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MQL5 Undeclared Identifier Error: Causes and Fixes

By Karan Rajput

Quick Answer

undeclared identifier is an MQL5 compile error. The compiler found a name it does not know: a variable, function, class or constant that was never declared, is out of scope, is spelled differently, or needs an #include you did not add.

Read the first error in the MetaEditor Errors tab, check the spelling and case of that name, check where it was declared, and check whether it comes from a library or from another language such as MQL4 or Pine Script.

Key Facts

| Item | Detail | |---|---| | Error type | Compile error, the program is not built | | Error text | 'name' - undeclared identifier | | Where to read it | The Errors tab in MetaEditor, with the file, line and column | | Case sensitivity | MQL5 is case sensitive, so trade and Trade are different names | | Most common causes | Typo, wrong case, missing include, variable out of scope | | Common for converters | MQL4 names such as Bid and Ask, and Pine names such as close |

What the Compiler Is Telling You

When you press F7 in MetaEditor, the compiler reads your file from top to bottom. When it meets a name that it has not seen before, it stops with a message like this:

'trade' - undeclared identifier    MyEA.mq5    42    8

The numbers are the line and the column. Press Ctrl+G, enter the line, and look at that name first.

One missing declaration often causes many errors that follow. Fix the first error in the list, compile again, and many of the others disappear.

7 Common Causes

1. Missing include

You use a class from the standard library but never included its file.

// Error: 'CTrade' - undeclared identifier
CTrade trade;

// Fixed
#include <Trade\Trade.mqh>

CTrade trade;

2. A typo

SymbolInfoDoubel instead of SymbolInfoDouble, or lotSize in one place and lotsize in another. The compiler sees two different names.

3. Wrong case

MQL5 distinguishes upper and lower case. If you declare CTrade trade; and later write Trade.Buy(...), the compiler reports Trade as undeclared.

4. Variable out of scope

A variable declared inside a block exists only inside that block.

void OnTick()
{
   if(IsNewBar())
   {
      double ma = 1.0; // exists only inside this if block
   }

   Print(ma); // Error: 'ma' - undeclared identifier
}

// Fixed: declare it where you need to read it
void OnTick()
{
   double ma = 0.0;

   if(IsNewBar())
   {
      ma = 1.0;
   }

   Print(ma);
}

5. Missing input or global declaration

A value used in several functions must be a global variable or an input declared outside any function.

input int MaPeriod = 20; // visible in every function in this file

6. MQL4 names that do not exist in MQL5

Code copied from an MQL4 Expert Advisor often uses Bid and Ask as predefined variables. They do not exist in MQL5.

// Error: 'Bid' - undeclared identifier
double price = Bid;

// Fixed
double bid = SymbolInfoDouble(_Symbol, SYMBOL_BID);
double ask = SymbolInfoDouble(_Symbol, SYMBOL_ASK);

7. Pine Script names used as if they were MQL5

Pine Script defines close, open, ta.ema() and strategy.entry() for you. MQL5 does not. These names must be replaced with explicit calls. Our Pine Script to MQL5 guide covers the full conversion process. The table below covers the names that cause this error most often.

| Pine Script or MQL4 habit | MQL5 replacement | |---|---| | Bid (MQL4) | SymbolInfoDouble(_Symbol, SYMBOL_BID) | | Ask (MQL4) | SymbolInfoDouble(_Symbol, SYMBOL_ASK) | | close[1] (Pine) | iClose(_Symbol, _Period, 1), mind the shift, see close[1] in MQL5 | | ta.ema(close, 20) (Pine) | iMA(...) handle, then CopyBuffer(...) | | strategy.entry(...) (Pine) | trade.Buy(...) or trade.Sell(...) with CTrade | | input.int(14) (Pine) | input int Period = 14; |

How to Debug It

  1. Compile with F7 and open the Errors tab.
  2. Double-click the first error, and MetaEditor jumps to the line.
  3. Check the spelling of the name against where you declared it.
  4. Check the case of every letter.
  5. Check whether the declaration is inside a block that has already closed.
  6. If the name belongs to a class, check the #include at the top of the file.
  7. Compile again, and repeat with the next first error.

Compile Error vs Runtime Error

An undeclared identifier stops the build, so you see it before you run anything. Errors such as array out of range are different, because the program compiles and fails later. Knowing which kind you have tells you where to look: the Errors tab for the first, and the Experts tab for the second.

FAQ

What does undeclared identifier mean in MQL5?

The compiler found a name it has not seen declared. It could be a typo, a name in the wrong scope, a missing #include, or a name from another language.

Why do I get 'Bid' undeclared identifier in MQL5?

Bid and Ask are predefined variables in MQL4, but not in MQL5. Read them with SymbolInfoDouble(_Symbol, SYMBOL_BID) and SymbolInfoDouble(_Symbol, SYMBOL_ASK).

Is MQL5 case sensitive?

Yes. trade and Trade are two different identifiers.

Why do I get many errors after fixing one?

One missing declaration can trigger errors on every line that uses it. Fix the first error, compile again, and check what remains.

Convert Without Chasing Compile Errors

When you move a Pine Script strategy into MQL5 by hand, every Pine built-in has to be rewritten, and each one you miss becomes a compile error. CodeFlowOS generates MQL5 code you can review before you compile and test it in MetaEditor and the MT5 Strategy Tester. You can try it on the converter page.

This article is educational. It does not describe trading performance and is not financial advice. Always compile and test code in MetaEditor and the Strategy Tester before using it on a live account.