code snippet 1

code snippet 2

code snippet 3

Above is my code of my game. Humanoid is like an NPC and player can sell item to him. So what I am trying to do in snippet 1 is to get all the items in the player inventory out first. At this point, I should be checking if the item can be sold or not. But, the problem I faced is that I could not use instanceOf/typecasting as it will violate SOLID Principle. How can I check if an Item is Sellable?

p/s: I cannot modify Item class as it is an engine code.

I had try to do type casting as previously I did not know that it will also violates SOLID Principle. At this point I really have no idea how to continue. The correct implementation should be able to add new SellAction(item) into the ActionList in code snippet 1.

Thank you in advance.