WPF datagrid and Item Container Recycling - wpf

Question

In WPF I have a dialog window with an embedded datagrid.
The purpose of the dialog is to select files to upload. I've added a column in the datagrid to allow selection. But there is a fairly complicated relationship between the rows -- uploading one file may require the upload a parent file, for example. Because of that, there is a handler that updates the checkboxes (checking/unchecking and setting isEnabled off/on)
I seem to have run into a problem with "Item Container Recycling" when I get many rows in the datagrid. Some of the definition looks like
<DataGrid Name="myGrid"
...
VirtualizingPanel.IsVirtualizing="True"
VirtualizingPanel.VirtualizationMode="Standard">
<DataGrid.Resources>
<Style TargetType="DataGridColumnHeader" x:Key="DataGridHeaderStyle" >
</Style>
<Style x:Key="SingleClickEditing" TargetType="{x:Type DataGridCell}">
<EventSetter Event="PreviewMouseLeftButtonDown" Handler="DataGridCell_PreviewMouseLeftButtonDown"></EventSetter>
<EventSetter Event="CheckBox.Checked" Handler="OnChecked"/>
<EventSetter Event="CheckBox.Unchecked" Handler="OnChecked"/>
</Style>
...
When I first created the datagrid I was not able to access all rows of the datagrid until I set:
VirtualizingPanel.VirtualizationMode="Standard"
Now on dialog initialization I find that I can access all rows of the grid and set them up OK.
But the odd thing is that later when the user clicks on a checkbox, the checkbox handler method attempts to access the checkbox from other rows to set them. But then again not all of the rows are available and I see strange results.
It is as if the following got turned off for the datagrid.
VirtualizingPanel.VirtualizationMode="Standard"
Programmatically in the cs code for the dialog I don't see that I am able to access a parameter grid.VirtualizingPanel to see what the value is.
I try to get all the rows of the grid as follows:
private List<DataGridRow> GetDataGridRows(DataGrid grid)
{
var itemsSource = grid.ItemsSource as IEnumerable;
List<DataGridRow> rows = new List<DataGridRow>();
if (null == itemsSource) return null;
foreach (var item in itemsSource)
{
DataGridRow row = grid.ItemContainerGenerator.ContainerFromItem(item) as DataGridRow;
if (row!=null) rows.Add(row);
}
return rows;
}
The problem is that sometimes the following returns null:
grid.ItemContainerGenerator.ContainerFromItem(item)
How I can I make sure that I can always get a list of all rows?
==================================
Added:
The XAML column definition for the checkbox column is as follows:
<DataGrid.Columns>
<DataGridCheckBoxColumn ElementStyle="{StaticResource CenteredCheckStyle}" MinWidth="15"
CellStyle="{StaticResource SingleClickEditing}" Visibility="{Binding exists}"
Binding="{Binding Path=toTransfer, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" IsReadOnly="False"
CanUserSort="False" CanUserResize="false" CanUserReorder="false">
<DataGridCheckBoxColumn.HeaderTemplate>
<DataTemplate>
<CheckBox Checked="CheckBox_Checked" Unchecked="CheckBox_Checked" Loaded="CheckBox_Loaded"
HorizontalContentAlignment="Center" HorizontalAlignment="Center"
IsThreeState="False" Margin="8 0 0 0" Padding="0 5 0 0"/>
</DataTemplate>
</DataGridCheckBoxColumn.HeaderTemplate>
</DataGridCheckBoxColumn>