5 Things I Check Before Debugging an MQL5 Program

When working with MQL5, I try not to jump straight into changing code as soon as something goes wrong. First, I want to understand what the program is supposed to do, what it actually does, and where the difference begins.

Here are five checks that help me approach debugging more systematically.

1. Read the compiler errors carefully

The MetaEditor compiler output is usually the first place I look when a program fails to compile.

I check the reported line number, error description, and surrounding code. Sometimes one missing semicolon or mismatched bracket can cause several additional errors to appear.

Rather than fixing every reported line blindly, I look for the earliest meaningful error first.

2. Check variable types and declarations

MQL5 has different data types for integers, floating-point values, strings, Boolean values, and more.

Using the wrong type or referencing an undeclared variable can cause compilation errors or unexpected behavior.

I also check whether variables are declared in the correct scope and whether their values are initialized before use.

3. Verify function parameters and return values

When a function doesn't behave as expected, I check its parameters, return type, and how it is called.

For built-in MQL5 functions, I refer to the official documentation rather than guessing the expected arguments.

This is especially useful when working with unfamiliar trading, order-management, or market-data functions.

4. Use logs to understand runtime behavior

A program can compile successfully and still behave incorrectly when running.

The Experts and Journal tabs in MetaTrader 5 can provide useful information about runtime errors, rejected requests, and other events.

Adding targeted Print() statements can also help reveal the values a program is receiving and the path its logic is following.

5. Test one change at a time

When several changes are made together, it becomes harder to identify which change fixed the problem—or introduced another one.

I prefer to make a focused change, compile again, and test the affected behavior before moving to the next issue.

For trading software, testing should be performed in an appropriate demo or controlled environment before considering live use.

Final thoughts

Debugging is not just about removing compiler errors. It's about understanding why the program behaves the way it does.

A systematic process makes it easier to isolate problems, verify fixes, and maintain code over time.

What's your first step when debugging an MQL5 program: checking compiler errors, reviewing the logic, or inspecting runtime logs?