Menu
Can someone explain why there is the need to add an out or in parameter to indicate that a generic type is Co or Contra variant in C# 4.0?
I've been trying to understand why this is important and why the compiler can't just figure it out..
Thanks,
Josh
303
In C#, covariance and contravariance enable implicit reference conversion for array types, delegate types, and generic type arguments. Covariance preserves assignment compatibility and contravariance reverses it.
Covariance permits a method to have return type that is more derived than that defined in the delegate. Contravariance permits a method that has parameter types that are less derived than those in the delegate type.
This is mainly useful when using already defined standard interfaces. Covariance means that you can use IEnumerable<string> in place where IEnumerable<object> is expected. Contravariance allows you to pass IComparable<object> as an argument of a method taking IComparable<string> .
Covariance means that a method can return a type that is derived from the delegate's return type. Contra-variance means that a method can take a parameter that is a base of the delegate's parameter type.
You can declare a generic type parameter contravariant by using the in keyword. The contravariant type can be used only as a type of method arguments and not as a return type of interface methods. The contravariant type can also be used for generic constraints.
Eric Lippert, who works on the langauge, has a series of posts on msdn that should help clarify the issues involved:
http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx
When reading the articles shown at that link, start at the bottom and work up.
Eventually you'll get to #7 (Why do we need a syntax at all?).
77
We don't actually need them, any more then we need abstract on classes or both out and ref. They exist just so that we, as programmers, can make our intention crystal clear, so that maintenance programmer know what we are doing, and the compiler can verify that we are doing it right.
42
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
