Nullable<T> type
or ? on variable
Represents a value type that can be assigned
null.
It's a
nullable value. Structs, by default, cannot be nullable, they must have a
value, so in C# 2.0, the
Nullable<T> type
was introduced to the .NET Framework.
C# implements the
Nullable<T> type with a piece of
syntactic sugar, which places a question mark after the type name, thus making
the previously non-nullable type, nullable.
Explanation
//Cannot Be
Null
DateTime
DateTime dt = null; // Error: Cannot convert null to 'System.DateTime'
//because it is a
non-nullable value type
//Can Be Null
DateTime? //Nullable <
DateTime >
DateTime? dt = null; // no problems
Example
1
using System;
using System.Collections.Generic;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Web;
using System;
class Sample
{
// Define the
"titleAuthor" table of the Microsoft "pubs" database.
public struct titleAuthor
{
// Author ID;
format ###-##-####
public string au_id;
// Title ID;
format AA####
public string
title_id;
// Author ORD
is nullable.
public short? au_ord;
// Royalty
Percent is nullable.
public int?
royaltyper;
}
public static void Main()
{
// Declare and
initialize the titleAuthor array.
titleAuthor[] ta = new titleAuthor[3];
ta[0].au_id = "712-32-1176";
ta[0].title_id = "PS3333";
ta[0].au_ord = 1;
ta[0].royaltyper = 100;
ta[1].au_id = "213-46-8915";
ta[1].title_id = "BU1032";
ta[1].au_ord = null;
ta[1].royaltyper = null;
ta[2].au_id = "672-71-3249";
ta[2].title_id = "TC7777";
ta[2].au_ord = null;
ta[2].royaltyper = 40;
// Display the
values of the titleAuthor array elements, and
// display a
legend.
Display("Title
Authors Table", ta);
Console.WriteLine("Legend:");
Console.WriteLine("An Author ORD of -1 means no
value is defined.");
Console.WriteLine("A Royalty % of 0 means no
value is defined.");
}
// Display the
values of the titleAuthor array elements.
public static void Display(string dspTitle,
titleAuthor[] dspAllTitleAuthors)
{
Console.WriteLine("*** {0} ***", dspTitle);
foreach (titleAuthor dspTA in
dspAllTitleAuthors)
{
Console.WriteLine("Author ID ... {0}", dspTA.au_id);
Console.WriteLine("Title ID .... {0}", dspTA.title_id);
Console.WriteLine("Author ORD .. {0}", dspTA.au_ord ?? -1);
Console.WriteLine("Royalty % ... {0}", dspTA.royaltyper ?? 0);
Console.WriteLine();
}
}
}
// The
example displays the following output:
// *** Title Authors Table ***
// Author ID ... 712-32-1176
// Title ID .... PS3333
// Author ORD .. 1
// Royalty % ... 100
//
// Author ID ... 213-46-8915
// Title ID .... BU1032
// Author ORD .. -1
// Royalty % ... 0
//
// Author ID ... 672-71-3249
// Title ID .... TC7777
// Author ORD .. -1
// Royalty % ... 40
//
// Legend:
// An Author ORD of -1 means no value is
defined.
// A Royalty % of 0 means no value is
defined.
