covariance

I have a List of objects, which are of my type QuoteHeader and I want to pass this list as a list of objects to a method which is able to accept a List<object> . My line of code reads... Tools.MyMethod((List<object>)MyListOfQuoteHeaders); But I get the following error at design time... Cannot convert type 'System.Collections.Generic.List<MyNameSpace.QuoteHeader>' to 'System.Collections.Generic.List<object>' Do I need to do anything to my class to allow this? I thought that all classes inherit from object so I can't understand why this wouldn't work? The reason this is not legal is because it

I feel like this one has been asked before, but I'm unable to find it on SO, nor can I find anything useful on Google. Maybe "covariant" isn't the word I'm looking for, but this concept is very similar to covariant return types on functions, so I think it's probably correct. Here's what I want to do and it gives me a compiler error: class Base; class Derived : public Base; SmartPtr<Derived> d = new Derived; SmartPtr<Base> b = d; // compiler error Assume those classes are fully fleshed out... I think you get the idea. It can't convert a SmartPtr<Derived> into a SmartPtr<Base> for some unclear

I watched a video about it on Channel 9 but I didn't really understand it much. Can someone please give me a simple example about these that's easy to understand? After that maybe how it would be used in practice? You may want to look at this blog, he does a fantastic job of explaining it, but I think it will take more examples to clear it up for people, as this gets into a very hard-to-understand area, but, the quote below from the article sums it up well. http://hestia.typepad.com/flatlander/2008/12/c-covariance-and-contravariance-by-example.html "covariance and contravariance" means that

tl;dr What strategies exist to overcome parameter type invariance for specializations, in a language ( PHP ) without support for generics? Note: I wish I could say my understanding of type theory/safety/variance/etc., was more complete; I'm no CS major. Situation You've got an abstract class, Consumer , that you'd like to extend. Consumer declares an abstract method consume(Argument $argument) which needs a definition. Shouldn't be a problem. Problem Your specialized Consumer , called SpecializedConsumer has no logical business working with every type of Argument . Instead, it should accept a

As far as i know it is not possible to do the following in C# 2.0 public class Father { public virtual Father SomePropertyName { get { return this; } } } public class Child : Father { public override Child SomePropertyName { get { return this; } } } I workaround the problem by creating the property in the derived class as "new", but of course that is not polymorphic. public new Child SomePropertyName Is there any solution in 2.0? What about any features in 3.5 that address this matter? Alex Lyman This is not possible in any .NET language because of type-safety concerns. In type-safe languages,

What is the different between the following Generics definitions in Scala: class Foo[T <: List[_]] and class Bar[T <: List[Any]] My gut tells me they are about the same but that the latter is more explicit. I am finding cases where the former compiles but the latter doesn't, but can't put my finger on the exact difference. Thanks! Edit: Can I throw another into the mix? class Baz[T <: List[_ <: Any]] OK, I figured I should have my take on it, instead of just posting comments. Sorry, this is going to be long, if you want the TL;DR skip to the end. As Randall Schulz said, here _ is a shorthand

There are quite a few questions & answers about hacking around the limitation of C# not allowing method return (and argument) types to be changed to compatible types on overrides, but why does this limitation exist, either in the C# compiler or in the CLR? As I an see, there is nothing that could break if co/contra-variance was allowed, so what is the reasoning behind it? A similar question could be asked for widening access parameters - eg overriding a protected internal method with a public method (something which Java supports, IIRC) Robert Kozak Eric Lippert already answered this way

I'm trying to expose a read-only dictionary that holds objects with a read-only interface. Internally, the dictionary is write-able, and so are the objects within (see below example code). My problem is that IReadOnlyDictionary doesn't support covariant conversions because of the reason outlined in the question here . This means I can't just expose my internal dictionary as a read only one. So my question is, is there an efficient way to convert my internal dictionary to an IReadOnlyDictionary, or some other way to handle this? The options I can think of are: Hold two internal dictionaries and