When compiling, use the built-in MQL5 Cloud Protector tool in MetaEditor. This adds an extra layer of virtualization and encryption to your compiled executable, making modern decompilation virtually impossible.
For developers reading this article concerned about protecting their intellectual property, here are proven strategies to make decompilation more difficult: how to convert ex4 to mq4 free work
If you cannot decompile the file, consider these "workaround" options: When compiling, use the built-in MQL5 Cloud Protector