New Post: Custom Text for CheckComboBox based on items selected

 public class MyCheckComboBox : CheckComboBox
  {
    protected override void UpdateText()
    {
      if( this.SelectedItems.Count == this.Items.Count )
      {
        this.SetCurrentValue( MyCheckComboBox.TextProperty, "All Items are selected." );
      }
      else if( this.SelectedItems.Count <= 1 )
      {
        base.UpdateText();
      }
      else
      {
        this.SetCurrentValue( MyCheckComboBox.TextProperty, this.SelectedItems.Count + " items are selected." );
      }
    }
  }