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Nullable(Of T) Structure

Represents a value type that can be assigned Nothing.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
<SerializableAttribute> _
Public Structure Nullable(Of T As Structure)

Type Parameters

T

The underlying value type of the Nullable(Of T) generic type.

The Nullable(Of T) type exposes the following members.

  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryNullable(Of T)Initializes a new instance of the Nullable(Of T) structure to the specified value.
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  NameDescription
Public propertySupported by the XNA FrameworkSupported by Portable Class LibraryHasValueGets a value indicating whether the current Nullable(Of T) object has a valid value of its underlying type.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibraryValueGets the value of the current Nullable(Of T) object if it has been assigned a valid underlying value.
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  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryEqualsIndicates whether the current Nullable(Of T) object is equal to a specified object. (Overrides ValueType.Equals(Object).)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetHashCodeRetrieves the hash code of the object returned by the Value property. (Overrides ValueType.GetHashCode.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetValueOrDefaultRetrieves the value of the current Nullable(Of T) object, or the object's default value.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetValueOrDefault(T)Retrieves the value of the current Nullable(Of T) object, or the specified default value.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryToStringReturns the text representation of the value of the current Nullable(Of T) object. (Overrides ValueType.ToString.)
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  NameDescription
Public operatorStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryNarrowing(Nullable(Of T) to T)Defines an explicit conversion of a Nullable(Of T) instance to its underlying value.
Public operatorStatic memberSupported by the XNA FrameworkSupported by Portable Class LibraryWidening(T to Nullable(Of T))Creates a new Nullable(Of T) object initialized to a specified value.
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A type is said to be nullable if it can be assigned a value or can be assigned Nothing, which means the type has no value whatsoever. By default, all reference types, such as String, are nullable, but all value types, such as Int32, are not.

In C# and Visual Basic, you mark a value type as nullable by using the ? notation after the value type. For example, int? in C# or Integer? in Visual Basic declares an integer value type that can be assigned Nothing.

The Nullable(Of T) 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(Of T) 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.

Fundamental Properties

The two fundamental members of the Nullable(Of T) structure are the HasValue and Value properties. If the HasValue property for a Nullable(Of T) 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(Of T) object, not the Nullable(Of T) 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(Of T) structure initialized to the underlying value.

If the HasValue property of a nullable type is false, the result of a boxing operation is Nothing. 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 Nothing. When Nothing is unboxed into a nullable type, the common language runtime creates a new Nullable(Of T) structure and initializes its HasValue property to false.

.NET Framework 4.5.1 and Windows Runtime Components

Starting with the .NET Framework 4.5.1, you can include a Nullable(Of T) type as a member of a structure exported in a WinMD library. Previously, this was not supported.

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. 
'

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
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