c# – 为什么我必须创建`IEnumerable`的具体实现才能修改其成员

This blog post (Exhibit 1)解释了这种行为,但我无法完全理解它.

我在这里有一个非常简单的代码片段来重现这个问题(C#4.0 / .NET 4.0).

class Person
{
    public int Age { get; set; }

    public Person()
    {

    }
}

class Program
{
    static void Main(string[] args)
    {
        //calling .ToList() on GetPeople() below will fix the issue
        var people = GetPeople();

        foreach (var item in people)
        {
            item.Age = DateTime.Now.Second;
        }

        foreach (var item in people)
        {
            Console.WriteLine("Age is {0}",item.Age);
        }

        Console.Read();
    }

    public static IEnumerable<Person> GetPeople()
    {
        int i = 0;
        while (i < 3)
        {
            i++;
            yield return new Person(); 
        }
    }
}