Iterate two Lists or Arrays with one ForEach statement in C

Question

This just for general knowledge   If I have two  let s say  List  and I want to iterate both with the same foreach loop  can we do that   Edit  Just to clarify  I wanted to do this   List lt String gt  listA   new List lt string gt     string    string     List lt String gt  listB   new List lt string gt     string    string      for int i   0  i  lt  listA Count  i        listB i    listA i     But with a foreach

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Since C  7  you can use Tuples     int   nums     1  2  3  4    string   words      one    two    three    four      foreach  var tuple in nums Zip words   x  y    gt   x  y          Console WriteLine    tuple Item1    tuple Item2           or    foreach  var tuple in nums Zip words   x  y    gt   Num  x  Word  y          Console WriteLine    tuple Num    tuple Word

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You can also do the following   var i   0  foreach  var itemA in listA      Console WriteLine itemA   listB i             Note  the length of listA must be the same with listB

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No  you would have to use a for-loop for that   for  int i   0  i  lt  lst1 Count  i            lst1 i           lst2 i         You can t do something like  foreach  var objCurrent1 int lst1  var objCurrent2 in lst2

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I often need to execute an action on each pair in two collections  The Zip method is not useful in this case  This extension method ForPair can be used  public static void ForPair lt TFirst  TSecond gt  this IEnumerable lt TFirst gt  first  IEnumerable lt TSecond gt  second      Action lt TFirst  TSecond gt  action        using  var enumFirst   first GetEnumerator        using  var enumSecond   second GetEnumerator                  while  enumFirst MoveNext    amp  amp  enumSecond MoveNext                          action enumFirst Current  enumSecond Current                      So for example  you could write  var people   new List lt Person gt    person1  person2    var wages   new List lt decimal gt    10  20     people ForPair wages   p  w    gt  p Wage   w    Note however that this method cannot be used to modify the collection itself  This for example will not work  List lt String gt  listA   new List lt string gt     quot string quot    quot string quot     List lt String gt  listB   new List lt string gt     quot string quot    quot string quot      listA ForPair listA   c1  c2    gt  c1   c2       Nothing will happen   So in this case  the example in your own question is probably the best way

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Here s a custom IEnumerable lt   extension method that can be used to loop through two lists simultaneously   using System  using System Collections Generic  using System Linq   namespace ConsoleApplication1       public static class LinqCombinedSort               public static void Test                         var a   new     a    b    c    d    e    f                var b   new    3  2  1  6  5  4                var sorted   from ab in a Combine b                           orderby ab Second                          select ab First               foreach char c in sorted                                Console WriteLine c                                    public static IEnumerable lt Pair lt TFirst  TSecond gt  gt  Combine lt TFirst  TSecond gt  this IEnumerable lt TFirst gt  s1  IEnumerable lt TSecond gt  s2                        using  var e1   s1 GetEnumerator                using  var e2   s2 GetEnumerator                                  while  e1 MoveNext    amp  amp  e2 MoveNext                                          yield return new Pair lt TFirst  TSecond gt  e1 Current  e2 Current                                                          public class Pair lt TFirst  TSecond gt                private readonly TFirst  first          private readonly TSecond  second          private int  hashCode           public Pair TFirst first  TSecond second                         first   first               second   second                     public TFirst First                       get                               return  first                                   public TSecond Second                       get                               return  second                                   public override int GetHashCode                         if   hashCode    0                                 hashCode    ReferenceEquals  first  null    213    first GetHashCode    37                                ReferenceEquals  second  null    213    second GetHashCode                               return  hashCode                     public override bool Equals object obj                        var other   obj as Pair lt TFirst  TSecond gt               if  other    null                                return false                            return Equals  first  other  first   amp  amp  Equals  second  other  second

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You can use Union or Concat  the former removes duplicates  the later doesn t  foreach  var item in List1 Union List1          TODO  Real code goes here    foreach  var item in List1 Concat List1          TODO  Real code goes here

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This method would work for a list implementation and could be implemented as an extension method   public void TestMethod         var first   new List lt int gt   1  2  3  4  5       var second   new List lt string gt    One    Two    Three    Four    Five         foreach var value in this Zip first  second   x  y    gt  new  Number   x  Text   y                  Console WriteLine   0  -  1   value Number  value Text            public IEnumerable lt TResult gt  Zip lt TFirst  TSecond  TResult gt  List lt TFirst gt  first  List lt TSecond gt  second  Func lt TFirst  TSecond  TResult gt  selector        if  first Count    second Count          throw new Exception           for var i   0  i  lt  first Count  i                  yield return selector Invoke first i   second i

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This is known as a Zip operation and will be supported in  NET 4   With that  you would be able to write something like   var numbers   new      1  2  3  4    var words   new       one    two    three    four      var numbersAndWords   numbers Zip words   n  w    gt  new   Number   n  Word   w     foreach var nw in numbersAndWords        Console WriteLine nw Number   nw Word         As an alternative to the anonymous type with the named fields  you can also save on braces by using a Tuple and its static Tuple Create helper   foreach  var nw in numbers Zip words  Tuple Create          Console WriteLine nw Item1   nw Item2

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I understand hope that the lists have the same length  No  your only bet is going with a plain old standard for loop

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If you want one element with the corresponding one you could do  Enumerable Range 0  List1 Count  All x   gt  List1 x     List2 x      That will return true if every item is equal to the corresponding one on the second list  If that s almost but not quite what you want it would help if you elaborated more

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You could also simply use a local integer variable if the lists have the same length   List lt classA gt  listA   fillListA    List lt classB gt  listB   fillListB     var i   0  foreach var itemA in listA        Console WriteLine itemA    listB i

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If you don t want to wait for  NET 4 0  you could implement your own Zip method  The following works with  NET 2 0  You can adjust the implementation depending on how you want to handle the case where the two enumerations  or lists  have different lengths  this one continues to the end of the longer enumeration  returning the default values for missing items from the shorter enumeration   static IEnumerable lt KeyValuePair lt T  U gt  gt  Zip lt T  U gt  IEnumerable lt T gt  first  IEnumerable lt U gt  second        IEnumerator lt T gt  firstEnumerator   first GetEnumerator        IEnumerator lt U gt  secondEnumerator   second GetEnumerator         while  firstEnumerator MoveNext                  if  secondEnumerator MoveNext                          yield return new KeyValuePair lt T  U gt  firstEnumerator Current  secondEnumerator Current                     else                       yield return new KeyValuePair lt T  U gt  firstEnumerator Current  default U                        while  secondEnumerator MoveNext                  yield return new KeyValuePair lt T  U gt  default T   secondEnumerator Current            static void Test         IList lt string gt  names   new string      one    two    three         IList lt int gt  ids   new int     1  2  3  4         foreach  KeyValuePair lt string  int gt  keyValuePair in ParallelEnumerate names  ids                 Console WriteLine keyValuePair Key      lt null gt       -     keyValuePair Value ToString

[c#] Iterate two Lists or Arrays with one ForEach statement in C#

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