covariance

问题 Please help me understand why add1() and add4() report errors and why add2() and add3() don't. Specifically, please show examples of undesired consequences if the compiler allowed each of these to compile. class InnerTypeConfusion { interface Animal {} class Dog implements Animal {} class Room<T> { void add(T t) {} } void add1(Room<? extends Animal> room) { // Error: The method add(capture#1-of ? extends Animal) in the type // Room<capture#1-of ? extends Animal> is not applicable for the //

问题 Updated with input from MarcinJuraszek I have a feeling I'm bumping into a co / contra variance problem here, but I'm not sure I understand how to fix it. I have a class like this: public interface ISomeClass<TEnum, out S> { TEnum Dim { get; } IEnumerable<S> Inc { get; } } public class SomeClass<TEnum, S> : ISomeClass<TEnum, S> where TEnum : struct, IConvertible where S : IMyInterface { public TEnum Dim { get; set; } public IEnumerable<S> Inc { get; set; } } and I have a class that implements

问题 i have these classes public abstract class Unit { public abstract UnitType getType(); ... } public class Item extends Unit { protected ItemType type; @Override public ItemType getType() { return type; } public void setType(ItemType type) { this.type = type; } ... } and obvoiusly ItemType extends UnitType . and i get: javax.el.PropertyNotWritableException: /WEB-INF/facelets/general.xhtml @23,165 value="#{bean.item.type}": The class 'com.example.Item' does not have a writable property 'type'. i

问题 weird problem... Ive implemented covariance from example . My target FW is 4.0. How ever . I wanted to see if it fails on 3.5 /2 -> but it wont. It cant be since covariance is from FW4.0. After changing to 3.0 i Build and see : (+wont fail on runtime) 回答1: Covariance was supported in framework 3, but not implemented in the compiler. You are using compiler 4 targeting framework 3. This is mentioned in an Eric Lippert blog post, I'll see if I can find it.. EDIT To further clarify,: The Common

问题 The new Guava 10 Optional states to be naturally covariant and thus may be casted. If I try so it looks a bit ugly: Optional<Integer> opti = Optional.of(42); Optional<Number> optn = (Optional) opti; I like to see some utility function like: static <T> Optional<T> transform(Optional<? extends T> opt, Class<T> clazz); (how to express this as a member function of Optional ?) Is it even possible to define a transformation function object like: static <T> Function<Optional<? extends T>, Optional<T

问题 I saw Jon Skeet's lecture at the NDC 2010 He mentioned something interesting : public Class Base { public void Foo(IEnumerable<string> strings){} } public Class Child:Base { publc void Foo(IEnumerable<object> objects) {} } Main : List<string> lst = new List<string>(); lst.Add("aaa"); Child c = new Child(); c.Foo(lst); With C# 3 it will call : Base.Foo With C# 4 it will call : Child.Foo I know it's because covariance Question : Isn't it a bit code breaking change ? Is there any workaround so

问题 Assuming that this applies: public class Cat : Animal { } and assuming that I have a method: public void Feed(Animal animal) { ... } And I can call it this way: var animal = new Cat(); Feed(animal); How can I get this working when Feed is refactored to only support Lazy<Animal> as parameter? I'd like to pass in my var lazyAnimal = new Lazy<Cat>(); somehow. This obviously doesnt work: var lazyAnimal = new Lazy<Cat>(); Feed(lazyAnimal); 回答1: Well, you won't be able to use it exactly as is.

问题 In the code: Interface ISelf(Of Out TMe) End Interface Class SomeBase Implements ISelf(Of SomeBase) End Class Class SomeDerived Inherits SomeBase Implements ISelf(Of SomeDerived) End Class Module ISelfTester Sub TestISelf() Dim z7 As New SomeDerived Dim z8 As ISelf(Of SomeDerived) Dim z9 As ISelf(Of ISelf(Of SomeDerived)) z8 = z7 z9 = z8 z9 = z7 ' Why is this illegal? End Sub End Module The assignment directly from Z7 to Z9 yields the message "Error 13 Option Strict On does not allow implicit

问题 to avoid confusion I summarised some code: namespace ConsoleApplication1 { class Program { static void Main() { IManager<ISpecificEntity> specificManager = new SpecificEntityManager(); IManager<IIdentifier> manager = (IManager<IIdentifier>) specificManager; manager.DoStuffWith(new SpecificEntity()); } } internal interface IIdentifier { } internal interface ISpecificEntity : IIdentifier { } internal class SpecificEntity : ISpecificEntity { } internal interface IManager<TIdentifier> where

问题 I have one abstract class named A, and other classes (B, C, D, E, ...) that implements A. My derived classes are holding values of different types. I also have a list of A objects. abstract class A { } class B : class A { public int val {get;private set;} } class C : class A { public double val {get;private set;} } class D : class A { public string val {get;private set;} } class Program { static void Main(string[] args) { List list = new List { new B(), new C(), new D(), new E() }; // ...