Add confirm before closing radwindow.

you can use previewclosed and set WindowPreviewClosedEventArgs's  property of cancel true  to implement to add confirm before closing radwindow.

void _win_PreviewClosed(object sender,Telerik.Windows.Controls.WindowPreviewClosedEventArgs e)
      {
         e.Cancel = true;
         if (_viewModel.IsEnabledForButtonSave)
         {
            string confirmText = "Are you sure you want to close without saving?";
            RadWindow.Confirm(confirmText, new EventHandler<WindowClosedEventArgs>(OnConfirmWindowClosed));
         }
         else
         {            
            _proxy.Close();
            _mProxy.Close();
            _viewModel.BtnCloseClick();
            e.Cancel = false;
         }
      }