Use LINQ to get items in one List that are not in another List

Question

I would assume there s a simple LINQ query to do this  I m just not exactly sure how   Given this piece of code   class Program       static void Main string   args                List lt Person gt  peopleList1   new List lt Person gt             peopleList1 Add new Person     ID   1             peopleList1 Add new Person     ID   2             peopleList1 Add new Person     ID   3              List lt Person gt  peopleList2   new List lt Person gt             peopleList2 Add new Person     ID   1             peopleList2 Add new Person     ID   2             peopleList2 Add new Person     ID   3             peopleList2 Add new Person     ID   4             peopleList2 Add new Person     ID   5              class Person       public int ID   get  set        I would like to perform a LINQ query to give me all of the people in peopleList2 that are not in peopleList1   This example should give me two people  ID   4  amp  ID   5

User · Answer

Here is a working example that get IT skills that a job candidate does not already have     Get a list of skills from the Skill table IEnumerable lt Skill gt  skillenum   skillrepository Skill    Get a list of skills the candidate has                    IEnumerable lt CandSkill gt  candskillenum   candskillrepository CandSkill         Where p   gt  p Candidate ID    Candidate ID                   Using the enum lists with LINQ filter out the skills not in the candidate skill list IEnumerable lt Skill gt  skillenumresult   skillenum Where p   gt   candskillenum Any p2   gt  p2 Skill ID    p Skill ID      Assign the selectable list to a viewBag ViewBag SelSkills   new SelectList skillenumresult   Skill ID    Skill Name   1

User · Answer

Bit late to the party but a good solution which is also Linq to SQL compatible is   List lt string gt  list1   new List lt string gt       1    2    3     List lt string gt  list2   new List lt string gt       2    4      List lt string gt  inList1ButNotList2    from o in list1                                    join p in list2 on o equals p into t                                    from od in t DefaultIfEmpty                                      where od    null                                    select o  ToList lt string gt      List lt string gt  inList2ButNotList1    from o in list2                                    join p in list1 on o equals p into t                                    from od in t DefaultIfEmpty                                      where od    null                                    select o  ToList lt string gt      List lt string gt  inBoth    from o in list1                        join p in list2 on o equals p into t                        from od in t DefaultIfEmpty                          where od    null                        select od  ToList lt string gt       Kudos to http   www dotnet-tricks com Tutorial linq UXPF181012-SQL-Joins-with-C

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Once you write a generic FuncEqualityComparer you can use it everywhere   peopleList2 Except peopleList1  new FuncEqualityComparer lt Person gt   p  q    gt  p ID    q ID     public class FuncEqualityComparer lt T gt    IEqualityComparer lt T gt        private readonly Func lt T  T  bool gt  comparer      private readonly Func lt T  int gt  hash       public FuncEqualityComparer Func lt T  T  bool gt  comparer                this comparer   comparer          if  typeof T  GetMethod nameof object GetHashCode   DeclaringType    typeof object               hash         gt  0          else             hash   t   gt  t GetHashCode                public bool Equals T x  T y    gt  comparer x  y       public int GetHashCode T obj    gt  hash obj

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This Enumerable Extension allow you to define a list of item to exclude and a function to use to find key to use to perform comparison   public static class EnumerableExtensions       public static IEnumerable lt TSource gt  Exclude lt TSource  TKey gt  this IEnumerable lt TSource gt  source      IEnumerable lt TSource gt  exclude  Func lt TSource  TKey gt  keySelector               var excludedSet   new HashSet lt TKey gt  exclude Select keySelector           return source Where item   gt   excludedSet Contains keySelector item               You can use it this way  list1 Exclude list2  i   gt  i ID

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first  extract ids from the collection where condition  List lt int gt  indexes Yes   this Contenido Where x   gt  x key     TEST   Select x   gt  x Id  ToList      second  use  compare  estament to select ids diffent to the selection  List lt int gt  indexes No   this Contenido Where x   gt   indexes Yes Contains x Id   Select x   gt  x Id  ToList      Obviously you can use x key     TEST   but is only a example

User · Answer

If you override the equality of People then you can also use   peopleList2 Except peopleList1    Except should be significantly faster than the Where    Any  variant since it can put the second list into a hashtable  Where    Any  has a runtime of O peopleList1 Count   peopleList2 Count  whereas variants based on HashSet lt T gt   almost  have a runtime of O peopleList1 Count   peopleList2 Count    Except implicitly removes duplicates  That shouldn t affect your case  but might be an issue for similar cases   Or if you want fast code but don t want to override the equality   var excludedIDs   new HashSet lt int gt  peopleList1 Select p   gt  p ID    var result   peopleList2 Where p   gt   excludedIDs Contains p ID      This variant does not remove duplicates

User · Answer

Or if you want it without negation   var result   peopleList2 Where p   gt  peopleList1 All p2   gt  p2 ID    p ID      Basically it says get all from peopleList2 where all ids in peopleList1 are different from id in peoplesList2    Just a little bit different approach from the accepted answer

User · Answer

This can be addressed using the following LINQ expression   var result   peopleList2 Where p   gt   peopleList1 Any p2   gt  p2 ID    p ID      An alternate way of expressing this via LINQ  which some developers find more readable   var result   peopleList2 Where p   gt  peopleList1 All p2   gt  p2 ID    p ID         Warning  As noted in the comments  these approaches mandate an O n m  operation  That may be fine  but could introduce performance issues  and especially if the data set is quite large  If this doesn t satisfy your performance requirements  you may need to evaluate other options  Since the stated requirement is for a solution in LINQ  however  those options aren t explored here  As always  evaluate any approach against the performance requirements your project might have

User · Answer

Klaus  answer was great  but ReSharper will ask you to  Simplify LINQ expression    var result   peopleList2 Where p   gt  peopleList1 All p2   gt  p2 ID    p ID

User · Answer

Since all of the solutions to date used fluent syntax  here is a solution in query expression syntax  for those interested   var peopleDifference      from person2 in peopleList2   where          from person1 in peopleList1        select person1 ID       Contains person2 ID    select person2    I think it is different enough from the answers given to be of interest to some  even thought it most likely would be suboptimal for Lists  Now for tables with indexed IDs  this would definitely be the way to go

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