silverlight - ComboBox Binding is lost when bound item is swapped for a new item containing null for the field -

I have a "user" screen where a user unit is bound I when it stops as a new user If the bound value for the combboxes is zero, then the kickboxes begin to empty and the purpose of the tasks is to use if I click on my "new user" button, then take any comb boxes from the empty null value to the value Go, while still behind New user Affairs unit is fully binding on. Before clicking on "New", any Cambobox was left taped from the beginning, which would continue to work properly. After that my cockbox is completely dead. Changing its value will not return the source, and while loading a new source, the combobox is not selecting the right data anymore.

Is there anything that can be done with this fact that there is no empty line in my blankbox, once I select something then I can not empty it again. I do not know anything can happen to him, but I can not figure out how to make an empty line in the list of options.

Once the binding items do not match the sauce material, it is difficult to return things to the track. . Take a look at this post where I show how to set up combobox for encex content loading. This is a great option when you are switching items and selectable etam bindings at the same time.