Exception during iteration on collection and remove items from that collection [duplicate]

Answer 1

You can't remove an item from a collection while iterating over it.

You can find the index of the item that needs to be removed and remove it after iteration has finished.

int indexToRemove = 0;

// Iteration start

if (fileName.Equals(names[i].Trim()))
{
    indexToRemove = i;
    break;
}

// End of iteration

attachmentsFilePath.RemoveAt(indexToRemove);

If, however, you need to remove more than one item, iterate over a copy of the list:

foreach(string fullFilePath in new List<string>(attachmentsFilePath))
{
    // check and remove from _original_ list
}