title: WPF Bind TextBox.Text to a property of the item bound to the SelectedItem of a ComboBox
body: <p>I have a combobox whose itemssource is a collection of objects of my domain model.  One of the properties of that object is 'Amount'
  How do I bind the 'Amount' property of the SelectedItem of the combobox to the text of a texdtbox?</p>