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

It denotes a covariant parameter. See also the description on MSDN. Essentially it says, that IGrouping can be regarded as IGrouping, hence you can

IGrouping gr = MakeGrouping(...);
IGrouping grBase = gr;

if Aderived is an interface or a type derived from Abase. This is a feature that comes in handy when you want to call a method that requires a parameter of type IGrouping, but you only got an object of type IGrouping. In this case, both types can be considered equivalent due to the covariance of their type parameters.