ComboBox's SelectedValue set to null when changing templates
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I am using the MVVM pattern, and have a multi-screen data entry form. There is a single VM with an ActiveScreen property that uses INotifyPropertyChanged events. Templates are used by the View based on the current ActiveScreen. On one of the screens, I have a ComboBox that is bound to a List<> for its ItemsSource. I am using the MVVM pattern, and the selected value of the Combobox is twoway bound to a property on my model. All works fine when you are on the screen entering data. When the user changes the ActiveScreen, and hence a new template is used in the View, a weird thing happens. The property on the model bound to the combobox is set to NULL. Looking at the call stack, I see the ActiveScreen property change event, then a bunch of WPF code, and then my property setter. The count in the collection on the combobox at that point is 0. It seems as if WPF does some cleanup of the combobox by removing all the items, which sets the selecteditem to null, and because of my binding, it changes my model. The net effect is I lose the data entered by the user. The other elements on that screen (textboxes, datepickers, etc) do not have this problem, so it's clearly something with the ComboBox. Has anyone hit this before, and know of a solution? The problem is, I want the user to be able to select a blank value, so I don't know if it's the user setting the value to null or this odd series of events.