LINQ s Distinct on a particular property

Question

I am playing with LINQ to learn about it  but I can t figure out how to use Distinct when I do not have a simple list  a simple list of integers is pretty easy to do  this is not the question   What I if want to use Distinct on a list of an Object on one or more properties of the object   Example  If an object is Person  with Property Id  How can I get all Person and use Distinct on them with the property Id of the object   Person1  Id 1  Name  Test1  Person2  Id 1  Name  Test1  Person3  Id 2  Name  Test2    How can I get just Person1 and Person3  Is that possible   If it s not possible with LINQ  what would be the best way to have a list of Person depending on some of its properties in  NET 3 5

User · Answer

The best way to do this that will be compatible with other  NET versions is to override Equals and GetHash to handle this  see Stack Overflow question This code returns distinct values  However  what I want is to return a strongly typed collection as opposed to an anonymous type   but if you need something that is generic throughout your code  the solutions in this article are great

User · Answer

You can do this with the standard Linq ToLookup    This will create a collection of values for each unique key  Just select the first item in the collection  Persons ToLookup p   gt  p Id  Select coll   gt  coll First

User · Answer

EDIT  This is now part of MoreLINQ   What you need is a  distinct-by  effectively  I don t believe it s part of LINQ as it stands  although it s fairly easy to write   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  seenKeys   new HashSet lt TKey gt         foreach  TSource element in source                if  seenKeys Add keySelector element                          yield return element                      So to find the distinct values using just the Id property  you could use   var query   people DistinctBy p   gt  p Id     And to use multiple properties  you can use anonymous types  which implement equality appropriately   var query   people DistinctBy p   gt  new   p Id  p Name       Untested  but it should work  and it now at least compiles    It assumes the default comparer for the keys though - if you want to pass in an equality comparer  just pass it on to the HashSet constructor

User · Answer

Override Equals object obj  and GetHashCode   methods   class Person       public int Id   get  set        public int Name   get  set         public override bool Equals object obj                return   Person obj  Id    Id             or              var o    Person obj             return o Id    Id  amp  amp  o Name    Name            public override int GetHashCode                 return Id GetHashCode              and then just call   List lt Person gt  distinctList   new     person1  person2  person3   Distinct   ToList

User · Answer

Override Equals object obj  and GetHashCode   methods   class Person       public int Id   get  set        public int Name   get  set         public override bool Equals object obj                return   Person obj  Id    Id             or              var o    Person obj             return o Id    Id  amp  amp  o Name    Name            public override int GetHashCode                 return Id GetHashCode              and then just call   List lt Person gt  distinctList   new     person1  person2  person3   Distinct   ToList

User · Answer

What if I want to obtain a distinct list based on one or more properties    Simple  You want to group them and pick a winner out of the group   List lt Person gt  distinctPeople   allPeople    GroupBy p   gt  p PersonId     Select g   gt  g First       ToList      If you want to define groups on multiple properties  here s how   List lt Person gt  distinctPeople   allPeople    GroupBy p   gt  new  p PersonId  p FavoriteColor       Select g   gt  g First       ToList

User · Answer

I ve written an article that explains how to extend the Distinct function so that you can do as follows  var people   new List lt Person gt      people Add new Person 1   quot a quot    quot b quot     people Add new Person 2   quot c quot    quot d quot     people Add new Person 1   quot a quot    quot b quot      foreach  var person in people Distinct p   gt  p ID          Do stuff with unique list here   Here s the article  now in the Web Archive   Extending LINQ - Specifying a Property in the Distinct Function

User · Answer

You can do it  albeit not lightning-quickly  like so   people Where p   gt   people Any q   gt   p    q  amp  amp  p Id    q Id       That is   select all people where there isn t another different person in the list with the same ID    Mind you  in your example  that would just select person 3   I m not sure how to tell which you want  out of the previous two

User · Answer

I think it is enough   list Select s   gt  s MyField  Distinct

User · Answer

List lt Person gt lst new List lt Person gt          var result1   lst OrderByDescending a   gt  a ID  Select a   gt new Player  ID a ID Name a Name    Distinct

User · Answer

You can use DistinctBy   for getting Distinct records by an object property  Just add the following statement before using it      using Microsoft Ajax Utilities    and then use it like following   var listToReturn   responseList DistinctBy x   gt  x Index  ToList      where  Index  is the property on which i want the data to be distinct

User · Answer

In case you need a Distinct method on multiple properties  you can check out my PowerfulExtensions library  Currently it s in a very young stage  but already you can use methods like Distinct  Union  Intersect  Except on any number of properties   This is how you use it   using PowerfulExtensions Linq      var distinct   myArray Distinct x   gt  x A  x   gt  x B

