Reset comboxbox selection after a selection is made - wpf

Question

I have a combobox that is created with data from a dataset
foreach(var item in ds.MiDESValues)
{
string comboboxtext = ds.MiDESValues.Rows[k][1].ToString();
sFactorCB.Items.Add(comboboxtext);
k++;
}
On a selectionchanged event it will populate a listbox with that selection
private void sFactors_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
string add = sFactorCB.SelectedValue.ToString();
var svalue = ds.MiDESValues.Rows[0][2].ToString();
int Svalue = int.Parse(svalue);
SValue.Add(svalue);
SelectionListBox.Items.Add(add);
SelectionBox.Add(add);
// when a new item is added to Selection list box, select it and show it
// this will keep the last item highlighted and as the list grows beyond
// the view of the list box, the last item will always be shown
SelectionListBox.SelectedIndex = SelectionListBox.Items.Count - 1;
SelectionListBox.ScrollIntoView(SelectionListBox.SelectedItem);
}
That list box then used to populate a listbox used on the next page. If I navigate to the next page and then navigate back, the combobox is still showing the last selection I made therefor the listbox is being populated with that value.
I have tried setting the selectedindex of the combobox to sFactorCB.SelectedIndex = -1;, at the end of the sFactors_SelectionChanged event but i get System.NullReferenceException. How can I get the combobox to reset back to a non-selected item state? Thanks

Answer 1

Actually, you are doing it correctly. To clear the selection either set the SelectedIndex to -1 or the SelectedItem to null.
The problem is that once you do that your sFactors_SelectionChanged gets called again and since there is no current selection, the SelectedValue property is null thus causing the following to fail:
string add = sFactorCB.SelectedValue.ToString();
How to solve this sort of depends on your intended results when nothing is selected. The simplest thing to do would be to just check for nothing selected at the start of the handler and simply return.
private void sFactors_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (sFactorCB.SelectedIndex == -1 || sFactorCB.SelectedValue == null)
return;
string add = sFactorCB.SelectedValue.ToString();
...