Virtual Mode ListView

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Introduction

A WindowsForms ListView control can be used in "virtual mode". When running in virtual mode, the ListView doesn't host any data, instead the only thing it needs to know is, how many rows/lines it has. A callback method has to be provided in order to pass over the data to display, whenever the visible lines the control is able to display at a given time, scroll into view. The example is able to display about 100 million lines and represents the line number as text (German).

Screenshot

Background

This simple prototype was created before I started creating the TreeListView as an extension to the ListView - also running in virtual mode - which is also available here at CodeProject: VirtualModeTreeListView.

Using the Code

In order for a ListView to operate in virtual mode, we will have to set the appropriate property "VirtualMode" to true. Once we place the WindowsForms ListView control on some form, the properties window of the ListView will appear. Be sure that the ListView object is selected.

After activating the virtual mode, it's a must to subscribe to the "RetrieveVirtualItem" event as well, due to the fact that the control gathers for data regarding the rows that are visible/in view. If we forget to subscribe to this event, the control will fire an exception. So we kindly fulfill the requirements.

Now we will have to provide some code within the callback handler of the event "RetrieveVirtualItem". Whenever lines are visible, the control gathers for data in order to display data within the columns of the control. Therefore the control expects a usual ListViewItem object, composed with any additional ListViewSubItem objects as child objects, depending on the configured columns of the control. Keep in mind that if we were not using the virtual mode, we'd have to fill the control with such objects so that the control could view data in its columns of each row. If not, the control would just be "empty".

This example just takes the line numbers that are passed into the RetrieveVirtualItem callback method and MakeText() creates the textual representation of the line numbers.

private void listView1_RetrieveVirtualItem(object sender, RetrieveVirtualItemEventArgs e)
   {
       ListViewItem lvi = new ListViewItem(); 	// create a listviewitem object
       lvi.Text = nt.MakeText(e.ItemIndex); 		// assign the text to the item
       ListViewItem.ListViewSubItem lvsi = new ListViewItem.ListViewSubItem(); // subitem
       NumberFormatInfo nfi = new CultureInfo("de-DE").NumberFormat;
       nfi.NumberDecimalDigits = 0;
       lvsi.Text = e.ItemIndex.ToString("n", nfi); 	// the subitem text
       lvi.SubItems.Add(lvsi); 			// assign subitem to item

       e.Item = lvi; 		// assign item to event argument's item-property
   } 

The MakeText() method just creates the appropriate string for a certain number, the text however is in German. There is a class called NumberText which is used to create the string. Please refer to the source file NumberText.cs for details.

One thing we still have to do is tell the ListView how many rows/lines it has. I do this within the Load event of the Form like this:

private void Form1_Load(object sender, EventArgs e)
        {
            listView1.VirtualMode = true; 		// switching virtual mode on
            listView1.VirtualListSize = 1000000000; 	// give it 1 million lines
        }

Points of Interest

You might also be interested in a treelistview running in virtual mode: VirtualModeTreeListView.

History

• 11^th September, 2009: First version