title: COMBOBOX filtering in WPF with MVVM
body: <p>I am developing an application using WPF mvvm approach.
  I have a requirement where I have to show a list of items in a combo box for selection.
  Based on some flag I need to filter out few items from the combo box for selection.</p>
  
  <p>I tried to use two different items sources one with full list and another with filtered list and based on the flag I wanted to change the items source.
  This does not seem to be working well. Is there any easy way to apply filters on the existing list based on some flag ?</p>