将集合传递给函数是否意味着函数可以更改集合的元素

我实际上知道问题的答案（我认为），但我不知道原因...

所以，我知道如果我有像下面这样的课程：

class Man
{
    public string Name;
    public int Height;
    public Man() { }
    public Man(string i_name, int i_height)
    {
        Name = i_name;
        Height = i_height;
    }
}    

我有以下程序类（具有主功能）：

class Program
{
    static void Main(string[] args)
    {
        Program p = new Program();
        Man g = new Man("greg", 175);
        //assigning null to g inside the function.
        p.ChangeMan(g);

        Console.WriteLine(g == null? "the function changed g out side the function" : "the function did not change g out side the function");
        //the output of course is that the function did not change g outside the function.
        //now I am creating a list of Man and adding 5 Man instances to it.
        List<Man> manList = new List<Man>();
        for (int i = 0; i < 5; i++)
        {
            manList.Add(new Man("Gadi" + i.ToString(), 10 * i));
        }
        //assigning null to the list insdie the function
        p.ChangeList(manList);
        Console.WriteLine(manList == null ? "the function changed the list out side the function" : "the function did not change the list out side the function");
        //the output of cousre again is the function did not change the list out side the function
        //now comes the part I dont understand...

        p.ChangeManInAList(manList);
        Console.WriteLine("list count = " + manList.Count());
        //count is now 6.
        Console.WriteLine(manList[0] == null ? "the function changed the element out side the function" : "the function did not change the element out side the function");
        //the out again - the function did not change...

    }
    public void ChangeMan(Man g)
    {
        g = null;
    }
    public void ChangeManInAList(IList<Man> gadiList)
    {
        Man g = gadiList.First<Man>();
        g = null;
        Console.WriteLine(g == null? "g is null" : "g is not null");
        gadiList.Add(new Man("a new gadi", 200));
        Console.WriteLine("list count = " + gadiList.Count());
    }
    public void ChangeList(List<Man> list)
    {
        list = null;
    }

}

我正在为列表的第一个元素分配 null + 向列表中添加一个人。我期望如果我能添加到列表中，我也可以更改元素，但我看到了不同的......

我能够将一个人添加到列表中，但无法为其中一个元素分配 null，怎么会这样？我知道列表是按值传递的，所以我不能更改列表本身（例如为其分配 null），但我可以添加到它吗？并且不能为元素分配 null？他们也被瓦尔通过了吗？

会很高兴得到一些良好而清晰的解释:)

 Man g = gadiList.First<Man>();
 g = null;

int a = 5;
int b = a;
b = 3;
//you wouldn't expect `a` to be 3 now, would you?

Man g = gadiList.First<Man>();
gadiList[gadiList.IndexOf(g)] = null;
//or specifically in this case:
gadiList[0] = null;