Exception Handling in TwinCAT: __TRY, __CATCH, __FINALLY

In TwinCAT 3.1.4026 and above, the __TRY → __CATCH → __FINALLY → __ENDTRY construct is available.

This construct allows you to execute a block of code, handle exceptions if they occur, and always run finalization logic regardless of the outcome.

🔹 Important notes:

• Not all exceptions can be handled. For example, __TRY → __CATCH will not protect against critical memory violations (e.g., invalid addressing or double memory release).

• Inside __CATCH, you can retrieve an error code (numeric value). Some codes are predefined in the __SYSTEM.ExceptionCode enumeration.

• Custom exceptions can be created and raised using Tc2_System.F_RaiseException.

• The construct __TRY → __FINALLY → __ENDTRY without a __CATCH block is currently not supported.

• As of TwinCAT 3.1.4026.21, the __FINALLY block is not executed if another exception occurs inside __CATCH. This appears to be a bug and is expected to be fixed in future versions.

• There are additional nuances not covered here, as they are expected to change. Overall, the mechanism is stable enough for practical use.

💡 What I built on top of this:

Based on this mechanism, in TwinCAT.OpenFramework I implemented an advanced exception handling system that significantly improves error processing.

It provides: • Detailed error message • Error code • Error location (namespace, class, method, line) • Timestamp of occurrence • Support of inner exception • Ability to create aggregate exceptions (multiple nested exceptions)

If you’re interested — check it out via the link. It’s powerful, free for commercial use, and open-source.

Concept description: ExceptionsConcept.md

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