string imgUrl = "";
                if (!string.IsNullOrEmpty(Request["picUrl"]))
                    imgUrl = Request.QueryString["picUrl"].ToString();
                if (imgUrl == "")
                {
                    imgUrl="aaa";
                }

文章题目：验证Request传过来的值是否为空
URL标题：http://scpingwu.com/article/gdeheh.html