如果 HasValue 属性为 true，则可以使用 Value 属性访问当前 Nullable 对象的值。

//官方关于nullable类的hasvalue属性的示例

using System;

class Sample 
{
    public static void Main() 
    {
      //声明变量 datatime类型的变量mynow,注：此时未初始化仅声明
     DateTime? myNow;

//  Assign the current date and time to myNow then display its value.
     //初始化变量mynow,以datatime.now 
    myNow = DateTime.Now;
 
        //调用display方法显示mynow变量的值
    Display(myNow, "1) ");

//  Assign null (Nothing in Visual Basic) to myNow then display its value.
   
      //mynow重置为null,可理解为没有提供值
    myNow = null;
    Display(myNow, "2) ");
    }

// Display the date and time.
  //display方法的定义体如下
  //方法参数为displaydatatime及title
 public static void Display(DateTime? displayDateTime, string title)
    {
// If a value is defined for the displayDatetime argument, display its value; otherwise, 
// display that no value is defined.
    Console.Write(title);
       //通过nullable对象的hasvalue属性判断方法display 的参数displaydatatime
       //是否传入值，若传入值则为true,否则为false
    if (displayDateTime.HasValue == true)
           //表明displaydatatime有值，显示下述内容
         Console.WriteLine("The date and time is {0:F}.", displayDateTime.Value);
    else
        Console.WriteLine("The date and time is not defined.");
    }
}

//示例代码的输出
/*
This code example produces the following results:

1) The date and time is Tuesday, April 19, 2005 4:16:06 PM.
2) The date and time is not defined.

*/