MQL4 to MQL5 Syntax Reference
Look up how a function, event handler, or predefined variable in MQL4 maps to its MQL5 equivalent, or paste your MQL4 source to flag what needs attention.
| MQL4 | MQL5 | Notes |
|---|---|---|
| init() | OnInit() | MQL5 requires the OnInit/OnDeinit/OnTick naming; MQL4 (build 600+) also accepts these names. |
| deinit() | OnDeinit(const int reason) | MQL5 passes a deinitialization reason code. |
| start() | OnTick() | Runs on every new tick for the chart symbol. |
| OrderSend(symbol, cmd, volume, price, slippage, sl, tp, ...) | OrderSend(MqlTradeRequest &request, MqlTradeResult &result) or CTrade::PositionOpen() | MQL5 uses a request/result struct pair, or the CTrade class from Trade.mqh for simpler calls. |
| OrderClose(ticket, lots, price, slippage) | CTrade::PositionClose(ticket) or OrderSend() with ORDER_TYPE_CLOSE_BY | MQL5 closes positions (netting) or specific deals (hedging), not raw order tickets. |
| OrderModify(ticket, price, sl, tp, expiration) | CTrade::PositionModify(ticket, sl, tp) | Pending order modification uses OrderModify() with a request struct in MQL5 too. |
| OrdersTotal() | PositionsTotal() or OrdersTotal() | MQL5 separates open positions (PositionsTotal) from pending orders (OrdersTotal). |
| OrderSelect(index, SELECT_BY_POS) | PositionGetTicket(index) then PositionSelectByTicket(ticket) | MQL5 has no order pool selection model; positions and history deals are queried separately. |
| OrderTicket() | PositionGetInteger(POSITION_TICKET) | For historical orders/deals use HistoryOrderGetInteger / HistoryDealGetInteger. |
| Bid / Ask | SymbolInfoDouble(_Symbol, SYMBOL_BID) / SYMBOL_ASK | MQL5 has no global Bid/Ask predefined variables for arbitrary symbols. |
| Digits | (int)SymbolInfoInteger(_Symbol, SYMBOL_DIGITS) | _Digits also works for the current chart symbol. |
| Point | SymbolInfoDouble(_Symbol, SYMBOL_POINT) | _Point also works for the current chart symbol. |
| MarketInfo(symbol, MODE_SPREAD) | SymbolInfoInteger(symbol, SYMBOL_SPREAD) | MarketInfo() is removed in MQL5; each MODE_* maps to a SymbolInfoDouble/Integer call. |
| iHigh(symbol, tf, shift) | CopyHigh(symbol, tf, shift, count, array) | MQL5 price/time/volume series are read into arrays via Copy* functions, not single-value calls. |
| iClose(symbol, tf, shift) | CopyClose(symbol, tf, shift, count, array) | Same pattern applies to iOpen/iLow/iTime/iVolume -> CopyOpen/CopyLow/CopyTime/CopyTickVolume. |
| iMA(symbol, tf, period, shift, method, price, shift) | handle = iMA(...); CopyBuffer(handle, 0, shift, count, array) | MQL5 indicator calls return a handle once; values are pulled with CopyBuffer, not per-call. |
| AccountBalance() | AccountInfoDouble(ACCOUNT_BALANCE) | Same pattern for AccountEquity, AccountMargin, AccountFreeMargin. |
| AccountCurrency() | AccountInfoString(ACCOUNT_CURRENCY) | |
| extern int Period = 14; | input int Period = 14; | MQL4 (build 600+) supports input too; extern still compiles but input is preferred. |
| ArrayResize + implicit series behavior | ArraySetAsSeries(array, true) required explicitly | MQL5 arrays are not series-ordered by default; you must opt in per array. |
| Procedural only (pre build 600) | Full OOP: classes, inheritance, CTrade/CPositionInfo/CSymbolInfo standard library | MQL5's Trade.mqh, PositionInfo.mqh, etc. remove most of the boilerplate above. |
| No OnTrade / OnTimer distinction in early builds | OnTrade(), OnTimer(), OnTradeTransaction(), OnBookEvent() | MQL5 exposes finer-grained event handlers with no MQL4 equivalent. |
| Always hedging — every OrderSend() creates an independent order, multiple same-symbol orders coexist | Netting (default) or Hedging, set per account type | Netting accounts collapse same-symbol trades into one position — code that assumes independent tickets per trade will double-count or fail on a netting account. |
| OrderMagicNumber() | PositionGetInteger(POSITION_MAGIC) (open) or HistoryDealGetInteger(ticket, DEAL_MAGIC) (closed) | Which one to call depends on whether you're reading an open position or a historical deal — there's no single call that covers both like MQL4's. |
| OrderProfit() | PositionGetDouble(POSITION_PROFIT) (open) or HistoryDealGetDouble(ticket, DEAL_PROFIT) (closed) | Same open/closed split as OrderMagicNumber(). |
