.net2.0新特性,允许为空:for example:// This code example demonstrates the Nullable<T>.HasValue // and Value properties.using System;class Sample { public static void Main() { DateTime? myNow;// Assign the current date and time to myNow then display its value. myNow = DateTime.Now; Display(myNow, "1) ");// Assign null (Nothing in Visual Basic) to myNow then display its value. myNow = null; Display(myNow, "2) "); }// Display the date and time. 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); if (displayDateTime.HasValue == true) Console.WriteLine("The date and time is {0:F}.", displayDateTime.Value); else Console.WriteLine("The date and time is not defined."); } }/* 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.*/
// and Value properties.using System;class Sample
{
public static void Main()
{
DateTime? myNow;// Assign the current date and time to myNow then display its value.
myNow = DateTime.Now;
Display(myNow, "1) ");// Assign null (Nothing in Visual Basic) to myNow then display its value.
myNow = null;
Display(myNow, "2) ");
}// Display the date and time.
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);
if (displayDateTime.HasValue == true)
Console.WriteLine("The date and time is {0:F}.", displayDateTime.Value);
else
Console.WriteLine("The date and time is not defined.");
}
}/*
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.*/