What does “out” mean before a Generic type parameter?

Question

I\'ve just saw an unfamiliar syntax while looking for GroupBy return type:

public interface IGrouping : IEnumerable<         


        

Answer 1

out just means that the type is only used for output e.g.

public interface Foo
{
   T Bar()
}

There also is a modifier in that means that the type is only used for input e.g.

public interface Foo
{
    int Bar(T x)
}

These are used because Interfaces with in are covariant in T and Interfaces with out are contravariant in T.