Nullable Generic Structure
Assembly: mscorlib (in mscorlib.dll)
'Declaration <SerializableAttribute> _ Public Structure Nullable(Of T As Structure) 'Usage Dim instance As Nullable(Of T)
J# supports the use of generic types and methods, but not the declaration of new ones.
JScript does not support generic types and methods.
Not applicable.
Type Parameters
- T
The underlying value type of the Nullable generic type.
A type is said to be nullable if it can be assigned a value or can be assigned a null reference (Nothing in Visual Basic), which means the type has no value whatsoever. Consequently, a nullable type can express a value, or that no value exists. For example, a reference type such as String is nullable, whereas a value type such as Int32 is not. A value type cannot be nullable because it has enough capacity to express only the values appropriate for that type; it does not have the additional capacity required to express a value of null.
The Nullable structure supports using only a value type as a nullable type because reference types are nullable by design.
The Nullable class provides complementary support for the Nullable structure. The Nullable class supports obtaining the underlying type of a nullable type, and comparison and equality operations on pairs of nullable types whose underlying value type does not support generic comparison and equality operations.
Scenario
Use nullable types to represent things that exist or do not exist depending on the circumstance. For example, an optional attribute of an HTML tag might exist in one tag but not another, or a nullable column of a database table might exist in one row of the table but not another.
You can represent the attribute or column as a field in a class and you can define the field as a value type. The field can contain all the valid values for the attribute or column, but cannot accommodate an additional value that means the attribute or column does not exist. In this case, define the field to be a nullable type instead of a value type.
Fundamental Properties
The two fundamental members of the Nullable structure are the HasValue and Value properties. If the HasValue property for a Nullable object is true, the value of the object can be accessed with the Value property. If the HasValue property is false, the value of the object is undefined and an attempt to access the Value property throws an InvalidOperationException.
Boxing and Unboxing
When a nullable type is boxed, the common language runtime automatically boxes the underlying value of the Nullable object, not the Nullable object itself. That is, if the HasValue property is true, the contents of the Value property is boxed. When the underlying value of a nullable type is unboxed, the common language runtime creates a new Nullable structure initialized to the underlying value.
If the HasValue property of a nullable type is false, the result of a boxing operation is a null reference (Nothing in Visual Basic). Consequently, if a boxed nullable type is passed to a method that expects an object argument, that method must be prepared to handle the case where the argument is a null reference (Nothing in Visual Basic). When a null reference (Nothing in Visual Basic) is unboxed into a nullable type, the common language runtime creates a new Nullable structure and initializes its HasValue property to false.
The following code example defines three rows of a table in the Microsoft Pubs sample database. The table contains two columns that are not nullable and two columns that are nullable.
' This code example demonstrates the Nullable(Of T) class. ' The code example defines a database table in which two columns ' are nullable. In the application, an array of rows is created ' and initialized. The table rows could subsequently be ' written to a database. Imports System Class Sample ' Define the "titleAuthor" table of the Microsoft "pubs" database. Public Structure titleAuthor ' Author ID; format ###-##-#### Public au_id As String ' Title ID; format AA#### Public title_id As String ' Author ORD is nullable. Public au_ord As Nullable(Of Short) ' Royalty Percent is nullable. Public royaltyper As Nullable(Of Integer) End Structure 'titleAuthor Public Shared Sub Main() ' Declare and initialize the titleAuthor array. Dim ta(2) As titleAuthor 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 = Nothing ta(1).royaltyper = Nothing ta(2).au_id = "672-71-3249" ta(2).title_id = "TC7777" ta(2).au_ord = Nothing 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.") End Sub 'Main ' Display the values of the titleAuthor array elements. Public Shared Sub Display(ByVal dspTitle As String, _ ByVal dspAllTitleAuthors() As titleAuthor) Console.WriteLine("*** {0} ***", dspTitle) Dim dspTA As titleAuthor For Each 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.GetValueOrDefault(-1)) Console.WriteLine("Royalty % ... {0}", dspTA.royaltyper.GetValueOrDefault(0)) Console.WriteLine() Next dspTA End Sub 'Display End Class 'Sample ' 'This code example produces the following results: ' '*** 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. '
Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition
The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.