Question

The msdn docs (http://msdn.microsoft.com/en-us/library/tfakywbh.aspx) report the syntax for the Comparison delgate with what looks like a keyword ‘in’.

public delegate int Comparison<in T>(
    T x,
    T y
)

Does the ‘in’ have any actual meaning? Are there other keywords that may appear there?

Answer 1

in means the generic parameter is contravariant. This means, in this case, that you can assign a Comparison<Base> to a Comparison<Derived>.

You can do this, because a Comparison<Derived> variable can accept a method which takes Base type parameters. When you call a Comparison<Derived>, you need to pass Derived variables, which happen to be valid parameters to a method accepting Base parameters. This means that it makes sense to assign a Comparison<Base> to a Comparison<Derived>.

The opposite of in is (naturally) out. This means the parameter is covariant, and can assign a Derived generic to a Base generic. This would be used, for example, in specifying the return type of a delegate.

A handy way of remembering which is which:

in should only be used for types that are only passed in. out should only be used for types that are only passed out.

Read more here:

in (Generic Modifier) (C# Reference)

Covariance and Contravariance (C# and Visual Basic)