Summary: Fix MT5 MQL5 compile errors including undeclared identifiers and missing include files. This guide provides systematic debugging steps for EA developers.




Step 1: Identify the Exact Compile Error Message
Open MetaEditor (F4 in MT5). Click 'Compile' (F7). The bottom 'Errors' panel will display red messages. Common examples: 'function not defined' or 'cannot open include file'. Screenshot: MetaEditor error panel with red error lines.

Step 2: Fix 'Function Not Defined' Error
This error means the function is either misspelled, missing, or out of scope. Check spelling first. If correct, verify the function exists in the same file or an included file. Add forward declaration at the top: `void MyFunction();`. Screenshot: forward declaration example above OnInit().

Step 3: Check Function Scope and Visibility
If the function is in another file, make sure that file is included. For library functions, ensure the .mqh file is properly referenced. Functions must be placed before they are called unless forward declared. Screenshot: correct order of functions in code.

Step 4: Fix 'Cannot Open Include File' Error
Check the include path. Use `#include ` for system includes (MQL5/Include folder). Use `#include "filename.mqh"` for local includes (same folder as your EA). Verify the file actually exists. Screenshot: folder structure showing Include and local folders.

Step 5: Verify File Location for Local Includes
Right-click in MetaEditor Navigator > Open Folder > MQL5 > Include for system files. For local files, place .mqh in the same folder as your .mq5 file. Screenshot: right-click context menu with Open Folder option.

Step 6: Fix 'Undeclared Identifier' for Built-in Functions
Make sure you have included the necessary core files. For most MT5 functions, add `#include ` for trading operations. `#include ` for chart objects. Screenshot: include statements at the top of the script.

Step 7: Clean and Rebuild the Project
Click 'Build' > 'Clean' then 'Build' > 'Compile' (or press F7 twice). This removes cached objects and forces full recompilation. Screenshot: Build menu dropdown with Clean highlighted.

Step 8: Check for Missing Semicolons or Brackets
Sometimes one missing semicolon causes multiple 'function not defined' errors. Scroll up from the first error. Fix the earliest error first. Common issues: missing `;` after variable declaration, unmatched `{` or `}`. Screenshot: line with missing semicolon highlighted.

Step 9: Use MetaEditor's Navigation Features
Double-click any error line. MetaEditor jumps directly to the problematic line. Hold Ctrl and click on any function name to jump to its definition. Screenshot: Ctrl+click on function name showing jump to definition.

Step 10: Test with Minimal Code
Comment out large blocks to isolate the error. Start with a working template: `void OnTick(){ Print("Test"); }`. Add your code gradually until the error reappears. This pinpoints the exact cause. Screenshot: commented code blocks with working test section.

Reference: MQL5 Documentation - Compilation Errors / MQL5 Community Forums - Debugging Guide.