Organizing Your Mql4 Code: A Guide to Building Reusable Libraries

To create a separate file for common functions in MQL4, the standard approach is to use Include files (extension .mqh).

This method allows you to write your logic once and import it into any Expert Advisor (EA), Script, or Indicator. When you compile the EA, the compiler takes the code from the .mqh file and inserts it into the EA, creating a single executable.

Here is the step-by-step implementation.

Step 1: Create the Include File (.mqh)

1. Open MetaEditor.
2. In the Navigator, right-click the Include folder and select New File.
3. Select Include (*.mqh), click Next, name it MyCommonLib, and click Finish.
4. Paste the following code. This library contains a function to handle 4/5 digit brokers and a risk-based lot size calculator.

File Path: MQL4\Include\MyCommonLib.mqh

//+------------------------------------------------------------------+
//|                                                  MyCommonLib.mqh |
//|                                 Copyright 2023, MetaQuotes Ltd.  |
//|                                             https://www.mql5.com |
//+------------------------------------------------------------------+
#property copyright "Copyright 2023, Your Name"
#property link      "https://www.yourwebsite.com"
#property strict

//+------------------------------------------------------------------+
//| Helper: Adjust points for 3/5 digit brokers                      |
//+------------------------------------------------------------------+
double GetPipMultiplier()
{
   if(Digits == 3 || Digits == 5) return(10.0);
   return(1.0);
}

//+------------------------------------------------------------------+
//| Common: Calculate Lot Size based on Risk Percentage              |
//+------------------------------------------------------------------+
double CalculateRiskLots(double riskPercent, double stopLossPips)
{
   // Prevent division by zero
   if(stopLossPips <= 0) 
   {
      Print("Error: StopLoss is 0, cannot calculate risk.");
      return(0.0);
   }

   double accountEquity = AccountEquity();
   double riskAmount = accountEquity * (riskPercent / 100.0);
   double tickValue = MarketInfo(Symbol(), MODE_TICKVALUE);
   
   // Adjust tick value if it returns 0 (rare server error protection)
   if(tickValue == 0) tickValue = Point;

   // Calculate raw lot size
   // Formula: Risk / (SL_Points * TickValue)
   // Note: We convert pips to points using the multiplier
   double points = stopLossPips * GetPipMultiplier();
   double lotSize = riskAmount / (points * tickValue);

   // Normalize to broker's step requirements
   double minLot = MarketInfo(Symbol(), MODE_MINLOT);
   double maxLot = MarketInfo(Symbol(), MODE_MAXLOT);
   double step   = MarketInfo(Symbol(), MODE_LOTSTEP);

   // Round down to nearest step
   lotSize = MathFloor(lotSize / step) * step;

   // Cap limits
   if(lotSize < minLot) lotSize = minLot;
   if(lotSize > maxLot) lotSize = maxLot;

   return(lotSize);
}

//+------------------------------------------------------------------+
//| Common: Print formatted error message                            |
//+------------------------------------------------------------------+
void LogError(string functionName, string message)
{
   int err = GetLastError();
   if(err > 0)
   {
      PrintFormat("Error in %s: %s | Code: %d", functionName, message, err);
   }
   else
   {
      PrintFormat("Log %s: %s", functionName, message);
   }
}

Step 2: Use the Include File in an EA

To use the functions created above, you must use the #include directive at the top of your Expert Advisor.

File Path: MQL4\Experts\MyTradingRobot.mq4

//+------------------------------------------------------------------+
//|                                               MyTradingRobot.mq4 |
//|                                 Copyright 2023, MetaQuotes Ltd.  |
//|                                             https://www.mql5.com |
//+------------------------------------------------------------------+
#property copyright "Copyright 2023, Your Name"
#property link      "https://www.yourwebsite.com"
#property version   "1.00"
#property strict

// --- IMPORT THE COMMON LIBRARY ---
// The < > brackets tell the compiler to look in the MQL4/Include folder
#include <MyCommonLib.mqh> 

// --- Inputs ---
input double RiskPerTrade = 2.0; // Risk in %
input int    StopLossPips = 50;  // Stop Loss in Pips
input int    MagicNumber  = 123456;

//+------------------------------------------------------------------+
//| Expert initialization function                                   |
//+------------------------------------------------------------------+
int OnInit()
{
   // We can use functions from the library immediately
   LogError("OnInit", "EA Initialized successfully.");
   return(INIT_SUCCEEDED);
}

//+------------------------------------------------------------------+
//| Expert tick function                                             |
//+------------------------------------------------------------------+
void OnTick()
{
   // Example logic: Open a trade if no orders exist
   if(OrdersTotal() == 0)
   {
      // 1. Use the library function to calculate lots
      double lots = CalculateRiskLots(RiskPerTrade, StopLossPips);
      
      // 2. Use the library function to get point multiplier for SL calculation
      double pipMult = GetPipMultiplier();
      double slPrice = Ask - (StopLossPips * Point * pipMult);
      double tpPrice = Ask + (StopLossPips * Point * pipMult); // 1:1 RR
      
      // Send Order
      int ticket = OrderSend(Symbol(), OP_BUY, lots, Ask, 3, slPrice, tpPrice, "My Trade", MagicNumber, 0, clrGreen);
      
      if(ticket < 0)
      {
         // 3. Use the library function to log errors
         LogError("OnTick", "OrderSend failed");
      }
      else
      {
         Print("Buy Order Opened. Ticket: ", ticket, " Lots: ", lots);
      }
   }
}

Key Mechanics Explained

1. #include <Filename.mqh>:
   • Using angle brackets < > tells the compiler to look specifically in the MQL4\Include\ directory.
   • Using quotes "Filename.mqh" tells the compiler to look in the same folder as the current source file (e.g., MQL4\Experts\).
2. Compilation: When you compile MyTradingRobot.mq4, the compiler literally copies the text from MyCommonLib.mqh and pastes it into the EA code in memory before generating the .ex4 file. You do not need to send the .mqh file to other users; only the compiled .ex4 EA file is required for trading.
3. Organization: You can create subfolders. If you save your file in MQL4\Include\MyStrategies\Risk.mqh, you would import it like this:
   #include <MyStrategies\Risk.mqh>

Q&A

Q: Can I include multiple .mqh files in one EA?
A: Yes, you can include as many files as you need. It is common practice to have separate files for RiskManagement.mqh, SignalLogic.mqh, and OrderExecution.mqh.

Q: What happens if I declare a variable with the same name in the EA and the .mqh file?
A: This will cause a compilation error (variable redefinition) or a warning (variable hiding), depending on the scope. It is best practice to prefix global variables in your library (e.g., Lib_RiskPercent) or encapsulate your library code inside a class to avoid naming conflicts.

Q: Do I need to recompile the .mqh file if I change it?
A: No, you cannot compile an .mqh file directly. However, after modifying an .mqh file, you must recompile every EA that uses it for the changes to take effect in those EAs.

Q: Can I use .ex4 libraries instead of .mqh files?
A: Yes, using #import. However, .ex4 libraries are harder to maintain because you must define the function headers in every EA that uses them. .mqh files are generally preferred for internal code reuse because the compiler can optimize the code better.