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MQL5 ReferenceCommon FunctionsSetUserError 

SetUserError

Sets the predefined variable _LastError into the value equal to ERR_USER_ERROR_FIRST + user_error

void  SetUserError(
   ushort user_error,   // error number
   );

Parameters

user_error

[in] Error number set by a user.

Return Value

No return value.

Note

After an error has been set using the SetUserError(user_error) function, GetLastError() returns value equal to ERR_USER_ERROR_FIRST + user_error.

Example:

void OnStart()
  {
//--- set error number 65537=(ERR_USER_ERROR_FIRST +1)
   SetUserError(1);
//--- get last error code
   Print("GetLastError = ",GetLastError());
/* 
   Result
   GetLastError = 65537
*/ 
  }