Nullable<T> Structure
Définition
Important
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Représente un type valeur qui peut avoir la valeur null
.
generic <typename T>
where T : value classpublic value class Nullable
public struct Nullable<T> where T : struct
[System.Serializable]
public struct Nullable<T> where T : struct
type Nullable<'T (requires 'T : struct)> = struct
[<System.Serializable>]
type Nullable<'T (requires 'T : struct)> = struct
Public Structure Nullable(Of T)
Paramètres de type
- T
Type valeur sous-jacent du type Nullable<T> générique.
- Héritage
- Attributs
Exemples
L’exemple de code suivant définit trois lignes d’une table dans l’exemple de base de données Microsoft Pubs. La table contient deux colonnes qui ne peuvent pas avoir la valeur Null et deux colonnes qui peuvent avoir la valeur Null.
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.
open System
// Define the "titleAuthor" table of the Microsoft "pubs" database.
type titleAuthor =
struct
// Author ID format ###-##-####
val mutable au_id: string
// Title ID format AA####
val mutable title_id: string
// Author ORD is nullable.
val mutable au_ord: Nullable<int16>
// Royalty Percent is nullable.
val mutable royaltyper: Nullable<int>
end
// Display the values of the titleAuthor array elements.
let display dspTitle (dspAllTitleAuthors: #seq<titleAuthor>) =
printfn $"*** {dspTitle} ***"
for dspTA in dspAllTitleAuthors do
printfn $"Author ID ... {dspTA.au_id}"
printfn $"Title ID .... {dspTA.title_id}"
printfn $"Author ORD .. {dspTA.au_ord.GetValueOrDefault -1s}"
printfn $"Royalty %% ... {dspTA.royaltyper.GetValueOrDefault -1}\n"
// Declare and initialize the titleAuthor array.
let ta = Array.zeroCreate<titleAuthor> 3
ta[0].au_id <- "712-32-1176"
ta[0].title_id <- "PS3333"
ta[0].au_ord <- Nullable 1s
ta[0].royaltyper <- Nullable 100
ta[1].au_id <- "213-46-8915"
ta[1].title_id <- "BU1032"
ta[1].au_ord <- Nullable()
ta[1].royaltyper <- Nullable()
ta[2].au_id <- "672-71-3249"
ta[2].title_id <- "TC7777"
ta[2].au_ord <- Nullable()
ta[2].royaltyper <- Nullable 40
// Display the values of the titleAuthor array elements, and
// display a legend.
display "Title Authors Table" ta
printfn "Legend:"
printfn "An Author ORD of -1 means no value is defined."
printfn "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.
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
Pour plus d’informations sur cette API, consultez Remarques supplémentaires sur l’API pour nullable<T>.
Constructeurs
Nullable<T>(T) |
Initialise une nouvelle instance de la structure Nullable<T> avec la valeur spécifiée. |
Propriétés
HasValue |
Obtient une valeur indiquant si l'objet Nullable<T> actuel a une valeur valide de son type sous-jacent. |
Value |
Obtient la valeur de l'objet Nullable<T> actuel si une valeur sous-jacente valide lui a été assignée. |
Méthodes
Equals(Object) |
Indique si l'objet Nullable<T> en cours est égal à un objet spécifié. |
GetHashCode() |
Récupère le code de hachage de l'objet retourné par la propriété Value. |
GetValueOrDefault() |
Récupère la valeur de l’objet Nullable<T> actuel ou la valeur par défaut du type sous-jacent. |
GetValueOrDefault(T) |
Récupère la valeur de l'objet Nullable<T> actuel ou la valeur par défaut spécifiée. |
ToString() |
Retourne la représentation textuelle de la valeur de l'objet Nullable<T> actuel. |
Opérateurs
Explicit(Nullable<T> to T) |
Définit une conversion explicite d’une instance Nullable<T> à sa valeur sous-jacente. |
Implicit(T to Nullable<T>) |
Crée un objet Nullable<T> initialisé à une valeur spécifiée. |