A couple of stupid questions about polymorphism and inheritance?

Question

User interface

Why do you need to write , and not like that ? I understand what the interface is for in a general sense, but I don’t understand why it should be used in this particular situation. What happens in this situation ? And how is it different from ? IList list = new List()List list = new List()
Base A = new Derived()Derived B = new Derived()

Answer 1

Reduces the chance of mistakenly accessing List and Derived fields and methods not listed in IList and Base.
That is, there is not a mandatory rule here, but a recommendation to reduce potential problems - hide the object from your crooked hands behind the minimum sufficient interface.