如何使用 MetaTrader4 错误代码刷新进行正确的错误处理？

Question

I have a "set up" on my EA to ping me an email when my Expert Advisor experiences an error and provides me the error code in accordance with the pre-determined 3-4 digit error code in the documentation on the MQL4 website .

/* technically speaking,
   error codes with 5+ digits are also possible,
   Using:
   ------                                                            */
   SetUserError( 1000000 );                                          /*

// this will set an error-state
// with a number 1065536 -- having a bit more than the said 3 ~ 4 digits
// composed as ( 1000000 + ERR_USER_ERROR_FIRST )
*/

This is to enable me to diagnose the problem.

Just wanting to clarify if I need to refresh the error code ( from a previous error ) I'm getting in my email, or does it do this automatically, when a new error is presented in the Journal of my MT4 Platform?

Answer 1

There is a helpful function ResetLastError() that explicitly sets _LastError to zero. Next, there is also a side effect of each call to a GetLastError() function, as it also sets zero to the _LastError variable.

Either way, it is a common practice to embed ( surround ) a section, where some error-states need to get handled accordingly, right by a:

// -------------------------------------------- // START_________________________
     GetLastError();                            // implicit  pre-reset _LastError
   ResetLastError();                            // explicit  pre-reset _LastError
   bool an_OK_flag = OrderModify( ... );        // XTO call  w ex-post _LastError
   int  anErrorSTATEtoHANDLE = GetLastError();  /* get a value of the  _LastError
                                                 + implicit post-reset _LastError */
// -------------------------------------------- // HANDLE ERROR-STATE(s)
   switch( anErrorSTATEtoHANDLE ){
         case ERR_NO_ERROR:                       break;
         case ERR_NO_RESULT:                      ...
                                                  break;
         case ERR_INVALID_TRADE_PARAMETERS:       ...
                                                  break;
         case ERR_SERVER_BUSY:                    ...
                                                  break;
         case ERR_BROKER_BUSY:                    ...
                                                  break;
         case ERR_TOO_MANY_REQUESTS:              ...
                                                  break;
         case ERR_TRADE_MODIFY_DENIED:            ...
                                                  break;
         ...
         default:                                 break;
   }
// -------------------------------------------- // FIN __________________________

This makes your code robust against any kind of some "forgotten" ( coincidentally un-reset ) last error ( which was not crashing your code-execution at the place, where such error has appeared but ... ) which would trigger later an unwanted behaviour in the "next" error-handling section, thus potentially sending you an email also in cases, where the email-signal was not present, but the "forgotten" ( un-reset ) value inside the _LastError system register co-incidentally matched the error-handling case, resulting in sending the said email ( as if there were an email-signal present ( which was not, as described above ).