public string Delete(string TenDangNhap, string ChucVu)
 {
     if (ChucVu != "Admin")
     {
         try
         {
             NguoiDung.XoaNguoiDung(TenDangNhap);
             if (NguoiDung.DSNguoiDungTheoTen(TenDangNhap).Count() == 0)
             {
                 return("successed");
             }
             else
             {
                 return("failed");
             }
         }
         catch
         {
             return("failed");
         }
     }
     else
     {
         return("failed");
     }
 }