如何在此代码示例中获得 NullReferenceException？

??? o = new ???();

Console.WriteLine("ToString() -> " + o.ToString() ); //<--- Prints 'ToString() -> '
Console.WriteLine("GetType() -> " + o.GetType()); //<--- NullReferenceException

Output:

ToString() -> 

Unhandled Exception: System.NullReferenceException: Object reference not set 
to an instance of an object.
at System.Object.GetType()
at Program.Main(String[] args)

Question

类型是什么???为什么o.ToString() return string.Empty and o.GetType()抛出一个NullReferenceException?

Note: GetType()没有重新定义??? type.

Any Nullable<T>.

Check Gravell 的 C# 中奇怪的极端情况示例 https://stackoverflow.com/questions/194484/whats-the-strangest-corner-case-youve-seen-in-c-or-net/194671#194671