ComboBox Text

Hi!

in a editable ComboBox that is binding to a Database, i want to check if  text exist in ComboBox items.

<ComboBox DataContext="{StaticResource vehiclesViewSource}" DisplayMemberPath="vehicle" ItemsSource="{Binding}"
TextBox.TextChanged="vehicleComboBox" IsEditable="True" IsTextSearchEnabled="True" IsTextSearchCaseSensitive="False"/>

bool itemExists =false;foreach(ComboBoxItem cbi in cb.Items){
            itemExists = cbi.Content.Equals(cb.Text);if(itemExists)break;
             ...}

but i get a runtime exception