在没有任何数据时进行无效的读取尝试？怎么回事？大家帮忙看一下

SqlConnection myconn = new SqlConnection("server=localhost;user id=sa;pwd=sa;database=zy");
        myconn.Open();
        SqlCommand cmd = myconn.CreateCommand();
        cmd.CommandText = "SELECT UserName,PassWord from [User]";
        SqlDataReader reader = cmd.ExecuteReader();
        string username = TextBox1.Text;
        string password = TextBox2.Text;
        while (reader.Read());
        
                       if (username == reader["UserName"].ToString())
            {
                if (password == reader["PassWord"].ToString())
                {
                    Response.Redirect("Default3.aspx");
                    return;
                   
                }
                else
                {
                    Response.Write("<script language='javascript'>alert('请检查密码!');</script>");
                    return;
                    
                }
            }
            else
                Response.Write ("<script language='javascript'>alert('用户名不存在!');</script>");
            return;
            
            }