Pineify logo

MQL5 diagnostic pattern

MQL5 Error Handling: Catch and Print the Right Error

MQL5 does not provide a universal try-catch wrapper for terminal errors. Prevent critical errors with precondition checks, inspect each function return value, call ResetLastError before the suspected runtime call, read GetLastError immediately after failure, and inspect MqlTradeResult.retcode separately for trade server outcomes. Error 0 means no runtime error was recorded, not that every business operation succeeded.

Open MQL5 Coding AgentReview the diagnostic example

Open the Coding Agent and choose MQL5 from the language menu.

Layered error logging.mq5
ResetLastError();
bool sent=OrderSend(request,result);
int runtime_error=GetLastError();

if(!sent || result.retcode!=TRADE_RETCODE_DONE)
  {
   PrintFormat("sent=%s runtime=%d retcode=%u comment=%s",
               sent ? "true" : "false",
               runtime_error,result.retcode,result.comment);
  }
  • Runtime channel: GetLastError
  • Trade channel: result.retcode
  • Error 0: no recorded runtime error
  • Critical errors: prevent with guards

Also searched as: mql5 error 0; mql5 print error.

Separate the diagnostic channels

Compiler diagnostics appear before execution. GetLastError exposes the terminal runtime layer. Trade requests add MqlTradeCheckResult and MqlTradeResult retcodes. A useful log names the channel instead of printing one unlabeled number.

LayerRead fromExample
CompilerMetaEditor build output256 undeclared identifier
Terminal runtimeGetLastError4806 indicator data not found
Trade precheckMqlTradeCheckResult.retcodeRequest validation result
Trade serverMqlTradeResult.retcode10030 invalid filling type

Prevent critical runtime failures

A critical runtime error can stop the current MQL5 program. Guard array indexes, denominators, object pointers, handles, and copied counts before use. Logging after an unsafe operation is not a substitute for the guard.

  • Require index >= 0 and index < ArraySize(array).
  • Reject INVALID_HANDLE and wait for calculated data.
  • Check divisors before division.
  • Treat partial copy results as partial data.

Interpret error 0 correctly

Runtime code 0 is ERR_SUCCESS in the official table. It only says no runtime error is recorded in that channel. A function may return false or a trade server may reject a request while a stale or reset GetLastError remains zero, so always retain direct returns and retcodes.

Use a focused diagnostic

The pattern records the direct return, terminal runtime state, and server retcode in one place. The acceptable retcodes must match the actual request type because not every successful outcome is TRADE_RETCODE_DONE.

A clean compile or a cleared error code does not prove strategy logic, broker compatibility, backtest quality, or live-trading safety. Reproduce the result in the target MetaTrader 5 terminal and account environment.

Layered error logging.mq5
ResetLastError();
bool sent=OrderSend(request,result);
int runtime_error=GetLastError();

if(!sent || result.retcode!=TRADE_RETCODE_DONE)
  {
   PrintFormat("sent=%s runtime=%d retcode=%u comment=%s",
               sent ? "true" : "false",
               runtime_error,result.retcode,result.comment);
  }
Where Pineify fits

Repair editable MQL5 source with compiler feedback

Pineify can generate or revise MQL5 source and use MQL5 compilation diagnostics during the coding workflow. Keep the exact error, source location, program type, and intended behavior in the request so the change remains reviewable.

Open MQL5 Coding Agent

Pineify does not run your broker terminal, Strategy Tester, market data, or live orders. Validate the final program in your own MetaTrader 5 environment.

Review MQL5 AI Coding Agent capabilities
Pineify MQL5 AI Coding Agent with an editable MetaTrader 5 code artifact

A practical verification workflow

  1. 1

    Name the operation

    Log the function, symbol, timeframe, ticket, or handle that identifies the attempted operation.

  2. 2

    Reset and call

    Call ResetLastError immediately before the suspected runtime operation and save its direct return value.

  3. 3

    Capture every result layer

    Read GetLastError immediately and, for trading, retain check and send retcodes plus comments.

  4. 4

    Repair and reproduce

    Fix the violated precondition or request field, then repeat the same controlled scenario.

Frequently asked questions

Can MQL5 catch a critical error with try-catch?

There is no universal try-catch mechanism for terminal runtime failures. Guard unsafe operations and check documented returns before using their outputs.

What does MQL5 error 0 mean?

The official runtime table defines 0 as success, meaning no runtime error is recorded. It does not replace checking a function return or a trade server retcode.

How should I print an MQL5 error?

Print the operation, inputs, direct return, GetLastError code, and any API-specific retcode or comment. Reset the last-error state before the operation you are diagnosing.

Last verified against the linked official references on . Compile and test the final source in the MetaTrader 5 build, broker, symbol, and account environment you plan to use.

Continue with reviewable MQL5 source

Provide the exact diagnostic and intended behavior, inspect the generated change, compile it, and verify it in your MT5 test environment.

Open MQL5 Coding Agent