1  c2    gt  c1 CustomerId    c2 CustomerId    Anyone know if some such extension exists  or some equivalent workaround  Or am I missing something  Alternatively  is there a way of specifying an IEqualityComparer inline  embarrass me   Update I found a reply by Anders Hejlsberg to a post in an MSDN forum on this subject  He says   The problem you re going to run into is that when two objects compare equal they must have the same GetHashCode return value  or else the hash table used internally by Distinct will not function correctly   We use IEqualityComparer because it packages compatible implementations of Equals and GetHashCode into a single interface   I suppose that makes sense

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It looks to me like you want DistinctBy from MoreLINQ  You can then write   var distinctValues   myCustomerList DistinctBy c   gt  c CustomerId     Here s a cut-down version of DistinctBy  no nullity checking and no option to specify your own key comparer    public static IEnumerable lt TSource gt  DistinctBy lt TSource  TKey gt        this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector        HashSet lt TKey gt  knownKeys   new HashSet lt TKey gt         foreach  TSource element in source                if  knownKeys Add keySelector element                          yield return element

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No there is no such extension method overload for this   I ve found this frustrating myself in the past and as such I usually write a helper class to deal with this problem   The goal is to convert a Func lt T T bool gt  to IEqualityComparer lt T T gt      Example  public class EqualityFactory     private sealed class Impl lt T gt    IEqualityComparer lt T T gt        private Func lt T T bool gt  m del      private IEqualityComparer lt T gt  m comp      public Impl Func lt T T bool gt  del           m del   del        m comp   EqualityComparer lt T gt  Default            public bool Equals T left  T right          return m del left  right              public int GetHashCode T value          return m comp GetHashCode value               public static IEqualityComparer lt T T gt  Create lt T gt  Func lt T T bool gt  del        return new Impl lt T gt  del           This allows you to write the following  var distinctValues   myCustomerList    Distinct EqualityFactory Create  c1  c2    gt  c1 CustomerId    c2 CustomerId

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IEnumerable lt Customer gt  filteredList   originalList    GroupBy customer   gt  customer CustomerId     Select group   gt  group First

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This will do what you want but I don t know about performance   var distinctValues       from cust in myCustomerList     group cust by cust CustomerId     into gcust     select gcust First      At least it s not verbose

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A tricky way to do this is use Aggregate   extension  using a dictionary as accumulator with the key-property values as keys   var customers   new List lt Customer gt      var distincts   customers Aggregate new Dictionary lt int  Customer gt                                           d  e    gt    d e CustomerId    e  return d                                         d   gt  d Values     And a GroupBy-style solution is using ToLookup     var distincts   customers ToLookup c   gt  c CustomerId  Select g   gt  g First

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IEnumerable lambda extension   public static class ListExtensions               public static IEnumerable lt T gt  Distinct lt T gt  this IEnumerable lt T gt  list  Func lt T  int gt  hashCode                Dictionary lt int  T gt  hashCodeDic   new Dictionary lt int  T gt              list ToList   ForEach t   gt                                    var key   hashCode t                   if   hashCodeDic ContainsKey key                       hashCodeDic Add key  t                            return hashCodeDic Select kvp   gt  kvp Value             Usage   class Employee       public string Name   get  set        public int EmployeeID   get  set         Add 5 employees to List List lt Employee gt  lst   new List lt Employee gt      Employee e   new Employee   Name    Shantanu   EmployeeID   123456    lst Add e   lst Add e    Employee e1   new Employee   Name    Adam Warren   EmployeeID   823456    lst Add e1     Add a space in the Name Employee e2   new Employee   Name    Adam  Warren   EmployeeID   823456    lst Add e2     Name is different case Employee e3   new Employee   Name    adam warren   EmployeeID   823456    lst Add e3                  Distinct  without IEqalityComparer lt T gt   - Returns 4 employees var lstDistinct1   lst Distinct       Lambda Extension - Return 2 employees var lstDistinct   lst Distinct employee   gt  employee EmployeeID GetHashCode     employee Name ToUpper   Replace          GetHashCode

