Quickest way to compare two generic lists for differences

Question

What is the quickest  and least resource intensive  to compare two massive   50 000 items  and as a result have two lists like the ones below    items that show up in the first list but not in the second items that show up in the second list but not in the first   Currently I m working with the List or IReadOnlyCollection and solve this issue in a linq query   var list1   list Where i   gt   list2 Contains i   ToList    var list2   list2 Where i   gt   list Contains i   ToList      But this doesn t perform as good as i would like  Any idea of making this quicker and less resource intensive as i need to process a lot of lists

User · Answer

I have used this code to compare two list which has million of records.

This method will not take much time

    //Method to compare two list of string
    private List<string> Contains(List<string> list1, List<string> list2)
    {
        List<string> result = new List<string>();

        result.AddRange(list1.Except(list2, StringComparer.OrdinalIgnoreCase));
        result.AddRange(list2.Except(list1, StringComparer.OrdinalIgnoreCase));

        return result;
    }

User · Answer

If only combined result needed  this will work too   var set1   new HashSet lt T gt  list1   var set2   new HashSet lt T gt  list2   var areEqual   set1 SetEquals set2     where T is type of lists element

User · Answer

Maybe it s funny  but this works for me  string Join  quot  quot  List1     string Join  quot  quot   List2

User · Answer

I think this is a simple and easy way to compare two lists element by element  x  1 2 3 5 4 8 7 11 12 45 96 25  y  2 4 5 6 8 7 88 9 6 55 44 23   tmp        for i in range len x   and range len y        if x i  gt y i           tmp append 1      else          tmp append 0  print tmp

User · Answer

More efficient would be using Enumerable Except   var inListButNotInList2   list Except list2   var inList2ButNotInList   list2 Except list     This method is implemented by using deferred execution  That means you could write for example   var first10   inListButNotInList2 Take 10     It is also efficient since it internally uses a Set lt T gt  to compare the objects  It works by first collecting all distinct values from the second sequence  and then streaming the results of the first  checking that they haven t been seen before

User · Answer

Use Except   var firstNotSecond   list1 Except list2  ToList    var secondNotFirst   list2 Except list1  ToList      I suspect there are approaches which would actually be marginally faster than this  but even this will be vastly faster than your O N   M  approach   If you want to combine these  you could create a method with the above and then a return statement   return  firstNotSecond Any    amp  amp   secondNotFirst Any      One point to note is that there is a difference in results between the original code in the question and the solution here  any duplicate elements which are only in one list will only be reported once with my code  whereas they d be reported as many times as they occur in the original code   For example  with lists of  1  2  2  2  3  and  1   the  elements in list1 but not list2  result in the original code would be  2  2  2  3   With my code it would just be  2  3   In many cases that won t be an issue  but it s worth being aware of

User · Answer

Not for this Problem  but here s some code to compare lists for equal and not  identical objects     public class EquatableList lt T gt    List lt T gt   IEquatable lt EquatableList lt T gt  gt  where    T   IEquatable lt T gt        lt summary gt      True  if this contains element with equal property-values      lt  summary gt       lt param name  element  gt element of Type T lt  param gt       lt returns gt True  if this contains element lt  returns gt  public new Boolean Contains T element        return this Any t   gt  t Equals element            lt summary gt      True  if list is equal to this      lt  summary gt       lt param name  list  gt list lt  param gt       lt returns gt True  if instance equals list lt  returns gt  public Boolean Equals EquatableList lt T gt  list        if  list    null  return false      return this All list Contains   amp  amp  list All this Contains

User · Answer

This is the best solution you ll found  var list3   list1 Where l   gt  list2 ToList   Contains l

User · Answer

If you want the results to be case insensitive  the following will work   List lt string gt  list1   new List lt string gt     a dll    b1 dll     List lt string gt  list2   new List lt string gt     A dll    b2 dll      var firstNotSecond   list1 Except list2  StringComparer OrdinalIgnoreCase  ToList    var secondNotFirst   list2 Except list1  StringComparer OrdinalIgnoreCase  ToList      firstNotSecond would contain b1 dll  secondNotFirst would contain b2 dll

User · Answer

try this way   var difList   list1 Where a   gt   list2 Any a1   gt  a1 id    a id                Union list2 Where a   gt   list1 Any a1   gt  a1 id    a id

User · Answer

using System Collections Generic  using System Linq   namespace YourProject Extensions       public static class ListExtensions               public static bool SetwiseEquivalentTo lt T gt  this List lt T gt  list  List lt T gt  other              where T  IEquatable lt T gt                        if  list Except other  Any                    return false              if  other Except list  Any                    return false              return true                      Sometimes you only need to know if two lists are different  and not what those differences are  In that case  consider adding this extension method to your project  Note that your listed objects should implement IEquatable   Usage   public sealed class Car   IEquatable lt Car gt        public Price Price   get        public List lt Component gt  Components   get                 public override bool Equals object obj            gt  obj is Car other  amp  amp  Equals other        public bool Equals Car other            gt  Price    other Price              amp  amp  Components SetwiseEquivalentTo other Components        public override int GetHashCode             gt  Components Aggregate              Price GetHashCode                 code  next    gt  code   next GetHashCode        Bitwise XOR     Whatever the Component class is  the methods shown here for Car should be implemented almost identically   It s very important to note how we ve written GetHashCode  In order to properly implement IEquatable  Equals and GetHashCode must operate on the instance s properties in a logically compatible way    Two lists with the same contents are still different objects  and will produce different hash codes  Since we want these two lists to be treated as equal  we must let GetHashCode produce the same value for each of them  We can accomplish this by delegating the hashcode to every element in the list  and using the standard bitwise XOR to combine them all  XOR is order-agnostic  so it doesn t matter if the lists are sorted differently  It only matters that they contain nothing but equivalent members   Note  the strange name is to imply the fact that the method does not consider the order of the elements in the list  If you do care about the order of the elements in the list  this method is not for you

User · Answer

Enumerable SequenceEqual Method Determines whether two sequences are equal according to an equality comparer  MS Docs  Enumerable SequenceEqual list1  list2    This works for all primitive data types  If you need to use it on custom objects you need to implement IEqualityComparer Defines methods to support the comparison of objects for equality   IEqualityComparer Interface Defines methods to support the comparison of objects for equality  MS Docs for IEqualityComparer

[c#] Quickest way to compare two generic lists for differences

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