User · Answer

EDIT  This is now part of MoreLINQ   What you need is a  distinct-by  effectively  I don t believe it s part of LINQ as it stands  although it s fairly easy to write   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  seenKeys   new HashSet lt TKey gt         foreach  TSource element in source                if  seenKeys Add keySelector element                          yield return element                      So to find the distinct values using just the Id property  you could use   var query   people DistinctBy p   gt  p Id     And to use multiple properties  you can use anonymous types  which implement equality appropriately   var query   people DistinctBy p   gt  new   p Id  p Name       Untested  but it should work  and it now at least compiles    It assumes the default comparer for the keys though - if you want to pass in an equality comparer  just pass it on to the HashSet constructor

User · Answer

What if I want to obtain a distinct list based on one or more properties    Simple  You want to group them and pick a winner out of the group   List lt Person gt  distinctPeople   allPeople    GroupBy p   gt  p PersonId     Select g   gt  g First       ToList      If you want to define groups on multiple properties  here s how   List lt Person gt  distinctPeople   allPeople    GroupBy p   gt  new  p PersonId  p FavoriteColor       Select g   gt  g First       ToList

User · Answer

You should be able to override Equals on person to actually do Equals on Person id   This ought to result in the behavior you re after

User · Answer

What if I want to obtain a distinct list based on one or more properties    Simple  You want to group them and pick a winner out of the group   List lt Person gt  distinctPeople   allPeople    GroupBy p   gt  p PersonId     Select g   gt  g First       ToList      If you want to define groups on multiple properties  here s how   List lt Person gt  distinctPeople   allPeople    GroupBy p   gt  new  p PersonId  p FavoriteColor       Select g   gt  g First       ToList

User · Answer

You can do it  albeit not lightning-quickly  like so   people Where p   gt   people Any q   gt   p    q  amp  amp  p Id    q Id       That is   select all people where there isn t another different person in the list with the same ID    Mind you  in your example  that would just select person 3   I m not sure how to tell which you want  out of the previous two

User · Answer

You should be able to override Equals on person to actually do Equals on Person id   This ought to result in the behavior you re after

User · Answer

EDIT  This is now part of MoreLINQ   What you need is a  distinct-by  effectively  I don t believe it s part of LINQ as it stands  although it s fairly easy to write   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  seenKeys   new HashSet lt TKey gt         foreach  TSource element in source                if  seenKeys Add keySelector element                          yield return element                      So to find the distinct values using just the Id property  you could use   var query   people DistinctBy p   gt  p Id     And to use multiple properties  you can use anonymous types  which implement equality appropriately   var query   people DistinctBy p   gt  new   p Id  p Name       Untested  but it should work  and it now at least compiles    It assumes the default comparer for the keys though - if you want to pass in an equality comparer  just pass it on to the HashSet constructor

User · Answer

EDIT  This is now part of MoreLINQ   What you need is a  distinct-by  effectively  I don t believe it s part of LINQ as it stands  although it s fairly easy to write   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  seenKeys   new HashSet lt TKey gt         foreach  TSource element in source                if  seenKeys Add keySelector element                          yield return element                      So to find the distinct values using just the Id property  you could use   var query   people DistinctBy p   gt  p Id     And to use multiple properties  you can use anonymous types  which implement equality appropriately   var query   people DistinctBy p   gt  new   p Id  p Name       Untested  but it should work  and it now at least compiles    It assumes the default comparer for the keys though - if you want to pass in an equality comparer  just pass it on to the HashSet constructor

User · Answer

What if I want to obtain a distinct list based on one or more properties    Simple  You want to group them and pick a winner out of the group   List lt Person gt  distinctPeople   allPeople    GroupBy p   gt  p PersonId     Select g   gt  g First       ToList      If you want to define groups on multiple properties  here s how   List lt Person gt  distinctPeople   allPeople    GroupBy p   gt  new  p PersonId  p FavoriteColor       Select g   gt  g First       ToList

User · Answer

You should be able to override Equals on person to actually do Equals on Person id   This ought to result in the behavior you re after

