Difference between two lists

Question

I Have two generic list filled with CustomsObjects   I need to retrieve the difference between those two lists Items who are in the first without the items in the second one   in a third one   I was thinking using  Except   was a good idea but I don t see how to use this   Help

User · Answer

var third   first Except second      you can also call ToList   after Except    if you don t like referencing lazy collections    The Except   method compares the values using the default comparer  if the values being compared are of base data types  such as int  string  decimal etc   Otherwise the comparison will be made by object address  which is probably not what you want    In that case  make your custom objects implement IComparable  or implement a custom IEqualityComparer and pass it to the Except   method

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You could do something like this   var result   customlist Where p   gt   otherlist Any l   gt  p someproperty    l someproperty

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var resultList   checklist Where p   gt  myList All l   gt  p value    l value   ToList

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bit late but here is working solution for me    var myBaseProperty    typeof BaseClass   GetProperties     get base code properties                     var allProperty   entity GetProperties   0  DeclaringType GetProperties     get derived class property plus base code as it is derived from it                     var declaredClassProperties   allProperty Where x   gt   myBaseProperty Any l   gt  l Name    x Name   ToList     get the difference   In above mention code I am getting the properties difference between my base class and derived class list

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If both your lists implement IEnumerable interface you can achieve this using LINQ   list3   list1 where i   gt   list2 contains i

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var list3   list1 Where x   gt   list2 Any z   gt  z Id    x Id   ToList      Note  list3 will contain the items or objects that are not in both lists  Note  Its ToList   not toList

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Using Except is exactly the right way to go  If your type overrides Equals and GetHashCode  or you re only interested in reference type equality  i e  two references are only  equal  if they refer to the exact same object   you can just use   var list3   list1 Except list2  ToList      If you need to express a custom idea of equality  e g  by ID  you ll need to implement IEqualityComparer lt T gt   For example   public class IdComparer   IEqualityComparer lt CustomObject gt        public int GetHashCode CustomObject co                if  co    null                        return 0                    return co Id GetHashCode               public bool Equals CustomObject x1  CustomObject x2                if  object ReferenceEquals x1  x2                         return true                    if  object ReferenceEquals x1  null                 object ReferenceEquals x2  null                         return false                    return x1 Id    x2 Id            Then use   var list3   list1 Except list2  new IdComparer    ToList      Note that this will remove any duplicate elements  If you need duplicates to be preserved  it would probably be easiest to create a set from list2 and use something like   var list3   list1 Where x   gt   set2 Contains x   ToList

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Following helper can be usefull if for such task  There are 2 collections local collection called oldValues and remote called newValues From time to time you get notification bout some elements on remote collection have changed and you want to know which elements were added  removed and updated  Remote collection always returns ALL elements that it has      public class ChangesTracker lt T1  T2 gt        private readonly IEnumerable lt T1 gt  oldValues      private readonly IEnumerable lt T2 gt  newValues      private readonly Func lt T1  T2  bool gt  areEqual       public ChangesTracker IEnumerable lt T1 gt  oldValues  IEnumerable lt T2 gt  newValues  Func lt T1  T2  bool gt  areEqual                this oldValues   oldValues          this newValues   newValues          this areEqual   areEqual             public IEnumerable lt T2 gt  AddedItems               get   gt  newValues Where n   gt  oldValues All o   gt   areEqual o  n                public IEnumerable lt T1 gt  RemovedItems               get   gt  oldValues Where n   gt  newValues All o   gt   areEqual n  o                public IEnumerable lt T1 gt  UpdatedItems               get   gt  oldValues Where n   gt  newValues Any o   gt  areEqual n  o              Usage          Test      public void AddRemoveAndUpdate                    Arrange         var listA   ChangesTrackerMockups GetAList 10      ids 1-10         var listB   ChangesTrackerMockups GetBList 11      ids 1-11              Where b   gt  b Iddd    7      Exclude element means it will be delete         var changesTracker   new ChangesTracker lt A  B gt  listA  listB  AreEqual               Assert         Assert AreEqual 1  changesTracker AddedItems Count        b id   11         Assert AreEqual 1  changesTracker RemovedItems Count        b id   7         Assert AreEqual 9  changesTracker UpdatedItems Count        all a id    b iddd            private bool AreEqual A a  B b                if  a    null  amp  amp  b    null              return true          if  a    null    b    null              return false          return a Id    b Iddd

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I think important to emphasize - using Except method will return you items who are in the first without the items in the second one only  It does not return those elements in second that do not appear in first   var list1   new List lt int gt    1  2  3  4  5   var list2   new List lt int gt    3  4  5  6  7     var list3   list1 Except list2  ToList      list3 contains only 1  2   But if you want get real difference between two lists   Items who are in the first without the items in the second one and items who are in the second without the items in the first one   You need using Except twice   var list1   new List lt int gt    1  2  3  4  5   var list2   new List lt int gt    3  4  5  6  7     var list3   list1 Except list2     list3 contains only 1  2 var list4   list2 Except list1     list4 contains only 6  7 var resultList   list3 Concat list4  ToList      resultList contains 1  2  6  7   Or you can use SymmetricExceptWith method of HashSet  But it changes the set on which called   var list1   new List lt int gt    1  2  3  4  5   var list2   new List lt int gt    3  4  5  6  7     var list1Set   list1 ToHashSet       net framework 4 7 2 and  net core 2 0 and above otherwise new HashSet list1  list1Set SymmetricExceptWith list2   var resultList   list1Set ToList      resultList contains 1  2  6  7

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List lt ObjectC gt   list DF BW ANB   new List lt ObjectC gt         List lt ObjectA gt   listA   new List lt ObjectA gt     List lt ObjectB gt   listB   new List lt ObjectB gt      foreach  var itemB in  listB              var flat   0      foreach var itemA in  listA                 if itemA ProductId  itemB ProductId                        flat   1              break                      if  flat    0                 list DF BW ANB Add itemB

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List lt int gt  list1   new List lt int gt             List lt int gt  list2   new List lt int gt             List lt int gt  listDifference   new List lt int gt              foreach  var item1 in list1                        foreach  var item2 in list2                                if  item1    item2                      listDifference Add item1

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Since the Except extension method operates on two IEumerables  it seems to me that it will be a O n 2  operation    If performance is an issue  if say your lists are large   I d suggest creating a HashSet from list1 and use HashSet s ExceptWith method

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here is my solution       List lt String gt  list1   new List lt String gt          List lt String gt  list2   new List lt String gt          List lt String gt  exceptValue   new List lt String gt      foreach String L1 in List1         if  List2 Contains L1                 exceptValue Add L1           foreach String L2 in List2         if  List1 Contains L2                 exceptValue Add L2

[c#] Difference between two lists

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