Get index number of ListViewItem in ListView.

ListViewHelper.cs

public class ListViewHelper
{
    public static int GetIndex(object item)
    {
        if (item is not ListViewItem listViewItem)
            return -1;

        ListView listView = (ListView)ItemsControl.ItemsControlFromItemContainer(listViewItem);
        return listView.ItemContainerGenerator.IndexFromContainer(listViewItem);
    }
}