Combobox SelectionChanged problem with IsEditable

Selection Changed event firing when we type on the  textbox of the combo box. How to prevent this?  I want to allow filtering by type. How to possible in wpf?

<ComboBox IsEditable="True" SelectionChanged="ComboBox_SelectionChanged"   IsTextSearchEnabled="True" Height="30" Width="200"><ComboBoxItem Name="cbi1">Item1</ComboBoxItem><ComboBoxItem Name="cbi2">Item2</ComboBoxItem><ComboBoxItem Name="cbi3">Item3</ComboBoxItem></ComboBox>

thanks