User · Answer

If you don t want to add the MoreLinq library to your project just to get the DistinctBy functionality then you can get the same end result using the overload of Linq s Distinct method that takes in an IEqualityComparer argument   You begin by creating a generic custom equality comparer class that uses lambda syntax to perform custom comparison of two instances of a generic class   public class CustomEqualityComparer lt T gt    IEqualityComparer lt T gt        Func lt T  T  bool gt   comparison      Func lt T  int gt   hashCodeFactory       public CustomEqualityComparer Func lt T  T  bool gt  comparison  Func lt T  int gt  hashCodeFactory                 comparison   comparison           hashCodeFactory   hashCodeFactory             public bool Equals T x  T y                return  comparison x  y              public int GetHashCode T obj                return  hashCodeFactory obj             Then in your main code you use it like so   Func lt Person  Person  bool gt  areEqual    p1  p2    gt  int Equals p1 Id  p2 Id    Func lt Person  int gt  getHashCode    p    gt  p Id GetHashCode     var query   people Distinct new CustomEqualityComparer lt Person gt  areEqual  getHashCode      Voila       The above assumes the following    Property Person Id is of type int The people collection does not contain any null elements   If the collection could contain nulls then simply rewrite the lambdas to check for null  e g    Func lt Person  Person  bool gt  areEqual    p1  p2    gt         return  p1    null  amp  amp  p2    null    int Equals p1 Id  p2 Id    false       EDIT  This approach is similar to the one in Vladimir Nesterovsky s answer but simpler    It is also similar to the one in Joel s answer but allows for complex comparison logic involving multiple properties    However  if your objects can only ever differ by Id then another user gave the correct answer that all you need to do is override the default implementations of GetHashCode   and Equals   in your Person class and then just use the out-of-the-box Distinct   method of Linq to filter out any duplicates

User · Answer

Please give a try with below code       var Item   GetAll   GroupBy x   gt  x  Id  ToList

User · Answer

Solution first group by your fields then select firstordefault item       List lt Person gt  distinctPeople   allPeople     GroupBy p   gt  p PersonId      Select g   gt  g FirstOrDefault        ToList

User · Answer

If you don t want to add the MoreLinq library to your project just to get the DistinctBy functionality then you can get the same end result using the overload of Linq s Distinct method that takes in an IEqualityComparer argument   You begin by creating a generic custom equality comparer class that uses lambda syntax to perform custom comparison of two instances of a generic class   public class CustomEqualityComparer lt T gt    IEqualityComparer lt T gt        Func lt T  T  bool gt   comparison      Func lt T  int gt   hashCodeFactory       public CustomEqualityComparer Func lt T  T  bool gt  comparison  Func lt T  int gt  hashCodeFactory                 comparison   comparison           hashCodeFactory   hashCodeFactory             public bool Equals T x  T y                return  comparison x  y              public int GetHashCode T obj                return  hashCodeFactory obj             Then in your main code you use it like so   Func lt Person  Person  bool gt  areEqual    p1  p2    gt  int Equals p1 Id  p2 Id    Func lt Person  int gt  getHashCode    p    gt  p Id GetHashCode     var query   people Distinct new CustomEqualityComparer lt Person gt  areEqual  getHashCode      Voila       The above assumes the following    Property Person Id is of type int The people collection does not contain any null elements   If the collection could contain nulls then simply rewrite the lambdas to check for null  e g    Func lt Person  Person  bool gt  areEqual    p1  p2    gt         return  p1    null  amp  amp  p2    null    int Equals p1 Id  p2 Id    false       EDIT  This approach is similar to the one in Vladimir Nesterovsky s answer but simpler    It is also similar to the one in Joel s answer but allows for complex comparison logic involving multiple properties    However  if your objects can only ever differ by Id then another user gave the correct answer that all you need to do is override the default implementations of GetHashCode   and Equals   in your Person class and then just use the out-of-the-box Distinct   method of Linq to filter out any duplicates

User · Answer

You should be able to override Equals on person to actually do Equals on Person id   This ought to result in the behavior you re after

User · Answer

You could also use query syntax if you want it to look all LINQ-like   var uniquePeople   from p in people                    group p by new  p ID    or group by new  p ID  p Name  p Whatever                     into mygroup                    select mygroup FirstOrDefault

User · Answer

You can do this with the standard Linq ToLookup    This will create a collection of values for each unique key  Just select the first item in the collection  Persons ToLookup p   gt  p Id  Select coll   gt  coll First

User · Answer

The following code is functionally equivalent to Jon Skeet s answer   Tested on  NET 4 5  should work on any earlier version of LINQ   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  seenKeys   new HashSet lt TKey gt       return source Where element   gt  seenKeys Add keySelector element         Incidentially  check out Jon Skeet s latest version of DistinctBy cs on Google Code

User · Answer

I think it is enough   list Select s   gt  s MyField  Distinct

User · Answer

You could also use query syntax if you want it to look all LINQ-like   var uniquePeople   from p in people                    group p by new  p ID    or group by new  p ID  p Name  p Whatever                     into mygroup                    select mygroup FirstOrDefault

