Lists and Virtualization

Lists and virtualization

ListBox, CheckedListBox and DragList are ItemsControls: panels whose children are made from data. Items holds the data; a container — a label by default — is made for each item it shows.

var list = new ListBox
{
    SelectionMode = SelectionMode.Extended,
    OverflowY = Overflow.Auto,
    Height = 300,
};

list.Items.AddRange(customers);
list.SelectionChanged += (_, _) => details.Show(list.SelectedItem);

Member
Items	The data. With a DataSource set, a mirror of it
ItemTemplate	Func<object, UIElement>: the container of an item. Null — a label with its ToString()
SelectionMode	Single, Multiple (a click toggles), Extended (Ctrl toggles, Shift selects a range)
SelectedIndex, SelectedItem	The lead item
SelectedIndices, SelectedItems	Every selected item
SetSelected, SelectAll, ClearSelection	From code

list.ItemTemplate = item => new StackPanel
{
    Orientation = Orientation.Horizontal,
    Spacing = 8,
    Children = { new SvgIcon { PathData = "M12 12a4 4 0 1 0 0-8 4 4 0 0 0 0 8z", IconSize = 16 }, new Label { Text = ((Customer)item).Name } },
};

Virtualization

A list box virtualizes by default: it makes containers only for the rows in sight and a few around them, and reuses its own labels as rows scroll out and in. A list of ten thousand customers has a couple of dozen labels, not ten thousand, measured on every layout.

The rows must be of one height for this: ItemHeight, or — left NaN — the height of the first row there is. A list of rows of different heights — cards, a feed — turns it off:

var feed = new ListBox { IsVirtualizing = false, ItemTemplate = item => new PostCard((Post)item) };

With virtualization the children are no longer one per item, and a child's index in Children is not its item's index. Find a row's container through the list:

Member
ContainerFromIndex(index)	The container of an item; null when it is out of sight
IndexFromContainer(container)	The item a container shows; −1 when it isn't one of this list's
ScrollIntoView(index)	Scrolls the item's row into sight; the container is made at the next layout

TreeView and VirtualizingStackPanel virtualize with the same engine.

Scrolling

The keys move the selection and bring the row they reach into sight: Home, End, the arrows, and Shift with them in Extended mode. ScrollPastEnd lets a long list scroll on until its last row is at the top — see Layout.