MQL4 error diagnosis
Reliable error handling in MQL4 starts with the function contract: test the return value, capture GetLastError immediately on failure, log the inputs and external state, then choose a code-specific action. Resetting errors blindly or retrying every failure produces misleading logs and unsafe loops.
Open the Coding Agent, then choose MQL4 from the language menu.
#property strict
bool SelectTicket(const int ticket)
{
ResetLastError();
bool selected = OrderSelect(ticket, SELECT_BY_TICKET);
if(selected)
return(true);
int error_code = GetLastError();
Print("OrderSelect failed. error=", error_code,
" ticket=", ticket);
return(false);
}
void OnStart()
{
int ticket = -1;
if(!SelectTicket(ticket))
return;
Print("Selected order ", OrderTicket());
}MQL4 APIs use different failure sentinels: OrderSend returns -1, FileOpen returns INVALID_HANDLE, and many operations return false. _LastError is supporting context after a failure, not a substitute for the return value.
A stale error can survive from an earlier operation. ResetLastError is useful immediately before a call when zero is also a possible successful value, but it should not replace a result check.
Record the function, result, error, important arguments, symbol, ticket, terminal build, server time, and relevant external state in one message. Keep secrets and personal data out of logs.
Invalid inputs should fail fast. A closed market or disabled permission should stop until state changes. Requotes, off quotes, and a busy trade context may receive a small retry budget with refreshed state and delay.
Cleanup belongs in the owning lifecycle. Close valid file handles, delete only owned objects, and do not start new trading work during deinitialization.
The wrapper checks the documented boolean return, stores GetLastError once, and logs the ticket that caused the failure. Other API wrappers should follow their own return contracts.
Logging and retries can create their own faults. Avoid credentials in logs, cap repeated messages, and never use an unbounded trade retry loop.
#property strict
bool SelectTicket(const int ticket)
{
ResetLastError();
bool selected = OrderSelect(ticket, SELECT_BY_TICKET);
if(selected)
return(true);
int error_code = GetLastError();
Print("OrderSelect failed. error=", error_code,
" ticket=", ticket);
return(false);
}
void OnStart()
{
int ticket = -1;
if(!SelectTicket(ticket))
return;
Print("Selected order ", OrderTicket());
}Pineify can generate or revise one self-contained .mq4 file with MQL4-specific reference context. Its independent static checker returns MetaEditor diagnostics so you can repair source issues before testing in your own MT4 environment.
Open MQL4 Coding AgentStatic diagnostics do not prove runtime behavior, backtest results, broker compatibility, or live-trading safety.
Identify success, failure sentinel, error-state behavior, and resource ownership.
Test the return and capture the immediate error with the call arguments.
Define correct, wait, retry, stop, alert, or clean-up behavior for each expected class.
Reproduce invalid inputs and unavailable external states without risking a live account.
Test the API result, call GetLastError immediately on failure, store the number once, and log it with the relevant arguments.
No. Use it when the API contract and diagnosis need a clean error state, especially when a successful result can be zero. Always check the return value.
The error may be read after another call, already consumed, or unrelated because the primary return value was not checked.
Use an error-specific attempt and time limit, refresh state, recheck the signal, and stop when the failure is not transient.
It can catch source diagnostics and help review control flow. It cannot reproduce broker, network, market, file, or account failures.
Last verified against the linked official references on . Compile and test the final source in the MetaTrader 4 build, broker, symbol, and account environment you plan to use.
Define the requirement, inspect the generated source, repair diagnostics, and test the behavior in your own MT4 environment.
Open Coding Agent