我可以创建一个列表与LT; WeakReference的< T>&GT ;? [英] Can I create a List<WeakReference<T>>?

问题描述

我试图创建的WeakReference 的表S使用4.5通用的实施，使我能避免类型检查和的WeakReference铸造目标。但是，的WeakReference< T> 似乎并不支持协方差，所以我试图建立一个解决方法。

I'm trying to create a List of WeakReferences using the 4.5 generic implementation so that I can avoid type checking and casting of the WeakReference target. But, WeakReference<T> doesn't appear to support covariance, so I'm trying to establish a workaround.

我想它应该是可行的，因为每个T公司将在一个特定的继承链的类型。所以，我在想什么会沿着这些路线：

I'm thinking it should be doable since each T would be of a type in a particular inheritance chain. So, what I'm thinking would be along these lines:

public class Animal { }
public class Tiger : Animal { }
public class Wolf : Animal { }

var mylist = new List<WeakReference<Animal>>();

mylist.Add(new WeakReference<Animal>(new Animal()));
mylist.Add(new WeakReference<Tiger>(new Tiger()));
mylist.Add(new WeakReference<Wolf>(new Wolf()));

我试图创造WeakReference的（因为它是不遗传的）一个包装类，但没有按'T的工作。不管是什么，该列表将不超过接受任何类型的的WeakReference 等的WeakReference<动物方式>

我可以创建自己的通用的WeakReference 的实现，但似乎被击败，因为我会做类型转换点在其中。我找不到任何文件，但我有种假设框架版本处理这更好的。

I could create my own generic WeakReference implementation, but that would seem to be defeating the point since I'd be doing type casting within it. I can't find any documentation, but I'm kind of assuming the framework version handles this better.

有没有办法，我没有想到处理这个法子的，还是我找错了树？

Is there another way of handling this that I'm not thinking of, or am I barking up the wrong tree?

推荐答案

的WeakReference 是不变的，因为它允许值被设置，并且将是无效的，如果它是协变。为了让协变的，你需要做一个只读围绕基准包装，并且还使用一个接口。

WeakReference is invariant, since it allows the value to be set, and that wouldn't be valid if it were covariant. To make it covariant you'll need to make a read only wrapper around the reference, and also use an interface.

public interface IReadOnlyWeakReference<out T>
{
    T Value { get; }
}

public class WeakReferenceWrapper<T> : IReadOnlyWeakReference<T>
    where T : class
{
    private WeakReference<T> reference;
    public WeakReferenceWrapper(WeakReference<T> reference)
    {
        this.reference = reference;
    }

    public T Value
    {
        get
        {
            T output;
            if (reference.TryGetTarget(out output))
                return output;
            else
                return default(T);
        }
    }
}

有关转换的扩展方法也有些方便：

An extension method for the conversion is also somewhat convenient:

public static IReadOnlyWeakReference<T> AsReadOnly<T>(
    this WeakReference<T> reference)
    where T : class
{
    return new WeakReferenceWrapper<T>(reference);
}

现在我们可以这样写：

var mylist = new List<IReadOnlyWeakReference<Animal>>();

mylist.Add(new WeakReference<Animal>(new Animal()).AsReadOnly());
mylist.Add(new WeakReference<Tiger>(new Tiger()).AsReadOnly());
mylist.Add(new WeakReference<Wolf>(new Wolf()).AsReadOnly());

这篇关于我可以创建一个列表与LT; WeakReference的< T>&GT ;?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！