WPF 应用程序中的模式消息框

我试图在我的应用程序中将标准 MessageBox 显示为模态窗口，但它最终显示为非模态。在第一次调用中，在下面的代码中，我显示了一个标准的 MessageBox，它应该显示为模态。在第二次调用中，它不会显示为模态，即使我抓住主窗口调度程序也是如此。

Dispatcher disp = Application.Current.MainWindow.Dispatcher;
//First call, shown MODAL
if (this.messageService.ShowYesNo("Do you want to update the Word document, this will regenerate inspectiondata for document", "") == MessageBoxResult.Yes)
{
    using (new WaitCursor())
    {
        _eventAggregator.GetEvent<ProgressBarRequestShow>().Publish("");
        worker = new BackgroundWorker();
        worker.DoWork += delegate(object s, DoWorkEventArgs args)
        {
            AITUpdateProgressDelegate update = new AITUpdateProgressDelegate(UpdateProgress);
            this.docService.UpdateWorddocument(this.docService.GetCurrentDocumentFilePath, update);
        };
        worker.RunWorkerCompleted += delegate(object s, RunWorkerCompletedEventArgs args)
        {
            try
            {
                // Second call NOT MODAL
                disp.Invoke((Action)delegate()
                {
                    this.messageService.ShowInformation("Document generated, choose Open in Word in main toolbar to show document", "");
                });
                _eventAggregator.GetEvent<ProgressBarRequestHide>().Publish("");
            }
            finally
            {
            }
        };
        worker.RunWorkerAsync();
    }
}