title: Prevent ComboBox from setting the selectedValue to null when it's not in the bound list
body: <p>I'm not quite sure how to deal with this problem. I'm using a bunch of comboboxes with dropdown lists of values we allow the user to set a property too. (i.e. Currencies = "USD, CAD, EUR").</p>
  
  <p>Every now and then, when we load data, we'll find the currency is something not in our list, like "AUD". In this case, we still want the combobox to display the loaded value, and the current selected Currency should remain "AUD" unless the user chooses to change it, in which case their only options will still be "USD, CAD, EUR".</p>
  
  <p>My problem is that as soon as the control becomes visible, the ComboBox is calling the setter on my SelectedCurrency property and setting it to <code>null</code>, presumably because the current value "AUD" isn't in it's list. How can I disable this behaviour without making it possible for the user to type whatever they want into the Currency field?</p>