User · Answer

When we faced such a task in our project we defined a small API to compose comparators   So  the use case was like this   var wordComparer   KeyEqualityComparer Null lt Word gt         ThenBy item   gt  item Text       ThenBy item   gt  item LangID       source Select      Distinct wordComparer     And API itself looks like this   using System  using System Collections  using System Collections Generic   public static class KeyEqualityComparer       public static IEqualityComparer lt T gt  Null lt T gt                  return null             public static IEqualityComparer lt T gt  EqualityComparerBy lt T  K gt           this IEnumerable lt T gt  source          Func lt T  K gt  keyFunc                return new KeyEqualityComparer lt T  K gt  keyFunc              public static KeyEqualityComparer lt T  K gt  ThenBy lt T  K gt           this IEqualityComparer lt T gt  equalityComparer          Func lt T  K gt  keyFunc                return new KeyEqualityComparer lt T  K gt  keyFunc  equalityComparer            public struct KeyEqualityComparer lt T  K gt   IEqualityComparer lt T gt        public KeyEqualityComparer          Func lt T  K gt  keyFunc          IEqualityComparer lt T gt  equalityComparer   null                KeyFunc   keyFunc          EqualityComparer   equalityComparer             public bool Equals T x  T y                return   EqualityComparer    null     EqualityComparer Equals x  y    amp  amp                  EqualityComparer lt K gt  Default Equals KeyFunc x   KeyFunc y               public int GetHashCode T obj                var hash   EqualityComparer lt K gt  Default GetHashCode KeyFunc obj             if  EqualityComparer    null                        var hash2   EqualityComparer GetHashCode obj                hash     hash2  lt  lt  5    hash2                     return hash             public readonly Func lt T  K gt  KeyFunc      public readonly IEqualityComparer lt T gt  EqualityComparer      More details is on our site  IEqualityComparer in LINQ

User · Answer

You can do it  albeit not lightning-quickly  like so   people Where p   gt   people Any q   gt   p    q  amp  amp  p Id    q Id       That is   select all people where there isn t another different person in the list with the same ID    Mind you  in your example  that would just select person 3   I m not sure how to tell which you want  out of the previous two

User · Answer

You can do it  albeit not lightning-quickly  like so   people Where p   gt   people Any q   gt   p    q  amp  amp  p Id    q Id       That is   select all people where there isn t another different person in the list with the same ID    Mind you  in your example  that would just select person 3   I m not sure how to tell which you want  out of the previous two

User · Answer

Personally I use the following class   public class LambdaEqualityComparer lt TSource  TDest gt         IEqualityComparer lt TSource gt        private Func lt TSource  TDest gt   selector       public LambdaEqualityComparer Func lt TSource  TDest gt  selector                 selector   selector             public bool Equals TSource obj  TSource other                return  selector obj  Equals  selector other               public int GetHashCode TSource obj                return  selector obj  GetHashCode              Then  an extension method   public static IEnumerable lt TSource gt  Distinct lt TSource  TCompare gt       this IEnumerable lt TSource gt  source  Func lt TSource  TCompare gt  selector        return source Distinct new LambdaEqualityComparer lt TSource  TCompare gt  selector        Finally  the intended usage   var dates   new List lt DateTime gt                  var distinctYears   dates Distinct date   gt  date Year     The advantage I found using this approach is the re-usage of LambdaEqualityComparer class for other methods that accept an IEqualityComparer   Oh  and I leave the yield stuff to the original LINQ implementation

User · Answer

Personally I use the following class   public class LambdaEqualityComparer lt TSource  TDest gt         IEqualityComparer lt TSource gt        private Func lt TSource  TDest gt   selector       public LambdaEqualityComparer Func lt TSource  TDest gt  selector                 selector   selector             public bool Equals TSource obj  TSource other                return  selector obj  Equals  selector other               public int GetHashCode TSource obj                return  selector obj  GetHashCode              Then  an extension method   public static IEnumerable lt TSource gt  Distinct lt TSource  TCompare gt       this IEnumerable lt TSource gt  source  Func lt TSource  TCompare gt  selector        return source Distinct new LambdaEqualityComparer lt TSource  TCompare gt  selector        Finally  the intended usage   var dates   new List lt DateTime gt                  var distinctYears   dates Distinct date   gt  date Year     The advantage I found using this approach is the re-usage of LambdaEqualityComparer class for other methods that accept an IEqualityComparer   Oh  and I leave the yield stuff to the original LINQ implementation

