Question

For example, suppose I want an ICar interface and that all implementations will contain the field Year. Does this mean that every implementation has to separately declare Year? Wouldn’t it be nicer to simply define this in the interface?

Answer 1

Though many of the other answers are correct at the semantic level, I find it interesting to also approach these sorts of questions from the implementation details level.

An interface can be thought of as a collection of slots, which contain methods. When a class implements an interface, the class is required to tell the runtime how to fill in all the required slots. When you say

interface IFoo { void M(); } 
class Foo : IFoo { public void M() { ... } }

the class says “when you create an instance of me, stuff a reference to Foo.M in the slot for IFoo.M.

Then when you do a call:

IFoo ifoo = new Foo();
ifoo.M();

the compiler generates code that says “ask the object what method is in the slot for IFoo.M, and call that method.

If an interface is a collection of slots that contain methods, then some of those slots can also contain the get and set methods of a property, the get and set methods of an indexer, and the add and remove methods of an event. But a field is not a method. There’s no “slot” associated with a field that you can then “fill in” with a reference to the field location. And therefore, interfaces can define methods, properties, indexers and events, but not fields.