private void pptToImg(string pptPath, string imgPath)
    {
      var app = new Microsoft.Office.Interop.PowerPoint.Application();

      var ppt = app.Presentations.Open(pptPath, Microsoft.Office.Core.MsoTriState.msoFalse, Microsoft.Office.Core.MsoTriState.msoFalse, Microsoft.Office.Core.MsoTriState.msoFalse);

      var index = 0;

      var fileName = Path.GetFileNameWithoutExtension(pptPath);

      foreach (Microsoft.Office.Interop.PowerPoint.Slide slid in ppt.Slides) 
      {
        ++index;
        //設置圖片大小
        slid.Export(imgPath+string.Format("page{0}.png",index.ToString()), "png", 1024, 768);
        //根據屏幕尺寸。設置圖片大小
        //slid.Export(imgPath+string.Format("page{0}.jpg",index.ToString()), "jpg", Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);
      }

      //釋放資源
      ppt.Close();
      app.Quit();
      GC.Collect();
    }