C# – How to modify or delete items from an enumerable collection while iterating through it in C#

ccollectionsiteration

I have to delete some rows from a data table. I've heard that it is not ok to change a collection while iterating through it. So instead of a for loop in which I check if a row meets the demands for deletion and then mark it as deleted, I should first iterate through the data table and add all of the rows in a list, then iterate through the list and mark the rows for deletions. What are the reasons for this, and what alternatives do I have (instead of using the rows list I mean)?.

Best Answer

Iterating Backwards through the List sounds like a better approach, because if you remove an element and other elements "fall into the gap", that does not matter because you have already looked at those. Also, you do not have to worry about your counter variable becoming larger than the .Count.

        List<int> test = new List<int>();
        test.Add(1);
        test.Add(2);
        test.Add(3);
        test.Add(4);
        test.Add(5);
        test.Add(6);
        test.Add(7);
        test.Add(8);
        for (int i = test.Count-1; i > -1; i--)
        {
            if(someCondition){
                test.RemoveAt(i);
            }
        }

Related Solutions

Java – Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

Iterator.remove() is safe, you can use it like this:

List<String> list = new ArrayList<>();

// This is a clever way to create the iterator and call iterator.hasNext() like
// you would do in a while-loop. It would be the same as doing:
//     Iterator<String> iterator = list.iterator();
//     while (iterator.hasNext()) {
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
    String string = iterator.next();
    if (string.isEmpty()) {
        // Remove the current element from the iterator and the list.
        iterator.remove();
    }
}

Note that Iterator.remove() is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

Source: docs.oracle > The Collection Interface

And similarly, if you have a ListIterator and want to add items, you can use ListIterator#add, for the same reason you can use Iterator#remove — it's designed to allow it.

In your case you tried to remove from a list, but the same restriction applies if trying to put into a Map while iterating its content.

C# – the most efficient way to delete all selected items in a ListViewItem collection

If you have a collecion of your selectedItems, you can simply call remove on each of them instead of iterating over the ListView.

There is a method on ListViewItem called Remove().

ListView listview = <reference to ListView>;
foreach (ListViewItem item in listView.SelectedItems)
{
  item.Remove();
}

This removes the iteration through all the items and only removes your selected items.

Best Answer

Related Solutions

Java – Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

C# – the most efficient way to delete all selected items in a ListViewItem collection

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