using unbound wpf datagrid as data entry tool

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VB

WPF

DataGrid

Hello every one,
trying to use a datagrid as data entry tool
the datagrid has 4 columns (code,name,price, quantity)
i try to add an empty row through code like this :

datagrid1.items.add(new datagridrow)

but i get null reference exception...i really don't know why !
then tried

datagrid1.items.add(new dataobject)

that does add a new empty row but doesn't allow me to edit the cells

so the question is how can i in runtime add editable empty rows ??
also how can i keep all the cells in edit mode all the time ??

(each column .isReadOnly property is set to false and also the datagrid same property)
(the exact message when trying to edit a cell is :
'EditItem' is not allowed for this view.)

Thanks for your time every one