协方差泛型修饰符

Question

 public interface IOInitializable<in ItemType>

What does contravariant: converting from narrower (circle) to wider (Shapes)

mean for an interface?

What are the consequence and the dangers?

I've read http://msdn.microsoft.com/en-us/library/dd469484.aspx but this does not help me.

Answer 1

I'd recommend reading this explaination from Tomas:

The theory behind covariance and contravariance in C# 4

It boils down to the fact that if you have a method call that is passing a number of "Circles", you can use a function that accepts a number of "Shapes" as long as it doesn't return a "Shape" (because that may or may not be a "Circle").

Answer 2

IComparer is a good example to demonstrate this. IComparer looks like this:

IComparer<in T>

Take the following:

IComparer<Primate>
IComparer<Chimpanzee>

where Chimpanzee : Primate (of course). A method which requires an IComparer<Chimpanzee> can take an IComparer<Primate> as an argument, because if the comparer can compare primates, it can also compare chimpanzees, as it uses traits common to the two types to affect the comparison .

A good way to think of this is in terms of functionality. Covariance allows more complex objects to be passed which implement a core functionality (like passing a string for an object). Contravariance does something similar... comparing all primates is more complex than just comparing chimpanzees. It allows you to replace a comparer for a specific type with one which compares a more general type. In this sense, the "in" applies more to the functionality of the method than the actual type passed.