What is the best way to modify a list in a foreach loop

Question

A new feature in C     NET 4 0 is that you can change your enumerable in a foreach without getting the exception  See Paul Jackson s blog entry An Interesting Side-Effect of Concurrency  Removing Items from a Collection While Enumerating for information on this change  What is the best way to do the following  foreach var item in Enumerable        foreach var item2 in item Enumerable                item Add new item2           Usually I use an IList as a cache buffer until the end of the foreach  but is there better way

User · Answer

LINQ is very effective for juggling with collections.

Your types and structure are unclear to me, but I will try to fit your example to the best of my ability.

From your code it appears that, for each item, you are adding to that item everything from its own 'Enumerable' property. This is very simple:

foreach (var item in Enumerable)
{
    item = item.AddRange(item.Enumerable));
}

As a more general example, let's say we want to iterate a collection and remove items where a certain condition is true. Avoiding foreach, using LINQ:

myCollection = myCollection.Where(item => item.ShouldBeKept);

Add an item based on each existing item? No problem:

myCollection = myCollection.Concat(myCollection.Select(item => new Item(item.SomeProp)));

User · Answer

As mentioned  but with a code sample   foreach var item in collection ToArray        collection Add new Item

User · Answer

To add to Timo s answer LINQ can be used like this as well   items   items Select i   gt                     perform some logic adding   updating        return i   return new Item                     To remove an item simply have logic to return null          Then attach the Where to filter out nulls       return null                Where i   gt  i    null

User · Answer

To illustrate Nippysaurus s answer  If you are going to add the new items to the list and want to process the newly added items too during the same enumeration then you can just use for loop instead of foreach loop  problem solved     var list   new List lt YourData gt         populate the list        foreach  var entryToProcess in list  for  int i   0  i  lt  list Count  i          var entryToProcess   list i        var resultOfProcessing   DoStuffToEntry entryToProcess        if      condition              list Add new YourData            For runnable example   void Main         var list   new List lt int gt         for  int i   0  i  lt  10  i            list Add i          foreach  var entry in list      for  int i   0  i  lt  list Count  i                  var entry   list i           if  entry   2    0              list Add entry   1            Console Write entry                     Console Write list       Output of last example   0  1  2  3  4  5  6  7  8  9  1  3  5  7  9    List  15 items   0  1  2  3  4  5  6  7  8  9  1  3  5  7  9

User · Answer

You should really use for   instead of foreach   in this case

User · Answer

The collection used in foreach is immutable  This is very much by design    As it says on MSDN      The foreach statement is used to   iterate through the collection to get   the information that you want  but can   not be used to add or remove items   from the source collection to avoid   unpredictable side effects  If you   need to add or remove items from the   source collection  use a for loop    The post in the link provided by Poko indicates that this is allowed in the new concurrent collections

User · Answer

Here s how you can do that  quick and dirty solution  If you really need this kind of behavior  you should either reconsider your design or override all IList lt T gt  members and aggregate the source list    using System  using System Collections Generic   namespace ConsoleApplication3       public class ModifiableList lt T gt    List lt T gt                private readonly IList lt T gt  pendingAdditions   new List lt T gt             private int activeEnumerators   0           public ModifiableList IEnumerable lt T gt  collection    base collection                               public ModifiableList                                public new void Add T t                        if activeEnumerators    0                  base Add t               else                 pendingAdditions Add t                      public new IEnumerator lt T gt  GetEnumerator                           activeEnumerators               foreach T t in   IList lt T gt  this                   yield return t               --activeEnumerators               AddRange pendingAdditions               pendingAdditions Clear                         class Program               static void Main string   args                        ModifiableList lt int gt  ints   new ModifiableList lt int gt  new int     2  4  6  8                  foreach int i in ints                  ints Add i   2                foreach int i in ints                  Console WriteLine i   2

User · Answer

Make a copy of the enumeration  using an IEnumerable extension method in this case  and enumerate over it   This would add a copy of every element in every inner enumerable to that enumeration   foreach var item in Enumerable        foreach var item2 in item Enumerable ToList                  item Add item2

User · Answer

The best approach from a performance perspective is probably to use a one or two arrays   Copy the list to an array  do operations on the array  and then build a new list from the array   Accessing an array element is faster than accessing a list item  and conversions between a List lt T gt  and a T   can use a fast  bulk copy  operation which avoids the overhead associated accessing individual items   For example  suppose you have a List lt string gt  and wish to have every string in the list which starts with T be followed by an item  Boo   while every string that starts with  U  is dropped entirely   An optimal approach would probably be something like   int srcPtr destPtr  string   arr   srcPtr   theList Count  arr   new string srcPtr 2   theList CopyTo arr  theList Count      Copy into second half of the array destPtr   0  for    srcPtr  lt  arr Length  srcPtr        string st   arr srcPtr     char ch    st         0      Get first character of string  or     if empty   if  ch     U       arr destPtr      st    if  ch     T       arr destPtr       Boo     if  destPtr  gt  arr Length 2     More than half of dest  array is used     theList   new List lt String gt  arr      Adds extra elements   if  destPtr    arr Length      theList RemoveRange destPtr  arr Length-destPtr      Chop to proper length   else     Array Resize ref arr  destPtr     theList   new List lt String gt  arr      Adds extra elements     It would have been helpful if List lt T gt  provided a method to construct a list from a portion of an array  but I m unaware of any efficient method for doing so   Still  operations on arrays are pretty fast   Of note is the fact that adding and removing items from the list does not require  pushing  around other items  each item gets written directly to its appropriate spot in the array

User · Answer

I have written one easy step  but because of this performance will be degraded  Here is my code snippet -  for  int tempReg   0  tempReg  lt  reg Matches lines  Count  tempReg                                                                  foreach  Match match in reg Matches lines                                                                         var aStringBuilder   new StringBuilder lines                                       aStringBuilder Insert startIndex  match ToString   Replace                                                lines k    aStringBuilder ToString                                        tempReg   0                                      break

User · Answer

You can t change the enumerable collection while it is being enumerated  so you will have to make your changes before or after enumerating   The for loop is a nice alternative  but if your IEnumerable collection does not implement ICollection  it is not possible   Either    1  Copy collection first  Enumerate the copied collection and change the original collection during the enumeration    tvanfosson   or  2  Keep a list of changes and commit them after the enumeration

[c#] What is the best way to modify a list in a 'foreach' loop?

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