1. nullable type: represents plus a null value within the normal range of values of type
(1) Structure: nullable type is an instance of a generic structure Nullable <T>, which is declared as: public struct Nullable <T> where T:? Struct, grammar T is Nullable <T> is shorthand for both forms They are equivalent:
Nullable<int> myNullableInt = new Nullable<int>()，等价于int ? myNullableInt = null;
(2) may include the following types of empty instance members:
1) .HasValue: read-only attribute, determines whether the value, if the current value is not empty, returns true, false otherwise
2) .Value: read-only attribute, if the current value is non-empty, can be a normal visit, or description Value does not contain a meaningful value, will throw an exception InvalidOperationException At this time access Value
3) .GetValueOrDefault (): method of example, if the current value is non-empty (the HasValue as to true), the return value Vlaue otherwise returns the default value of type T (which is the default value of the private field)
4) .GetValueOrDefault (T defaultValue): example method, if the current value is non-empty (the HasValue as to false), to return Vlaue value, otherwise the default value defaultValue