Collection was modified enumeration operation may not execute in ArrayList

Question

I m trying to remove an item from an ArrayList and I get this Exception  Collection was modified  enumeration operation may not execute    Any ideas

User · Answer

One way is to add the item s  to be deleted to a new list  Then go through and delete those items

User · Answer

Here s an example  sorry for any typos   var itemsToRemove   new ArrayList        should use generic List if you can  foreach  var item in originalArrayList      if             itemsToRemove Add item          foreach  var item in itemsToRemove      originalArrayList Remove item       OR if you re using 3 5  Linq makes the first bit easier   itemsToRemove   originalArrayList    Where item   gt          ToArray     foreach  var item in itemsToRemove      originalArrayList Remove item       Replace       with your condition that determines if item should be removed

User · Answer

You are removing the item during a foreach  yes  Simply  you can t  There are a few common options here    use List lt T gt  and RemoveAll with a predicate iterate backwards by index  removing matching items  for int i   list Count - 1  i  gt   0  i--        if  some test   list RemoveAt i      use foreach  and put matching items into a second list  now enumerate the second list and remove those items from the first  if you see what I mean

User · Answer

Don t modify the list inside of a loop which iterates through the list    Instead  use a for   or while   with an index  going backwards through the list   This will let you delete things without getting an invalid index    var foo   new List lt Bar gt      for int i   foo Count-1  i  gt   0  --i      var item   foo i        do something with item

User · Answer

I agree with several of the points I ve read in this post and I ve incorporated them into my solution to solve the exact same issue as the original posting   That said  the comments I appreciated are     unless you are using  NET 1 0 or 1 1  use List lt T gt  instead of ArrayList      Also  add the item s  to be deleted to a new list  Then go through and delete those items      in my case I just created a new List and the populated it with the valid data values     e g    private List lt string gt  managedLocationIDList   new List lt string gt     string managedLocationIDs     1321 1235       user input  should be semicolon seperated list of values  managedLocationIDList AddRange managedLocationIDs Split new char              List lt string gt  checkLocationIDs   new List lt string gt         Remove any duplicate ID s and cleanup the string holding the list if ID s Functions helper   new Functions    checkLocationIDs   helper ParseList managedLocationIDList        public List lt string gt  ParseList List lt string gt  checkList        List lt string gt  verifiedList   new List lt string gt          foreach  string listItem in checkList      if   verifiedList Contains listItem Trim     amp  amp  listItem    string Empty          verifiedList Add listItem Trim          verifiedList Sort        return verifiedList

User · Answer

Am I missing something   Somebody correct me if I m wrong     list RemoveAll s   gt  s Name     Fred

User · Answer

I like to iterate backward using a for loop  but this can get tedious compared to foreach   One solution I like is to create an enumerator that traverses the list backward   You can implement this as an extension method on ArrayList or List lt T gt    The implementation for ArrayList is below       public static IEnumerable GetRemoveSafeEnumerator this ArrayList list                for  int i   list Count - 1  i  gt   0  i--                           Reset the value of i if it is invalid                 This occurs when more than one item                is removed from the list during the enumeration              if  i  gt   list Count                                if  list Count    0                      yield break                   i   list Count - 1                             yield return list i                     The implementation for List lt T gt  is similar       public static IEnumerable lt T gt  GetRemoveSafeEnumerator lt T gt  this List lt T gt  list                for  int i   list Count - 1  i  gt   0  i--                           Reset the value of i if it is invalid                 This occurs when more than one item                is removed from the list during the enumeration              if  i  gt   list Count                                if  list Count    0                      yield break                   i   list Count - 1                             yield return list i                     The example below uses the enumerator to remove all even integers from an ArrayList       ArrayList list   new ArrayList    1  2  3  4  5  6  7  8  9  10        foreach  int item in list GetRemoveSafeEnumerator                  if  item   2    0              list Remove item

User · Answer

Instead of foreach    use a for   loop with a numeric index

User · Answer

using ArrayList also you can try like this   ArrayList arraylist          myobject data list  ArrayList temp    ArrayList arraylist Clone     foreach  var item in temp          if                arraylist Remove item

[c#] Collection was modified; enumeration operation may not execute in ArrayList

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