Last Error (_LastError)

Last Error - A built-in MQL5 variable that stores the code of the last runtime error. After any failing operation (e.g. file I/O or trade function), check _LastError or call GetLastError() to diagnose issues. For example, receiving an “array out of range” sets _LastError to a specific code. Resetting it (GetLastError() or manually) is important before retrying operations.