编组。NET泛型类型

下面是一个C#程序,它尝试 Marshal.SizeOf 关于几种不同的类型:

using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential)]
class AClass { }

[StructLayout(LayoutKind.Sequential)] 
struct AStruct { }

[StructLayout(LayoutKind.Sequential)]
class B { AClass value; }

[StructLayout(LayoutKind.Sequential)]
class C<T> { T value; }

class Program
{
    static void M(object o) { Console.WriteLine(Marshal.SizeOf(o)); }

    static void Main()
    {
        M(new AClass());
        M(new AStruct());
        M(new B());
        M(new C<AStruct>());
        M(new C<AClass>());
    }
}

"Type 'C`1[AClass]' cannot be marshaled as an unmanaged structure; no meaningful size or offset can be computed."

为什么?具体来说,为什么SizeOf会堵塞 C<AClass> B C<AStruct> ?

编辑: 因为这是在评论中被问到的,所以这是激发这个主要是学术问题的“现实世界”问题:

C<T> 以及一个C#外部声明 M(C<T>) ,然后致电 M(C<int>) 在一条线上,以及 M(C<double>)