User · Answer

Use   List lt Person gt  pList   new List lt Person gt        Fill list     var result   pList Where p   gt  p Name    null  GroupBy p   gt  p Id  Select grp   gt  grp FirstOrDefault       The where helps you filter the entries  could be more complex  and the groupby and select perform the distinct function

User · Answer

Solution first group by your fields then select firstordefault item       List lt Person gt  distinctPeople   allPeople     GroupBy p   gt  p PersonId      Select g   gt  g FirstOrDefault        ToList

User · Answer

Use   List lt Person gt  pList   new List lt Person gt        Fill list     var result   pList Where p   gt  p Name    null  GroupBy p   gt  p Id  Select grp   gt  grp FirstOrDefault       The where helps you filter the entries  could be more complex  and the groupby and select perform the distinct function

User · Answer

List lt Person gt lst new List lt Person gt          var result1   lst OrderByDescending a   gt  a ID  Select a   gt new Player  ID a ID Name a Name    Distinct

User · Answer

I ve written an article that explains how to extend the Distinct function so that you can do as follows  var people   new List lt Person gt      people Add new Person 1   quot a quot    quot b quot     people Add new Person 2   quot c quot    quot d quot     people Add new Person 1   quot a quot    quot b quot      foreach  var person in people Distinct p   gt  p ID          Do stuff with unique list here   Here s the article  now in the Web Archive   Extending LINQ - Specifying a Property in the Distinct Function

User · Answer

Please give a try with below code       var Item   GetAll   GroupBy x   gt  x  Id  ToList

User · Answer

The best way to do this that will be compatible with other  NET versions is to override Equals and GetHash to handle this  see Stack Overflow question This code returns distinct values  However  what I want is to return a strongly typed collection as opposed to an anonymous type   but if you need something that is generic throughout your code  the solutions in this article are great

User · Answer

When we faced such a task in our project we defined a small API to compose comparators   So  the use case was like this   var wordComparer   KeyEqualityComparer Null lt Word gt         ThenBy item   gt  item Text       ThenBy item   gt  item LangID       source Select      Distinct wordComparer     And API itself looks like this   using System  using System Collections  using System Collections Generic   public static class KeyEqualityComparer       public static IEqualityComparer lt T gt  Null lt T gt                  return null             public static IEqualityComparer lt T gt  EqualityComparerBy lt T  K gt           this IEnumerable lt T gt  source          Func lt T  K gt  keyFunc                return new KeyEqualityComparer lt T  K gt  keyFunc              public static KeyEqualityComparer lt T  K gt  ThenBy lt T  K gt           this IEqualityComparer lt T gt  equalityComparer          Func lt T  K gt  keyFunc                return new KeyEqualityComparer lt T  K gt  keyFunc  equalityComparer            public struct KeyEqualityComparer lt T  K gt   IEqualityComparer lt T gt        public KeyEqualityComparer          Func lt T  K gt  keyFunc          IEqualityComparer lt T gt  equalityComparer   null                KeyFunc   keyFunc          EqualityComparer   equalityComparer             public bool Equals T x  T y                return   EqualityComparer    null     EqualityComparer Equals x  y    amp  amp                  EqualityComparer lt K gt  Default Equals KeyFunc x   KeyFunc y               public int GetHashCode T obj                var hash   EqualityComparer lt K gt  Default GetHashCode KeyFunc obj             if  EqualityComparer    null                        var hash2   EqualityComparer GetHashCode obj                hash     hash2  lt  lt  5    hash2                     return hash             public readonly Func lt T  K gt  KeyFunc      public readonly IEqualityComparer lt T gt  EqualityComparer      More details is on our site  IEqualityComparer in LINQ

User · Answer

You can use DistinctBy   for getting Distinct records by an object property  Just add the following statement before using it      using Microsoft Ajax Utilities    and then use it like following   var listToReturn   responseList DistinctBy x   gt  x Index  ToList      where  Index  is the property on which i want the data to be distinct

User · Answer

In case you need a Distinct method on multiple properties  you can check out my PowerfulExtensions library  Currently it s in a very young stage  but already you can use methods like Distinct  Union  Intersect  Except on any number of properties   This is how you use it   using PowerfulExtensions Linq      var distinct   myArray Distinct x   gt  x A  x   gt  x B

User · Answer

The following code is functionally equivalent to Jon Skeet s answer   Tested on  NET 4 5  should work on any earlier version of LINQ   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  seenKeys   new HashSet lt TKey gt       return source Where element   gt  seenKeys Add keySelector element         Incidentially  check out Jon Skeet s latest version of DistinctBy cs on Google Code

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