Question

I need to change the functionality in a component. What do you do when you get a “you can’t override this” message, or you need to change the code in a private method (“method does not exist in base class” message), rendering it impossible to descend the component?

Answer 1

If I face that problem,

• I first try to inherit from the component, or its CustomXXX ancestor and see if that fixes the problem. If that doesn’t,
• I go deeper, i.e. try to intercept the messages that come in. That can be done dynamically. If that turns out to be too deep, because the code that has to be built on that is too extensive, or if I still have to access items I can’t access,
• I try hacks. One hack is to copy the component and the dependent code to a new unit with a different name, rename the component and modify what needs to be modified.
• Sometimes I only need to redo one or two methods to make my new behaviour possible.

Never forget to give the unit a different name and the component a different name too (possibly inheriting from the original component or one of its ancestors, so they remain in the same hierarchy). Do never modify the original sources and then recompile the VCL. That is a maintenance nightmare.

I am no fan of interposer classes, i.e. classes that get the same name but different behaviour than the original classes, inheriting from the original. Their functionality depends on the order of inclusion in the uses clause, and that seems rahter flaky to me. I can’t recommend that.

But what I do greatly depends on the problem. I don’t think one can (or should) give a blanket advice that covers all situations.

But my main advice: do not modify the original units, always put the new code in a new unit and use a new class name. That way the original and the modified versions can peacefully co-exist, also in the IDE.