Nullable<T> Estructura

Definición

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)

Parámetros de tipo

Herencia

Object

ValueType

Nullable<T>

Atributos

SerializableAttribute

Ejemplos

En el ejemplo de código siguiente se definen tres filas de una tabla en la base de datos de ejemplo Microsoft Pubs. La tabla contiene dos columnas que no admiten valores NULL y dos columnas que aceptan valores NULL.

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.

Comentarios

Para obtener más información sobre esta API, consulte Comentarios complementarios de la API para T> que admite<valores NULL.

Consulte también

• Nullable
• Tipos que aceptan valores NULL (Guía de programación de C#)
• Tipos de valor que aceptan valores NULL (Visual Basic)