System.Nullable struct, essentials

Let's take Nullable<double > as a case study.

Physically, represented by a 64-bit object + some bytes for the boolean flag. See http://stackoverflow.com/questions/3559843/what-is-the-size-of-a-nullableint32

Accessing the "Value" property triggers exception if HasValue == false, but GetValueOrDefault property would return 0.0. I guess we usually don't want to use GetValueOrDefault when it's null.