为什么不能 IEnumerable&lt;struct&gt;被强制转换为 IEnumerable<object>? [英] Why cannot IEnumerable&lt;struct&gt; be cast as IEnumerable&lt;object&gt;?

问题描述

为什么不允许最后一行?

Why is the last line not allowed?

IEnumerable<double> doubleenumerable = new List<double> { 1, 2 };
IEnumerable<string> stringenumerable = new List<string> { "a", "b" };
IEnumerable<object> objects1 = stringenumerable; // OK
IEnumerable<object> objects2 = doubleenumerable; // Not allowed

这是不是因为 double 是不是从对象派生的值类型，因此协方差不起作用?

Is this because double is a value type that doesn't derive from object, hence the covariance doesn't work?

这是否意味着无法完成这项工作:

Does that mean that there is no way to make this work:

public interface IMyInterface<out T>
{
    string Method(); 
}

public class MyClass<U> : IMyInterface<U>
{
    public string Method()
    {
        return "test";
    }
}

public class Test
{
    public static object test2() 
    {
        IMyInterface<double> a = new MyClass<double>();
        IMyInterface<object> b = a; // Invalid cast!
        return b.Method();
    }
}

而且我需要编写我自己的 IMyInterface.Cast() 来做到这一点?

And that I need to write my very own IMyInterface<T>.Cast<U>() to do that?

推荐答案

为什么不允许最后一行?

Why is the last line not allowed?

因为double是值类型而object是引用类型；协方差仅在两种类型都是引用类型时有效.

Because double is a value type and object is a reference type; covariance only works when both types are reference types.

这是不是因为 double 是不是从对象派生的值类型，因此协方差不起作用?

Is this because double is a value type that doesn't derive from object, hence the covariance doesn't work?

没有.Double 确实从对象派生.所有值类型都派生自对象.

No. Double does derive from object. All value types derive from object.

现在你应该问的问题:

为什么协方差无法将 IEnumerable 转换为 IEnumerable