我错误Mssage，如何解决上面的方法

Question qs = new Question();

                        string name = Request.QueryString["UserName"];

                        TextBox txtQuestionNo = (TextBox)row.FindControl("Question No");

                        

                        

                        TextBox txtComments = (TextBox)row.FindControl("Comments");

                        //TextBox txtMarks = (TextBox)row.FindControl("Marks");