Question

I was wondering , why does IEnumerable<T> has ONLY the out and not the in contravariant flag ?

public interface IEnumerable<out T>

I can understand the out by this example :

IEnumerable<string> strings = new List<string>(){"a","b","c"};
IEnumerable<object> objects = strings;

object is bigger than string , so the compiler afraid that well do something like:

 objects = objects.First().Select(x=>5);// ( we cant insert int to string...)

fine and understood.

but what about if i want to use the IEnumerable as insertion?

something like :

IEnumerable<object> objects = ...;
IEnumerable<string> strings = objects

so i can also INSERT into objects …

but the problem is that there is no IEnumerable<in T> …

am I missing something here?

Answer 1

You can’t insert into IEnumerable. There is, unfortunately, no “IInsertable”.

There is no interface to tell that you can only add items to a collection. This would be an in generic type. Because you can get the items on every collection interface, it is not possible to have in generic arguments.

Edit

The interface IEnumerable<T> has only getters, so you can’t insert any elements to the collection using that interface. Because of that, out is allowed in the declaration of IEnumerable<out T>. It tells the compiler, that values of type T are only going in one direction, the are “coming out”. This makes it compatible in one direction. You can use IEnumerable<string>as an IEnumerable<object>, because you are only reading from it. You can read a string as an object. But you couldn’t write to it, you couldn’t pass an object as a string. It might be an integer or any other type.