c# - 为什么 List 不是 IEnumerable？

Question

[编辑:我很抱歉......原来的问题措辞含糊不清，我没有得到我正在寻找的答复]

对于任何继承自类 Y 的类 X，new List<X>() is IEnumerable<Y>是真的。但是，这不适用于结构:new List<int>() is IEnumerable<ValueType>是假的。我的问题是:为什么？

这是一个示例程序:

class Program
{
    class Y { }
    class X : Y { }
    struct Z { }

    static void Main(string[] args)
    {
        Test(new List<X>());
        Test(new List<string>());
        Test(new List<Z>());
        Test(new List<int>());
        Test("blah");
        Test(1);
        Console.ReadLine();
    }

    static void Test(object o)
    {
        if (o is IEnumerable<Y>)
        {
            Console.WriteLine(o + " is a list of Ys");
        }
        else if (o is IEnumerable<ValueType>)
        {
            Console.WriteLine(o + " is a list of ValueTypes");
        }
        else if (o is IEnumerable<object>)
        {
            Console.WriteLine(o + " is a list of objects");
        }
        else if (o is System.Collections.IEnumerable)
        {
            Console.WriteLine(o + " is most likely a list of ValueTypes or a string");
        }
        else
        {
            Console.WriteLine(o + " is not a list");
        }

    }
}

输出:

System.Collections.Generic.List`1[ConsoleApplication1.Program+X] is a list of Ys

System.Collections.Generic.List`1[System.String] is a list of objects

System.Collections.Generic.List`1[ConsoleApplication1.Program+Z] is most likely a list of ValueTypes or a string

System.Collections.Generic.List`1[System.Int32] is most likely a list of ValueTypes or a string

blah is most likely a list of ValueTypes or a string

1 is not a list

那为什么是new List<int>不是IEnumerable<ValueType> ？

Answer 1

协变仅适用于引用类型，不适用于值类型。所以一个List<string>可分配给 IEnumerable<object>因为string是一个引用类型，但是一个 List<int>不可分配给 IEnumerable<ValueType> .详见C#语言规范13.1.3.2节