Nullable<T> Struktur

Referenz

Definition

Namespace:: System

Assembly:: System.Runtime.dll

Assembly:: mscorlib.dll

Assembly:: netstandard.dll

Quelle:: Nullable.cs

Quelle:: Nullable.cs

Quelle:: Nullable.cs

Wichtig

Einige Informationen beziehen sich auf Vorabversionen, die vor dem Release ggf. grundlegend überarbeitet werden. Microsoft übernimmt hinsichtlich der hier bereitgestellten Informationen keine Gewährleistungen, seien sie ausdrücklich oder konkludent.

Stellt einen Werttyp dar, der null zugewiesen werden kann.

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)

Typparameter

T

Der zugrunde liegende Werttyp des generischen Nullable<T>-Typs.

Vererbung: Object

ValueType
Nullable<T>

Attribute: SerializableAttribute

Beispiele

Im folgenden Codebeispiel werden drei Zeilen einer Tabelle in der Microsoft Pubs-Beispieldatenbank definiert. Die Tabelle enthält zwei Spalten, für die kein NULL-Wert zulässig ist, und zwei Spalten, die NULL-Werte zulassen.

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.

Hinweise

Weitere Informationen zu dieser API finden Sie unter Ergänzende API-Hinweise für Nullable<T>.

Konstruktoren

Nullable<T>(T)

Initialisiert eine neue Instanz der Nullable<T>-Struktur mit dem angegebenen Wert.

Eigenschaften

HasValue	Ruft einen Wert ab, der angibt, ob das aktuelle Nullable<T>-Objekt einen gültigen Wert des zugrunde liegenden Typs hat.
Value	Ruft den Wert des aktuellen Nullable<T>-Objekts ab, wenn ihm ein gültiger zugrunde liegender Wert zugewiesen wurde.

Methoden

Equals(Object)	Gibt an, ob das aktuelle Nullable<T>-Objekt einem angegebenen Objekt entspricht.
GetHashCode()	Ruft den Hashcode des Objekts ab, das von der Value-Eigenschaft zurückgegeben wird.
GetValueOrDefault()	Ruft den Wert des aktuellen Nullable<T>-Objekts oder den Standardwert des zugrunde liegenden Typs ab.
GetValueOrDefault(T)	Ruft den Wert des aktuellen Nullable<T>-Objekts oder den angegebenen Standardwert ab.
ToString()	Gibt die Textdarstellung des Werts des aktuellen Nullable<T>-Objekts zurück.

Operatoren

Explicit(Nullable<T> to T)	Definiert eine explizite Konvertierung einer Nullable<T>-Instanz in den zugrunde liegenden Wert.
Implicit(T to Nullable<T>)	Erstellt ein neues Nullable<T>-Objekt, das mit einem angegebenen Wert initialisiert wurde.

Gilt für:

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