mono touch中UIAlertView对话框多选项处理

//"取消"=>0, "标清"=>1, "高清"=>2
			UIAlertView alert = new UIAlertView ("提示", "请选择视频版本", null, "取消", new string[] { "标清", "高清" });
			alert.Clicked += (s, b) => {
				if (b.ButtonIndex != 0) {
					string _url = b.ButtonIndex == 1 ?
					              TableItems [indexPath.Row].DOWNLOAD_URL_LOW :
					              TableItems [indexPath.Row].DOWNLOAD_URL;

					string _cp = FileHelper.GetMyDocumentPath (KeyCenter.Key_CoursePath);//课程路径
					string _sub = TableItems [indexPath.Row].Course_Number;
					string _name = StringHelper.GetUrlFileName (_url);

					int tag = 1; //检查是否下载过了
					if (FileHelper.FileUploaded (_cp + _sub, _name)) {
						tag = 2;
					}
					base.RowTouch2View = new CourseCenter (TableItems [indexPath.Row], tag, 2);
					base.RowSelected (tableView, indexPath);
				}
				tableView.DeselectRow (indexPath, true);
			};
			alert.Show ();