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

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

输出：

ToString() -> 

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

题

什么是???为什么o.ToString()返回string.Empty和o.GetType()会抛出NullReferenceException？

Note: GetType() is not redefined in the ??? type.