Introduction
If you're reading this, you probably have a working MQL4 EA or indicator that you need to port to MQL5. Maybe your broker is phasing out MT4, maybe you want access to MQL5's better backtesting, or maybe a client is paying you to convert their legacy code.
Whatever your reason, I've been there. I've migrated dozens of EAs and indicators, and I can tell you: it's not a simple find-and-replace job. But it's also not as scary as it looks.
This guide gives you the practical knowledge I wish I had when I started.
The Big Picture: What Actually Changed?
Before diving into code, understand the architectural shift:
MQL4: Simple and Direct
// Want the RSI? Just call it. double rsi = iRSI(NULL, 0, 14, PRICE_CLOSE, 0); // Want to open a trade? One function. int ticket = OrderSend(Symbol(), OP_BUY, 0.1, Ask, 3, 0, 0);
MQL5: Handle-Based and Object-Oriented
// RSI requires a handle (create once, use many times) int rsi_handle = iRSI(_Symbol, _Period, 14, PRICE_CLOSE); double rsi_buffer[]; ArraySetAsSeries(rsi_buffer, true); CopyBuffer(rsi_handle, 0, 0, 1, rsi_buffer); double rsi = rsi_buffer[0]; // Trading uses the CTrade class #include <Trade\Trade.mqh> CTrade trade; trade.Buy(0.1, _Symbol);
Why the change? Performance. MQL5's handle system lets the terminal cache indicator calculations. In MQL4, iRSI() recalculates every time you call it. In MQL5, you calculate once and read from memory.
Step 1: The Entry Points
This is the easy part. Find and replace:
| MQL4 | MQL5 | Notes |
|---|---|---|
| init() | OnInit() | Must return INIT_SUCCEEDED or INIT_FAILED |
| deinit() | OnDeinit(const int reason) | Now receives a reason code |
| start() | OnTick() | For EAs |
| start() | OnCalculate(...) | For indicators (different signature!) |
Common mistake: Forgetting that OnInit() must return an int. Your MQL4 init() that returned 0 needs to return INIT_SUCCEEDED.
int OnInit() { // Setup code... if(something_failed) return(INIT_FAILED); return(INIT_SUCCEEDED); }
Step 2: Add the Trade Libraries
At the top of your file, add:
#include <Trade\Trade.mqh> #include <Trade\PositionInfo.mqh> #include <Trade\SymbolInfo.mqh>
Then declare global instances:
CTrade trade; CPositionInfo position; CSymbolInfo symbol_info;
In OnInit():
trade.SetExpertMagicNumber(MagicNumber); trade.SetDeviationInPoints(10); symbol_info.Name(_Symbol);
Step 3: Converting Order Functions
This is where most people struggle. MQL5 separates orders (pending) from positions (open trades).
Opening Trades
MQL4:
int ticket = OrderSend(Symbol(), OP_BUY, 0.1, Ask, 3, sl, tp, "Comment", Magic); if(ticket < 0) Print("Error: ", GetLastError());
MQL5:
if(!trade.Buy(0.1, _Symbol, 0, sl, tp, "Comment"))
{
Print("Error: ", trade.ResultRetcodeDescription());
}
Note: In MQL5, passing 0 as price means "use current market price" — the class handles Ask/Bid automatically.
Closing Trades
MQL4:
OrderClose(ticket, OrderLots(), Bid, 3);
MQL5:
trade.PositionClose(ticket);
Checking Open Positions
MQL4:
for(int i = OrdersTotal() - 1; i >= 0; i--) { if(OrderSelect(i, SELECT_BY_POS, MODE_TRADES)) { if(OrderSymbol() == Symbol() && OrderMagicNumber() == Magic) { // Found our position } } }
MQL5:
for(int i = PositionsTotal() - 1; i >= 0; i--)
{
if(position.SelectByIndex(i))
{
if(position.Symbol() == _Symbol && position.Magic() == Magic)
{
// Found our position
}
}
}
Step 4: Converting Indicators
This is the biggest mental shift. Create handles once, read values many times.
Example: Moving Average
MQL4:
double ma = iMA(NULL, 0, 20, 0, MODE_SMA, PRICE_CLOSE, 0); double ma_prev = iMA(NULL, 0, 20, 0, MODE_SMA, PRICE_CLOSE, 1);
MQL5:
// Declare at global level
int ma_handle;
double ma_buffer[];
// In OnInit()
ma_handle = iMA(_Symbol, _Period, 20, 0, MODE_SMA, PRICE_CLOSE);
if(ma_handle == INVALID_HANDLE)
{
Print("Error creating MA handle");
return(INIT_FAILED);
}
ArraySetAsSeries(ma_buffer, true);
// In OnTick()
if(CopyBuffer(ma_handle, 0, 0, 2, ma_buffer) < 2)
return; // Not enough data
double ma = ma_buffer[0];
double ma_prev = ma_buffer[1];
// In OnDeinit() - IMPORTANT!