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Something I have used which worked well for me        lt summary gt      A class to wrap the IEqualityComparer interface into matching functions for simple implementation      lt  summary gt       lt typeparam name  T  gt The type of object to be compared lt  typeparam gt  public class MyIEqualityComparer lt T gt    IEqualityComparer lt T gt             lt summary gt          Create a new comparer based on the given Equals and GetHashCode methods          lt  summary gt           lt param name  equals  gt The method to compute equals of two T instances lt  param gt           lt param name  getHashCode  gt The method to compute a hashcode for a T instance lt  param gt      public MyIEqualityComparer Func lt T  T  bool gt  equals  Func lt T  int gt  getHashCode                if  equals    null              throw new ArgumentNullException  equals    Equals parameter is required for all MyIEqualityComparer instances            EqualsMethod   equals          GetHashCodeMethod   getHashCode                 lt summary gt          Gets the method used to compute equals          lt  summary gt      public Func lt T  T  bool gt  EqualsMethod   get  private set             lt summary gt          Gets the method used to compute a hash code          lt  summary gt      public Func lt T  int gt  GetHashCodeMethod   get  private set         bool IEqualityComparer lt T gt  Equals T x  T y                return EqualsMethod x  y              int IEqualityComparer lt T gt  GetHashCode T obj                if  GetHashCodeMethod    null              return obj GetHashCode            return GetHashCodeMethod obj

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Here s a simple extension method that does what I need     public static class EnumerableExtensions       public static IEnumerable lt TKey gt  Distinct lt T  TKey gt  this IEnumerable lt T gt  source  Func lt T  TKey gt  selector                return source GroupBy selector  Select x   gt  x Key             It s a shame they didn t bake a distinct method like this into the framework  but hey ho

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You can use InlineComparer  public class InlineComparer lt T gt    IEqualityComparer lt T gt          private readonly Func lt T  T  bool gt  equalsMethod        private readonly Func lt T  int gt  getHashCodeMethod      public Func lt T  T  bool gt  EqualsMethod   get  private set        public Func lt T  int gt  GetHashCodeMethod   get  private set         public InlineComparer Func lt T  T  bool gt  equals  Func lt T  int gt  hashCode                if  equals    null  throw new ArgumentNullException  equals    Equals parameter is required for all InlineComparer instances            EqualsMethod   equals          GetHashCodeMethod   hashCode             public bool Equals T x  T y                return EqualsMethod x  y              public int GetHashCode T obj                if  GetHashCodeMethod    null  return obj GetHashCode            return GetHashCodeMethod obj             Usage sample     var comparer   new InlineComparer lt DetalleLog gt   i1  i2    gt  i1 PeticionEV    i2 PeticionEV  amp  amp  i1 Etiqueta    i2 Etiqueta  i   gt  i PeticionEV GetHashCode     i Etiqueta GetHashCode       var peticionesEV   listaLogs Distinct comparer  ToList      Assert IsNotNull peticionesEV     Assert AreNotEqual 0  peticionesEV Count     Source  https   stackoverflow com a 5969691 206730 Using IEqualityComparer for Union Can I specify my explicit type comparator inline

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The Microsoft System Interactive package has a version of Distinct that takes a key selector lambda  This is effectively the same as Jon Skeet s solution  but it may be helpful for people to know  and to check out the rest of the library

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All solutions I ve seen here rely on selecting an already comparable field  If one needs to compare in a different way  though  this solution here seems to work generally  for something like   somedoubles Distinct new LambdaComparer lt double gt   x  y    gt  Math Abs x - y   lt  double Epsilon   Count

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Take another way    var distinctValues   myCustomerList  Select x   gt  x  myCaustomerProperty  Distinct      The sequence return distinct elements compare them by property   myCaustomerProperty

