Joining two lists together

Question

If I have two lists of type string  or any other type   what is a quick way of joining the two lists   The order should stay the same  Duplicates should be removed  though every item in both links are unique   I didn t find much on this when googling and didn t want to implement any  NET interfaces for speed of delivery

User · Accepted Answer

You could try   List lt string gt  a   new List lt string gt     List lt string gt  b   new List lt string gt      a AddRange b     MSDN page for AddRange  This preserves the order of the lists  but it doesn t remove any duplicates which Union would do   This does change list a  If you wanted to preserve the original lists then you should use Concat  as pointed out in the other answers    var newList   a Concat b     This returns an IEnumerable as long as a is not null

User · Answer

The AddRange method   aList AddRange  anotherList

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One way  I haven t seen mentioned that can be a bit more robust  particularly if you wanted to alter each element in some way  e g  you wanted to  Trim   all of the elements    List lt string gt  a   new List lt string gt     List lt string gt  b   new List lt string gt            b ForEach x  gt a Add x Trim

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var bigList   new List lt int gt    1  2  3        Concat new List lt int gt    4  5  6         ToList        yields   1  2  3  4  5  6

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As long as they are of the same type  it s very simple with AddRange   list2 AddRange list1

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one way  List AddRange   depending on the types

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targetList   list1 Concat list2  ToList      It s working fine I think so  As previously said  Concat returns a new sequence and while converting the result to List  it does the job perfectly  Implicit conversions may fail sometimes when using the AddRange method

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Something like this   firstList AddRange  secondList     Or  you can use the  Union  extension method that is defined in System Linq  With  Union   you can also specify a comparer  which can be used to specify whether an item should be unioned or not   Like this   List lt int gt  one   new List lt int gt    1  2  3  4  5    List lt int gt  second new List lt int gt    1  2  5  6     var result   one Union  second  new EqComparer       foreach  int x in result         Console WriteLine  x     Console ReadLine       region IEqualityComparer lt int gt  Members public class EqComparer   IEqualityComparer lt int gt        public bool Equals  int x  int y                 return x    y             public int GetHashCode  int obj                 return obj GetHashCode              endregion

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The two options I use are   list1 AddRange list2     or  list1 Concat list2     However I noticed as I used that when using the AddRange method with a recursive function  that calls itself very often I got an SystemOutOfMemoryException because the maximum number of dimensions was reached    Message Google Translated  The array dimensions exceeded the supported range   Using Concat solved that issue

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If some item s  exist in both lists you may use    var all   list1 Concat list2  Concat list3      Concat listN  Distinct   ToList

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I just wanted to test how Union works with the default comparer on overlapping collections of reference type objects    My object is   class MyInt       public int val       public override string ToString                 return val ToString              My test code is   MyInt   myInts1   new MyInt 10   MyInt   myInts2   new MyInt 10   int overlapFrom   4  Console WriteLine  overlapFrom   0    overlapFrom    Action lt IEnumerable lt MyInt gt   string gt  printMyInts    myInts  myIntsName    gt  Console WriteLine   2    0     1    myInts Count    string Join      myInts   myIntsName    for  int i   0  i  lt  myInts1 Length  i        myInts1 i    new MyInt   val   i    printMyInts myInts1  nameof myInts1     int j   0  for    j   overlapFrom  lt  myInts1 Length  j        myInts2 j    myInts1 j   overlapFrom   for    j  lt  myInts2 Length  j        myInts2 j    new MyInt   val   j   overlapFrom    printMyInts myInts2  nameof myInts2     IEnumerable lt MyInt gt  myUnion   myInts1 Union myInts2   printMyInts myUnion  nameof myUnion     for  int i   0  i  lt  myInts2 Length  i        myInts2 i  val    10  printMyInts myInts2  nameof myInts2    printMyInts myUnion  nameof myUnion     for  int i   0  i  lt  myInts1 Length  i        myInts1 i  val   i  printMyInts myInts1  nameof myInts1    printMyInts myUnion  nameof myUnion      The output is   overlapFrom  4 myInts1  10   0 1 2 3 4 5 6 7 8 9 myInts2  10   4 5 6 7 8 9 10 11 12 13 myUnion  14   0 1 2 3 4 5 6 7 8 9 10 11 12 13 myInts2  10   14 15 16 17 18 19 20 21 22 23 myUnion  14   0 1 2 3 14 15 16 17 18 19 20 21 22 23 myInts1  10   0 1 2 3 4 5 6 7 8 9 myUnion  14   0 1 2 3 4 5 6 7 8 9 20 21 22 23   So  everything works fine

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List lt string gt  list1   new List lt string gt     list1 Add  dot    list1 Add  net     List lt string gt  list2   new List lt string gt     list2 Add  pearls    list2 Add        var result   list1 Concat list2

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See this link   public class ProductA    public string Name   get  set    public int Code   get  set       public class ProductComparer   IEqualityComparer lt ProductA gt     public bool Equals ProductA x  ProductA y          Check whether the objects are the same object       if  Object ReferenceEquals x  y   return true         Check whether the products  properties are equal       return x    null  amp  amp  y    null  amp  amp  x Code Equals y Code   amp  amp  x Name Equals y Name          public int GetHashCode ProductA obj          Get hash code for the Name field if it is not null       int hashProductName   obj Name    null   0   obj Name GetHashCode           Get hash code for the Code field       int hashProductCode   obj Code GetHashCode           Calculate the hash code for the product       return hashProductName   hashProductCode            ProductA   store1     new ProductA   Name    apple   Code   9                        new ProductA   Name    orange   Code   4           ProductA   store2     new ProductA   Name    apple   Code   9                        new ProductA   Name    lemon   Code   12          Get the products from the both arrays   excluding duplicates   IEnumerable lt ProductA gt  union     store1 Union store2    foreach  var product in union      Console WriteLine product Name         product Code           This code produces the following output       apple 9     orange 4     lemon 12

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The way with the least space overhead is to use the Concat extension method    var combined   list1 Concat list2     It creates an instance of IEnumerable lt T gt  which will enumerate the elements of list1 and list2 in that order

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The Union method might address your needs   You didn t specify whether order or duplicates was important   Take two IEnumerables and perform a union as seen here   int   ints1     5  3  9  7  5  9  3  7    int   ints2     8  3  6  4  4  9  1  0     IEnumerable lt int gt  union   ints1 Union ints2       yields   5  3  9  7  8  6  4  1  0

[c#] Joining two lists together

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