C＃PowerPoint播放例外

Question

我有一个方法，它有一个线程，下一个幻灯片有powerpoint持续时间。但在关闭演示文稿后，我需要检查它是否有效。 但我无法弄明白。你能帮我解决这个问题吗？

private void Play()
{
    try
    {
        while (mainForm._slideIndex <= mainForm._slidescount)
        {
            if (mainForm._presentation.Slides._Index(1) != null)
            {
                Thread.Sleep((int)mainForm._oSlideShowView.Slide.SlideShowTransition.Duration * 500);
            }

            if (mainForm._slideIndex == mainForm._slidescount)
            {
                if (mainForm._oSlideShowView != null)
                {
                    mainForm._oSlideShowView.First();
                    mainForm._slideIndex = 1;
                }
            }

            if (mainForm._slideIndex <= mainForm._slidescount)
            {
                if (mainForm._oSlideShowView != null)
                {
                    mainForm._oSlideShowView.Next();
                    mainForm._slideIndex++;
                }
            }
            if (mainForm._presentation.Slides._Index(1) != null)
            {
                Thread.Sleep((int)mainForm._oSlideShowView.Slide.SlideShowTransition.Duration * 500);
            }
        }
    }
    catch (Exception exp)
    {
        throw new Exception();
    }
}

Exception