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Here s how you can do it   public static class Extensions       public static IEnumerable lt T gt  MyDistinct lt T  V gt  this IEnumerable lt T gt  query                                                      Func lt T  V gt  f                                                       Func lt IGrouping lt V T gt  T gt  h null                if  h  null  h  x   gt  x First             return query GroupBy f  Select h             This method allows you to use it by specifying one parameter like  MyDistinct d   gt  d Name   but it also allows you to specify a having condition as a second parameter like so   var myQuery    from x in  myObject select x  MyDistinct d   gt  d Name          x   gt  x FirstOrDefault y  gt y Name Contains  1      y Name Contains  2                 N B  This would also allow you to specify other functions like for example  LastOrDefault      as well     If you want to expose just the condition  you can have it even simpler by implementing it as   public static IEnumerable lt T gt  MyDistinct2 lt T  V gt  this IEnumerable lt T gt  query                                                  Func lt T  V gt  f                                                  Func lt T bool gt  h null                                                         if  h    null  h    y   gt  true       return query GroupBy f  Select x  gt x FirstOrDefault h        In this case  the query would just look like   var myQuery2    from x in  myObject select x  MyDistinct2 d   gt  d Name                      y   gt  y Name Contains  1      y Name Contains  2                            N B  Here  the expression is simpler  but note  MyDistinct2 uses  FirstOrDefault      implicitly     Note  The examples above are using the following demo class  class MyObject       public string Name      public string Code     private MyObject    myObject         new MyObject     Name    Test1   Code    T        new MyObject     Name    Test2   Code    Q        new MyObject     Name    Test2   Code    T        new MyObject     Name    Test5   Code    Q

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If Distinct   doesn t produce unique results  try this one   var filteredWC   tblWorkCenter GroupBy cc   gt  cc WCID I  Select grp   gt  grp First    Select cc   gt  new Model WorkCenter   WCID   cc WCID I    OrderBy cc   gt  cc WCID     ObservableCollection lt Model WorkCenter gt  WorkCenter   new ObservableCollection lt Model WorkCenter gt  filteredWC

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Shorthand solution   myCustomerList GroupBy c   gt  c CustomerId   key  c    gt  c FirstOrDefault

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I m assuming you have an IEnumerable  and in your example delegate  you would like c1 and c2 to be referring to two elements in this list   I believe you could achieve this with a self join var distinctResults   from c1 in myList                       join c2 in myList on

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To Wrap things up   I think most of the people which came here like me want the simplest solution possible without using any libraries and with best possible performance    The accepted group by method for me i think is an overkill in terms of performance     Here is a simple extension method using the IEqualityComparer interface which works also for null values   Usage   var filtered   taskList DistinctBy t   gt  t TaskExternalId  ToArray      Extension Method Code  public static class LinqExtensions       public static IEnumerable lt T gt  DistinctBy lt T  TKey gt  this IEnumerable lt T gt  items  Func lt T  TKey gt  property                GeneralPropertyComparer lt T  TKey gt  comparer   new GeneralPropertyComparer lt T TKey gt  property           return items Distinct comparer              public class GeneralPropertyComparer lt T TKey gt    IEqualityComparer lt T gt        private Func lt T  TKey gt  expr   get  set        public GeneralPropertyComparer  Func lt T  TKey gt  expr                this expr   expr            public bool Equals T left  T right                var leftProp   expr Invoke left           var rightProp   expr Invoke right           if  leftProp    null  amp  amp  rightProp    null              return true          else if  leftProp    null   rightProp    null              return false          else             return leftProp Equals rightProp             public int GetHashCode T obj                var prop   expr Invoke obj           return  prop  null   0 prop GetHashCode

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You can use LambdaEqualityComparer   var distinctValues       myCustomerList Distinct new LambdaEqualityComparer lt OurType gt   c1  c2    gt  c1 CustomerId    c2 CustomerId      public class LambdaEqualityComparer lt T gt    IEqualityComparer lt T gt                public LambdaEqualityComparer Func lt T  T  bool gt  equalsFunction                         equalsFunction   equalsFunction                     public bool Equals T x  T y                        return  equalsFunction x  y                      public int GetHashCode T obj                        return obj GetHashCode                       private readonly Func lt T  T  bool gt   equalsFunction

[c#] Distinct() with lambda?

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