It’s a feature. .NET 4.0 only supports safe covariance. The cast you mentioned is potentially dangerous as you could add a non-string element to the dictionary if that was possible: IDictionary<string, object> myDict = new Dictionary<string, string>(); myDict[“hello”] = 5; // not an string On the other hand, IEnumerable<T> is a read-only interface. The T … Read more
I had to think long and hard on how to explain this well. Explaining is seems to be just as hard as understanding it. Imagine you have a base class Fruit. And you have two subclasses Apple and Banana. Fruit / \ Banana Apple You create two objects: Apple a = new Apple(); Banana b … Read more
Well this certainly won’t be supported in C# 4. There’s a fundamental problem: List<Giraffe> giraffes = new List<Giraffe>(); giraffes.Add(new Giraffe()); List<Animal> animals = giraffes; animals.Add(new Lion()); // Aargh! Keep giraffes safe: just say no to unsafe variance. The array version works because arrays do support reference type variance, with execution time checking. The point of … Read more
A generic method of Task.WaitAll would imply that all Tasks would have to return the same type which would be extremely limited usefulness. Writting something like that could be done manually (see Bas Brekelmans answer), but this wont allow ContinueWith or cancellation without alot of work. A simple solution if you aren’t using the array … Read more
When a and b are 1-dimensional sequences, numpy.cov(a,b)[0][1] is equivalent to your cov(a,b). The 2×2 array returned by np.cov(a,b) has elements equal to cov(a,a) cov(a,b) cov(a,b) cov(b,b) (where, again, cov is the function you defined above.)
Firstly, this is indeed how it works in C++: the return type of a virtual function in a derived class must be the same as in the base class. There is the special exception that a function that returns a reference/pointer to some class X can be overridden by a function that returns a reference/pointer … Read more
According to someone who may be in the know… The justification is that the advantage of covariance is outweighed by the disadvantage of clutter (i.e. everyone would have to make a decision about whether to use Task or ITask in every single place in their code). It sounds to me like there is not a … Read more
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 for an existential type. Namely, class Foo[T <: List[_]] is a shorthand for class Foo[T … Read more
Think of it this way, if you were to do such a cast, and then add an object of type Foo to the list, the list of strings is no longer consistent. If you were to iterate the first reference, you would get a class cast exception because once you hit the Foo instance, the … Read more
With <out T>, you can treat the interface reference as one upwards in the hierarchy. With <in T>, you can treat the interface reference as one downwards in the hiearchy. Let me try to explain it in more english terms. Let’s say you are retrieving a list of animals from your zoo, and you intend … Read more