Interfaces and virtual behavior

The idea

An interface gives a shared contract to otherwise different classes. A client can receive an interface value and use its members without knowing the concrete class. An implementation must honor the meaning of the contract as well as its signatures. For example, a duration method should use consistent units across all implementations, not seconds in one class and minutes in another.

Class inheritance offers another form of polymorphism. A base instance method must be virtual, abstract, or already overridden to support an override in a derived class. Runtime dispatch then chooses the appropriate implementation for the actual object. Merely declaring a same-named method can hide a member instead, so override makes the intended behavior explicit.

using System;
IPrice price = new FixedPrice();
Console.WriteLine(price.Cents());
interface IPrice { int Cents(); }
class FixedPrice : IPrice {
  public int Cents() => 325;
}

325

The variable exposes the IPrice contract, while its object supplies FixedPrice.Cents. The returned amount follows the interface’s cents-based meaning.