1.Nullable Types in C#

书目：MCTS 70-536: TS: .NET Framework 2.0-Application Development Foundation 
章节：Chapter 1: Framework Fundamentals -- Lesson 1: Using Value Types -- How to Declare Value Types
原文内容：
Declare the variable as nullable if you want to be able to determine whether a value has
not been assigned. For example, if you are storing data from a yes/no question on a
form and the user did not answer the question, you should store a null value. The following
code allows a Boolean variable to be true, false, or other:

// C#
Nullable<bool> b = null;
// Shorthand notation, only for C#
bool? b = null;

Declaring a variable as nullable enables the HasValue and Value members. Use Has-
Value to detect whether or not a value has been set:

// C#
if (b.HasValue)Console.WriteLine("b is {0}.", b.Value);
else Console.WriteLine("b is not set.");

参考文章：
http://www.codeguru.com/Csharp/.NET/net_data/datagrid/article.php/c10393
http://www.codeguru.com/csharp/.net/net_data/datagrid/article.php/c10393__2/