Nullable<T> Structure

Définition

generic <typename T>
 where T : value classpublic value class Nullable

[System.Serializable]
public struct Nullable<T> where T : struct

type Nullable<'T (requires 'T : struct)> = struct

Public Structure Nullable(Of T)

Paramètres de type

Héritage

Object

ValueType

Nullable<T>

Attributs

SerializableAttribute

Exemples

L’exemple de code suivant définit trois lignes d’une table dans l’exemple de base de données pubs Microsoft.The following code example defines three rows of a table in the Microsoft Pubs sample database. La table contient deux colonnes qui n’acceptent pas les valeurs NULL et deux colonnes qui acceptent les valeurs NULL.The table contains two columns that are not nullable and two columns that are nullable.

using namespace System;

// Define the "titleAuthor" table of the Microsoft "pubs" database.
value struct titleAuthor
{
    public:
       // Author ID; format ###-##-####
       String^ au_id;
       // Title ID; format AA####
       String^ title_id;
       // Author ORD is nullable.
       Nullable<short> au_ord;
       // Royalty Percent is nullable.
       Nullable<int> royaltyper;

    // Display the values of the titleAuthor array elements.
    static void Display(String^ dspTitle,
                        array<titleAuthor>^ dspAllTitleAuthors)
    {
       Console::WriteLine("*** {0} ***", dspTitle);
       for each (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.HasValue ?
                             dspTA.au_ord.Value : -1);
          Console::WriteLine("Royalty % ... {0}", dspTA.royaltyper.HasValue ?
                             dspTA.royaltyper.Value : 0);
          Console::WriteLine();
       }
    }
};

void main()
{
    // Declare and initialize the titleAuthor array.
    array<titleAuthor>^ ta = gcnew array<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 = nullptr;
//    ta[1].royaltyper = nullptr;

    ta[2].au_id = "672-71-3249";
    ta[2].title_id = "TC7777";
//    ta[2].au_ord = nullptr;
    ta[2].royaltyper = 40;

   // Display the values of the array elements, and
   // display a legend.
    titleAuthor::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.");
}
// 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.

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.

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 
    
    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
    
    ' 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 
    End Sub
End Class 
'This 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.

Remarques

Un type est considéré comme Nullable s’il peut être assigné à une valeur ou peut être assigné null, ce qui signifie que le type n’a aucune valeur.A type is said to be nullable if it can be assigned a value or can be assigned null, which means the type has no value whatsoever. Par défaut, tous les types référence, tels Stringque, sont Nullable, mais tous les types valeur, Int32tels que, ne le sont pas.By default, all reference types, such as String, are nullable, but all value types, such as Int32, are not.

Dans C# et Visual Basic, vous marquez un type valeur comme Nullable à l' ? aide de la notation après le type valeur.In C# and Visual Basic, you mark a value type as nullable by using the ? notation after the value type. Par exemple, int? dans C# ou Integer? dans Visual Basic déclare un type valeur entière qui peut être assigné null.For example, int? in C# or Integer? in Visual Basic declares an integer value type that can be assigned null.

La Nullable<T> structure prend en charge l’utilisation d’un seul type valeur comme type Nullable, car les types référence acceptent les valeurs NULL par conception.The Nullable<T> structure supports using only a value type as a nullable type because reference types are nullable by design.

La Nullable classe fournit une prise en charge Nullable<T> complémentaire pour la structure.The Nullable class provides complementary support for the Nullable<T> structure. La Nullable classe prend en charge l’obtention du type sous-jacent d’un type Nullable, ainsi que les opérations de comparaison et d’égalité sur les paires de types Nullable dont le type valeur sous-jacent ne prend pas en charge les opérations de comparaison et d’égalité génériques.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.

Propriétés fondamentalesFundamental Properties

Les deux membres fondamentaux de la Nullable<T> structure sont les HasValue propriétés Value et.The two fundamental members of the Nullable<T> structure are the HasValue and Value properties. Si la HasValue propriété d’un Nullable<T> objet est true, la valeur de l’objet est accessible à l’aide Value de la propriété.If the HasValue property for a Nullable<T> object is true, the value of the object can be accessed with the Value property. Si la HasValue propriété est false, la valeur de l’objet n’est pas définie et une tentative d’accès à Value la propriété lève une InvalidOperationException.If the HasValue property is false, the value of the object is undefined and an attempt to access the Value property throws an InvalidOperationException.

Conversion boxing et unboxingBoxing and Unboxing

Quand un type Nullable est boxed, le Common Language Runtime convertit automatiquement la valeur sous Nullable<T> -jacente de l' Nullable<T> objet, et non l’objet lui-même.When a nullable type is boxed, the common language runtime automatically boxes the underlying value of the Nullable<T> object, not the Nullable<T> object itself. Autrement dit, si la HasValue propriété a truela valeur, le contenu Value de la propriété est boxed.That is, if the HasValue property is true, the contents of the Value property is boxed. Lorsque la valeur sous-jacente d’un type Nullable est unboxed, le Common Language Runtime Nullable<T> crée une nouvelle structure initialisée à la valeur sous-jacente.When the underlying value of a nullable type is unboxed, the common language runtime creates a new Nullable<T> structure initialized to the underlying value.

Si la HasValue propriété d’un type Nullable est false, le résultat d’une opération de boxing nullest.If the HasValue property of a nullable type is false, the result of a boxing operation is null. Par conséquent, si un type Nullable boxed est passé à une méthode qui attend un argument d’objet, cette méthode doit être préparée pour gérer le cas où l’argument nullest.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 null. Lorsque null est unboxed dans un type Nullable, le Common Language Runtime crée une Nullable<T> nouvelle structure et initialise sa HasValue propriété avec falsela valeur.When null is unboxed into a nullable type, the common language runtime creates a new Nullable<T> structure and initializes its HasValue property to false.

.NET Framework 4.5.1.NET Framework 4.5.1et composants Windows Runtimeand Windows Runtime Components

À .NET Framework 4.5.1.NET Framework 4.5.1compter de, vous pouvez inclure un Nullable<T> type en tant que membre d’une structure exportée dans une bibliothèque WinMD.Starting with the .NET Framework 4.5.1.NET Framework 4.5.1, you can include a Nullable<T> type as a member of a structure exported in a WinMD library. Auparavant, cela n’était pas pris en charge.Previously, this was not supported.

Constructeurs

Propriétés

Méthodes

Opérateurs

S’applique à

Voir aussi

• Nullable
• Types Nullable (Guide de programmation C#)Nullable Types (C# Programming Guide)
• Types valeur Nullable (Visual Basic)Nullable Value Types (Visual Basic)