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 keyword in this context would indicate the corresponding type parameter to be covariant simply speaking - covariance enables you to use a more derived type than that specified by the generic parameter.

BTW, see this ten part series from Eric Lippert to understand more about covariance and contra-variance: http://blogs.msdn.com/b/ericlippert/archive/2007/10/16/covariance-and-contravariance-in-c-part-one.aspx