If you want to model a user's transactions you can create a List<Transaction> rather than just a List, and then - The compiler will fail to compile your code i

If you want to model a user's transactions you can create a List<Transaction> rather than just a List, and then - The compiler will fail to compile your code if you put anything in the list apart from a Transaction - The compiler will fail to compile your code if you try to do anything with an object from that list which a Transaction can't do - It also makes the code more readable as if my function takes a List<Transaction> you know what you can pass to it. Does it take a List<Transaction> or a List<TransactionId>? It's obvious. - If you try to pass a List<TransactionId> then the compiler won't compile your code. (Otherwise you'll only find out about it at runtime.) This means a lot fewer fails at run-time. Which in turn means a lot less time writing (and maintaining) unit tests.