covariance

This question already has an answer here: Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic? 16 answers I know why one shouldn't do that. But is there way to explain to a layman why this is not possible. You can explain this to a layman easily : Animal animal = new Dog(); . A dog is a kind of animal but a list of dogs is not a list of animals. Imagine you create a list of Dogs . You then declare this as List<Animal> and hand it to a colleague. He, not unreasonably , believes he can put a Cat in it. He then gives it back to you, and you now have a list of

I have a base WPF UserControl that handles some common functionality for derived UserControls. In the code-behind of any derived UserControl I call an event private void SomeClick(object sender, RoutedEventArgs e) { HandleClick(sender); MyDataGrid.Items.Refresh(); } In my base UserControl I do public class BaseUserControl : UserControl { protected void HandleClick(object sender) { var vm = (BaseViewModel<Part>)DataContext; ... } } This throws an InvalidCastException since DataContext is of type BaseViewModel but of a derived type like BaseViewModel<Wire> or BaseViewModel<Connector> . How can I

Is it possible to cast a List<Subclass> to List<Superclass> in C# 4.0? Something along these lines: class joe : human {} List<joe> joes = GetJoes(); List<human> humanJoes = joes; Isn't this what covariance is for? if you can do: human h = joe1 as human; why shouldn't you be able to do List<human> humans = joes as List<human>; than it wouldn't be legal to do (joe)humans[0] because that item has been down casted.. and everyone would be happy. Now the only alternative is to create a new List You can't do this, because it wouldn't be safe. Consider: List<Joe> joes = GetJoes(); List<Human>

Is the code below the only way to implement covariant return types? public abstract class BaseApplication<T> { public T Employee{ get; set; } } public class Application : BaseApplication<ExistingEmployee> {} public class NewApplication : BaseApplication<NewEmployee> {} I want to be able to construct an Application or a NewApplication and have it return the appropriate Employee type from the Employee property. var app = new Application(); var employee = app.Employee; // this should be of type ExistingEmployee I believe this code works fine, but it gets really nasty when I have several

I am just trying to understand the extends keyword in Java Generics. List<? extends Animal> means we can stuff any object in the List which IS A Animal then won't the following also mean the same thing: List<Animal> Can someone help me know the difference between the above two? To me extends just sound redundant here. Thanks! List<Dog> is a subtype of List<? extends Animal> , but not a subtype of List<Animal> . Why is List<Dog> not a subtype of List<Animal> ? Consider the following example: void mySub(List<Animal> myList) { myList.add(new Cat()); } If you were allowed to pass a List<Dog> to