private static void OnCanExecute(object sender, CanExecuteRoutedEventArgs e)
        {
            EditorCommandInterface page = sender as EditorCommandInterface;

            // The e.CanExecute provides feedback to the associated caller whether the 
            // e.Command can be executed. If the value is false, the UI will be automatically disabled
            // thus enables consistent and reliable command interface.

            if (page == null)
            {
                e.CanExecute = false;
                return;
            }

            if (e.Command == EditingCommands.Cut || e.Command == EditingCommands.Copy)
            {
                // Enable Cut or Copy command if the textbox has selection
                e.CanExecute = page.textBox1.Selection != null && !string.IsNullOrEmpty(page.textBox1.Selection.Text);
            }
            else if (e.Command == EditingCommands.Paste)
            {
                // Enable Paste command if the Clipboard has a valid text
                e.CanExecute = page.textBox1.Selection != null && Clipboard.ContainsText();
            }
            else
            {
                e.CanExecute = true;
            }
        }