将 IEnumerable 隐式转换为泛型类型

Question

我在使用隐式转换和 IEnumerable 时遇到了一个有趣的案例 - 请查看附加代码的最后一行 - 它无法编译。

public class Outer<T>
{
    private T field;
    public Outer(T it)
    {
        field = it;
    }

    public static implicit operator Outer<T> (T source)
    {
        return new Outer<T>(source);
    }
}

void Main()
{
    Outer<string> intsample = "aa";
    Outer<IList<object>> listsample = new List<object>();

    //The line below fails with:
    //CS0266 Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<object>'
    //to 'UserQuery.Outer<System.Collections.Generic.IEnumerable<object>>'.
    //An explicit conversion exists (are you missing a cast?)
    Outer<IEnumerable<object>> enumerablesample = Enumerable.Empty<object>();
}

我有一种非常强烈的直觉，很高兴 IEnumerable 是协变的，但是有人可以更正式地解释一下吗？

Answer 1

The docs 状态：

A class or struct is permitted to declare a conversion from a source type S to a target type T provided all of the following are true:

S and T are different types.

Either S or T is the class or struct type in which the operator declaration takes place.

Neither S nor T is object or an interface-type. T is not a base class of S, and S is not a base class of T.

简单的解决方法是更改：

Enumerable.Empty<object>();

至：

new List<object>();

因为 List 不是接口（感谢@PatrickHofman！）。

将 IEnumerable 隐式转换为泛型类型

Implicit casting IEnumerable to generic type

c#

generics

covariance