子类与协变返回类型的方法，解决方法？

Question

I'm trying to neatly organize my code and came across and oddity as primarily Java-focused programmer. Here's the simplified version:

class ReturnParent { ... }
class ReturnChild : ReturnParent { ... }

class Parent
{
    abstract ReturnParent SomeMethod();    
}

class Child : Parent
{
    // Note the return type
    override ReturnChild SomeMethod() {}
}

Now I've looked up this issue and I know this doesn't work in C# because covariance isn't supported in this way. My question is, is there a workaround to this issue?

The method can be a parameter instead; the classes, including the return classes, MUST be classes, they cannot be interfaces; they can implement interfaces though, if that helps.

Also, the main class must be castable, which was an issue when I tried generics:

    Parent p = new Child(); // This works
    Child c = p as Child; // This doesn't work with generics

Answer 1

You can let the subclasses decide the return type of SomeMethod .

abstract class Parent<TReturn> where TReturn : ReturnParent
{
    public abstract TReturn SomeMethod();    
}

class Child : Parent<ReturnChild>
{
    public override ReturnChild SomeMethod(){}
}

If you make Parent<T> implement the following covariant interface, then you could cast Child to IParent<ReturnParent> .

interface IParent<out T>
{
    T SomeMethod(); 
} 

If you want to make it castable specifically to Parent<ReturnParent> , then no, that can't be done, and you may need a redesign.

Answer 2

This works with Linqpad

abstract class Parent<T>
{
    public abstract T SomeMethod();    
}

class Child : Parent<ReturnChild>
{
    // Note the return type
    public override ReturnChild SomeMethod()
    {
       return new ReturnChild();
    }
}

void Main()
{ 
    var p = new Child(); // This works
    Child c = p as Child; // This doesn't work with generics
}