IndicatorRelease(ma_handle);
Why ArraySetAsSeries()? MQL5 arrays index from oldest to newest by default. Setting as series flips it so index 0 is the current bar (like MQL4).
Step 5: Price Data
MQL4:
double close = Close[0]; double high = High[1]; datetime time = Time[0];
MQL5:
double close = iClose(_Symbol, _Period, 0); double high = iHigh(_Symbol, _Period, 1); datetime time = iTime(_Symbol, _Period, 0);
Or using arrays:
double close[], high[]; ArraySetAsSeries(close, true); ArraySetAsSeries(high, true); CopyClose(_Symbol, _Period, 0, 10, close); CopyHigh(_Symbol, _Period, 0, 10, high); // Now close[0] is current bar, close[1] is previous, etc.
Step 6: Account and Symbol Info
MQL4:
double balance = AccountBalance(); double equity = AccountEquity(); double point = Point; int digits = Digits; double bid = Bid; double ask = Ask;
MQL5:
double balance = AccountInfoDouble(ACCOUNT_BALANCE); double equity = AccountInfoDouble(ACCOUNT_EQUITY); double point = SymbolInfoDouble(_Symbol, SYMBOL_POINT); int digits = (int)SymbolInfoInteger(_Symbol, SYMBOL_DIGITS); double bid = SymbolInfoDouble(_Symbol, SYMBOL_BID); double ask = SymbolInfoDouble(_Symbol, SYMBOL_ASK); // Or using CSymbolInfo class: symbol_info.RefreshRates(); double bid = symbol_info.Bid(); double ask = symbol_info.Ask();
Common Pitfalls (Learn from My Mistakes)
1. Forgetting to Release Handles
Memory leak alert! Always release indicator handles in OnDeinit():
void OnDeinit(const int reason)
{
IndicatorRelease(ma_handle);
IndicatorRelease(rsi_handle);
// etc.
}
2. Arrays Not Set as Series
Your code looks right but gives wrong values? Check ArraySetAsSeries(). MQL5 arrays are "as series" = false by default.
3. Bid/Ask Not Updating
symbol_info.Bid() returns cached values. Call symbol_info.RefreshRates() first, or use SymbolInfoDouble() directly.
4. Position vs Order Confusion
- OrdersTotal() = pending orders only
- PositionsTotal() = open positions only
In MQL4, OrdersTotal() included both. In MQL5, they're separate.
5. ENUM Changes
Some enums changed names:
- OP_BUY → ORDER_TYPE_BUY (but trade.Buy() handles this)
- MODE_SMA → Same name, but check the value matches
- PRICE_CLOSE → Same name
Quick Reference Card
| Task | MQL4 | MQL5 |
|---|---|---|
| Current bid | Bid | SymbolInfoDouble(_Symbol, SYMBOL_BID) |
| Current ask | Ask | SymbolInfoDouble(_Symbol, SYMBOL_ASK) |
| Point size | Point | SymbolInfoDouble(_Symbol, SYMBOL_POINT) |
| Digits | Digits | SymbolInfoInteger(_Symbol, SYMBOL_DIGITS) |
| Current close | Close[0] | iClose(_Symbol, _Period, 0) |
| Bar count | Bars | Bars(_Symbol, _Period) |
| Open buy | OrderSend(..., OP_BUY, ...) | trade.Buy(...) |
| Close position | OrderClose(ticket, ...) | trade.PositionClose(ticket) |
| Count positions | OrdersTotal() | PositionsTotal() |
| Get MA value | iMA(..., shift) | CopyBuffer(ma_handle, ...) |
Migration Checklist
- ☐ Rename init() → OnInit() (return INIT_SUCCEEDED)
- ☐ Rename deinit() → OnDeinit(const int reason)
- ☐ Rename start() → OnTick() (EA) or OnCalculate() (indicator)
- ☐ Add #include <Trade\Trade.mqh> and friends
- ☐ Create CTrade trade; global variable
- ☐ Set magic number in OnInit(): trade.SetExpertMagicNumber(Magic)
- ☐ Convert all OrderSend() → trade.Buy()/Sell()
- ☐ Convert all OrderClose() → trade.PositionClose()
- ☐ Convert order loops to position loops
- ☐ Convert indicators to handle-based system
- ☐ Add IndicatorRelease() calls in OnDeinit()
- ☐ Replace Bid/Ask with SymbolInfoDouble()
- ☐ Replace Close[]/High[]/etc. with iClose()/iHigh()/etc.
- ☐ Test thoroughly in Strategy Tester
Final Thoughts
MQL5 migration isn't just about making code compile — it's about understanding the new architecture. Once you internalize the handle-based indicator system and the position-based order management, everything clicks.
The payoff is worth it: faster backtesting, cleaner code, better debugging tools, and access to the growing MQL5 marketplace.
Need help migrating your EA? Check my GitHub portfolio for examples: https://github.com/jimmer89/mql5-portfolio
© 2026 Jaume Sancho. Free to use and share with attribution.
