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Collection was modified; enumeration operation may not execute, enumerating through FileInfo

Posted on 2009-05-20
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Last Modified: 2013-12-17
I have a collection of zipfiles in a FileInfo object, and as I loop through them, I determine if I need it or not. When I run the code, I get following runtime error message...

Collection was modified; enumeration operation may not execute

Here is a sample pseudo, of what I have going on. Is there a simple way to work around this without making things too hairy?  :-)
foreach (FileInfo zp in ZipFiles)
            {
			
					// Do some calculations. Determine if I don't need the zip file.
                    ZipFiles.Remove(zp);//remove zipfiles that I don't need
					
					// Do some calculations. Determine if I do need the zip file.
                    ZipFiles.Add(fi);//add some new zipfiles that I do need.
					
 
            }//end of foreach loop

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Question by:silentthread2k
9 Comments
 
LVL 11

Assisted Solution

by:srikanthreddyn143
srikanthreddyn143 earned 200 total points
ID: 24433460
You cant add or remove from a collection while doing a loop on it. You have to clone that collection and make add/remove on it.
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LVL 75

Assisted Solution

by:käµfm³d 👽
käµfm³d   👽 earned 800 total points
ID: 24433501
You cannot modify a list (ZipFiles) for which you are stepping through via an iterator. Try copying the list and modifying the copy.
public List<FileInfo> EditList(List<FileInfo> ZipFiles)
{
    List<FileInfo> tempList = new List<FileInfo>(ZipFiles);
 
    foreach (FileInfo zp in ZipFiles)
    {
        // Do some calculations. Determine if I don't need the zip file.
        tempList.Remove(zp);//remove zipfiles that I don't need
 
        // Do some calculations. Determine if I do need the zip file.
        tempList.Add(fi);//add some new zipfiles that I do need.
    }
 
    return tempList;
}

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Assisted Solution

by:Anurag Thakur
Anurag Thakur earned 200 total points
ID: 24433821
to modify a collection you cannot use a foreach loop as if you remove the collection the indexers will be modified and and exception will be thrown

to achieve use a for loop and start from the last (highest count) and come back to 0 and if your calculations prove that it needs to be removed then you can achieve it easily
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LVL 3

Assisted Solution

by:mpf1748
mpf1748 earned 400 total points
ID: 24433912
Another way that I have used in the past is to traverse the list backwards and remove as a I go, but you will have to use a for loop instead of a foreach loop. This solution works better for removing, but you could also add items at the end of the list.
for (int i = ZipFiles.Count-1; i >= 0; i--)
{
  FileInfo zf = ZipFiles[i];
  if (<logic to remove>)
  {
    ZipFiles.Remove(zp);
  }
  if (<logic to add>)
  {
    ZipFiles.Add(<new zip file>);
  }
}

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LVL 22

Assisted Solution

by:p_davis
p_davis earned 400 total points
ID: 24434135
you can use a foreach with a delegate

ZipFiles.ForEach(delegate (FileInfo zp)
            {
                       
                                        // Do some calculations. Determine if I don't need the zip file.
                    ZipFiles.Remove(zp);//remove zipfiles that I don't need
                                       
                                        // Do some calculations. Determine if I do need the zip file.
                    ZipFiles.Add(fi);//add some new zipfiles that I do need.
                                       
 
            });
//end of foreach loop
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LVL 22

Expert Comment

by:p_davis
ID: 24434148
i really wouldn't recommend removing or adding in a for loop though
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Accepted Solution

by:
silentthread2k earned 0 total points
ID: 24434197
Thanks, I will just modify the files and then just build the collection.
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Author Comment

by:silentthread2k
ID: 24434217
Thanks.
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Author Comment

by:silentthread2k
ID: 24434613
I accepted my solution, but let me know if you get awarded points, which is what I tried doing.
Thanks again.
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