Question

With respect to a Virtual ListView control in a Winforms App, what are ALL the functions of RetrieveVirtualItem event?

Scenario: There is a Dictionary object which acts as my ListView cache. And its items are displayed on the ListView. On click of a column, the cache dictionary is sorted in memory and the ListView is Refresh()ed. The effect on UI, the ListView is sorted well. 🙂

But I want to understand what the role of RetrieveVirtualItem Event is, which gets triggered on Refresh().

In the RetrieveVirtualItem event handler [ Someone else’s code, 🙁 ], following are done:

1. Based on RetrieveVirtualItemEventArgs.ItemIndex, get the message from Cache
2. Set RetrieveVirtualItemEventArgs.Item = retreived item above

It appears that what’s done in the event handler is important because, if I take it out, ListView cries. What is the significance of this event?

EDIT
Or let me re-phrase the question… My concern is, Why, after Sorting (and RetrieveVirtualItem event handler), the selected item remains at the location where it was before sorting. I.e, If I select item #5 and sort, and if sorting makes this item as the last item, I would prefer the LAST item to be selected after Sort. But in my case 5th item is selected after sort. So what is the default behavior of RetrieveVirtualItem WRT selected item?

Answer 1

Virtual listviews only deal with indices. So, if the 5th item is selected before the sort, the 5th item will still be selected after the sort. The control itself has no way of knowing that the data that used to be on the 5th row is now on the first row.

You will have to program that for yourself in your sorting method:

1. remember which items were selected (remember: you can’t use SelectedItems property when in virtual mode)
2. do the sort
3. find the indices of the previously selected item now live
4. select those indices

You can see all this in action in ObjectListView — a wrapper around a standard .NET ListView.