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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 50 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 200 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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LVL 26

Assisted Solution

by:Anurag Thakur
Anurag Thakur earned 50 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 100 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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3 Use Cases for Connected Systems

Our Dev teams are like yours. They’re continually cranking out code for new features/bugs fixes, testing, deploying, testing some more, responding to production monitoring events and more. It’s complex. So, we thought you’d like to see what’s working for us.

 
LVL 22

Assisted Solution

by:p_davis
p_davis earned 100 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
0
 

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.
0
 

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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3 Use Cases for Connected Systems

Our Dev teams are like yours. They’re continually cranking out code for new features/bugs fixes, testing, deploying, testing some more, responding to production monitoring events and more. It’s complex. So, we thought you’d like to see what’s working for us.

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