Collection modified exception thrown when the collection wasn't modified

Question

I'm getting an exception which I don't understand in this bit of code here:

Dim folderList As List(Of String) = _folderList

For Each folder In folderList

    destinationFolder = destinationFolder + "/" + folderName
    localFilePath = lbl_folderPath.Text + "/" + folder

    alterFolderList(localFilePath)

    ...

Next

I've got a global variable _folderList which I copy to another variable, folderList, as seen in the first line of my code. When the last method (alterFolderList) is called, it alters the variable _folderList. When debugging, as I reach the end of the for each for the first time (at Next) I get the exception that the collection was modified, when it actually wasn't because the method called doesn't change it. When debugging, after the method is called, I hover above the variable folderList and I see it changed and is now the same as _folderList but it shouldn't because the variable folderList is equaled to _folderList outside the For Each enumeration.

How does this happen? And how to work around this?

Answer 1

To copy the list you cannot assign it. You should copy the elements. Try this.

Dim folderList As New List(Of String)
folderList.AddRange(_folderList)