title: How to register the TextChanged event for the TextBox which is inside ComboBox
body: <p>We know that comboBox is a combination of TextBox, Button and other UIElements. 
  My question is How to register the TextChanged event for the TextBox which is inside ComboBox.
  ComboBox contains only PreviewTextInput and TextInput events but I want to handle TextChangedEvent.
  Edit:1 My comboBox is an Editable Combobox</p>
  
  <p>Edit:2 When the user enters text in ComboBox, I want to check whether it is a double value or not. I allow only double values in my comboBox.</p>
  
  <p>Thanks in Advance.</p>