| OrdersHistoryTotal() + OrderSelect(i, SELECT_BY_POS, MODE_HISTORY) | HistorySelect(fromDate, toDate) then HistoryDealsTotal() / HistoryDealGetTicket(i) | MQL5 requires populating a history cache with HistorySelect() before querying past deals — skipping it silently returns empty results, not an error. |
| Manual OrderSend/OrderModify/OrderClose calls throughout | CPositionInfo / CTrade classes from PositionInfo.mqh / Trade.mqh | CPositionInfo wraps PositionGetX() calls into methods like .Ticket(), .Volume(), .Profit() — most MQL5 code uses these instead of raw Position*() calls. |
| SetIndexBuffer(index, array) | SetIndexBuffer(index, array, INDICATOR_DATA) | MQL5 adds a required buffer-type argument (INDICATOR_DATA, INDICATOR_COLOR_INDEX, INDICATOR_CALCULATIONS) — omitting it is a compile error, not a silent default. |
| IndicatorCounted() | prev_calculated parameter of OnCalculate(...) | Both tell you how many bars were already processed on the last call, so you only need to recalculate the new ones — MQL5 passes it as a parameter instead of a function call. |
| #property indicator_buffers N (buffer count only) | #property indicator_buffers N plus #property indicator_label1/type1/color1 per plotted line | MQL5 wants each plotted buffer explicitly declared with PlotIndexSetString/PlotIndexSetInteger or the #property shortcuts — indicators with unlabeled buffers won't show a legend. |
| Series arrays (Bid/Ask history) are series-ordered by default | ArrayGetAsSeries(array) to check, ArraySetAsSeries(array, true) to opt in per array | Mixing a series-ordered array with a normal-ordered one in the same calculation is a common source of off-by-one bugs when porting loop logic. |
Scan your MQL4 code
Paste MQL4 source below to flag the constructs that need attention when porting to MQL5. This is pattern matching against known MQL4 syntax, not a parser or a compiler — it won't catch everything and can't verify your logic, but it's a fast first pass. Nothing leaves your browser.
MQL4 and MQL5 share the same C-like syntax, but MetaQuotes changed how trading, price series, and indicator data are accessed between the two languages. There is no fully automatic converter that produces correct MQL5 for every MQL4 Expert Advisor or indicator — order handling, array series behavior, and indicator buffers all changed enough that a straight find-and-replace breaks on anything beyond trivial scripts.
Why porting MQL4 to MQL5 is not a simple find-and-replace
The biggest source of bugs when porting is the trading model. MQL4's order-ticket system (OrderSend, OrderSelect, OrderModify) assumes every trade is an independently addressable ticket. MQL5's default netting accounts work with aggregated positions instead, so code that loops over open orders by ticket needs to be rewritten around PositionGetTicket and the CTrade class. Price and indicator access changed too: MQL4's single-value calls like iClose() and iMA() become buffer copies (CopyClose, CopyBuffer) in MQL5, which return arrays instead of one number at a time.
The table above covers the mappings that come up most often when converting an EA or custom indicator: event handlers, order/position management, market and price-series access, account info, and input declarations. Search by function name or filter by category to find the MQL5 equivalent for a specific line of MQL4 code.
Why this doesn't auto-convert your code
Every rule-based converter reviewed for this tool handles basic syntax fine and then hits the same wall: order/position management and indicator buffers aren't mechanical find-and-replace jobs, they're model changes. A tool that pretends otherwise produces code that compiles and trades wrong. The scanner below takes the more honest approach instead: paste your MQL4 source and it flags the specific lines that map to real behavior changes — particularly order handling and indicator buffers — for you to fix by hand, rather than silently "converting" the parts most likely to introduce bugs.
If you'd rather have a full Expert Advisor ported and tested properly — against live and historical data, not just syntax-swapped — that's exactly the kind of work I take on through the Fiverr gig linked below.