How to speed up the removal of ListView items ListViews?

Question

I already use listView.BeginUpdate() and listView.EndUpdate() , but it still takes 10 seconds when I delete, for example, 100 items from 25k.

Any ideas, tricks to make it faster?

EDIT:

 this.listView.BeginUpdate(); for (int i = this.listView.CheckedItems.Count - 1; i > -1; --i) { this.listView.CheckedItems[i].Remove(); } this.listView.EndUpdate(); 

Answer 1

You can start your optimization here:

 List<int> toRemove = new List<int>(); foreach (ListViewItem item in this.listView.Items) { if (item.Checked) // check other remove conditions here toRemove.Add(item.Index); } /* sort indices descending, so you'll remove items with higher indices first and they will not be shifted when you remove items with lower indices */ toRemove.Sort((x, y) => y.CompareTo(x)); /* in this specific case you can simply use toRemove.Reverse(); or iterate thru toRemove in reverse order because it is already sorted ascending. But you might want to force sort it descending in some other cases. */ this.listView.BeginUpdate(); foreach (int itemIndex in toRemove) this.listView.Items.RemoveAt(itemIndex); // use RemoveAt when possible. It much faster with large collections this.listView.EndUpdate(); 

Answer 2

The ListView fires an event for each individual item that is removed from the list. You can try to avoid this by clearing the entire ListView, and then adding immediately a new list of items that lacks the 100 items you want to remove. This will trigger only a few events.

Answer 3

This is because every time you delete an item from Items, the ListView must find that item ( going through the list for this ) and update the CheckedItems collection (which repeats all the other items again), so the complexity is O ^ 2.

The easiest way is to cache SelectedIndices and use listItem.Items.RemoveAt ():

 var selectedIndices = listView.SelectedIndices.Cast<int>().Reverse().ToList(); listView.BeginUpdate(); foreach (var index in selectedIndices) { listView.Items.RemoveAt(index); } listView.EndUpdate(); 

If you do not want to use the Cast <> extension method, you can replace the first line:

 List<int> oToDelete = new List<int>(SelectedIndices.Count); foreach (int iX in SelectedIndices) { oToDelete.Add(iX); } oToDelete.Reverse();