问题
Some time ago I thought that Nullable<> value types are classes, encapsulating value types and a bool to HasValue. With some implicit cast operador for null, just implemented at BCL.
But being a struct, how this can be achieved? Nullable<> struct is "special" for CLR?
回答1:
Nullable<T> is defined as a normal struct, but there's special hooks within the CLR to box/unbox an instance of [mscorlib]System.Nullable`1 to null according to the HasValue property. There's more details on this here
回答2:
Here's the MSDN article on Nullable Types.
http://msdn.microsoft.com/en-us/library/1t3y8s4s(v=VS.100).aspx
I'm not sure what you're trying to get at with Nullable<>, unless you're mis-understanding that Nullable types are instances of the System.Nullable<T> struct.
回答3:
Nullable<> is a struct implemented within mscorlib.
One special thing is in the C# compiler that recognizes X? as an alias for Nullable<X>.
来源：https://stackoverflow.com/questions/4694264/nullable-types-are-a-bcl-clr-or-both-implementation