无法在事件处理程序combobox_selectionChanged中访问datagrid.itemsSource？

i am developing a wpf application , in which when the user performs selection changed action on a combobox , selection changed event is triggered, but in the event handler when i update datagrid.itemsSource, error is shown to use a new key word to create the object, through break point i have seen that inside the event handler datagrid object itself is null, but it is not null in the MainWindow function. why is it so?i am developing a wpf application , in which wh