How do you clear a dropdown list box in silverlight

I am using an AUTOCOMPLETEBOX defined in XAML. I load it by having its ItemSource point to

private List<string> lstUserNames;

then, after this list is loaded I just assign the autocompletebox to the list:

UserNameComboBox.ItemsSource = lstUserNames; 

so how do I clear out the AUTOCOMPLETEBOX if I want 开发者_JAVA技巧to reuse it?

I see nothing here that helps http://msdn.microsoft.com/en-us/library/system.windows.controls.autocompletebox(v=vs.95).aspx

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Set the ItemsSource to null to clear it.