Nullable<T> Struktur
Definisi
Penting
Beberapa informasi terkait produk prarilis yang dapat diubah secara signifikan sebelum dirilis. Microsoft tidak memberikan jaminan, tersirat maupun tersurat, sehubungan dengan informasi yang diberikan di sini.
Mewakili jenis nilai yang dapat ditetapkan 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)
Jenis parameter
- T
Jenis nilai yang mendasar dari Nullable<T> jenis generik.
- Warisan
- Atribut
Contoh
Contoh kode berikut menentukan tiga baris tabel dalam database sampel Microsoft Pubs. Tabel berisi dua kolom yang tidak dapat diubah ke null dan dua kolom yang dapat diubah ke 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.
Keterangan
Untuk informasi selengkapnya tentang API ini, lihat Keterangan API Tambahan untuk Nullable<T>.
Konstruktor
Nullable<T>(T) |
Menginisialisasi instans Nullable<T> baru struktur ke nilai yang ditentukan. |
Properti
HasValue |
Mendapatkan nilai yang menunjukkan apakah objek saat ini Nullable<T> memiliki nilai yang valid dari jenis yang mendasarnya. |
Value |
Mendapatkan nilai objek saat ini Nullable<T> jika telah diberi nilai yang mendasar yang valid. |
Metode
Equals(Object) |
Menunjukkan apakah objek saat ini Nullable<T> sama dengan objek yang ditentukan. |
GetHashCode() |
Mengambil kode hash objek yang dikembalikan oleh Value properti . |
GetValueOrDefault() |
Mengambil nilai objek saat ini Nullable<T> , atau nilai default dari jenis yang mendasar. |
GetValueOrDefault(T) |
Mengambil nilai objek saat ini Nullable<T> , atau nilai default yang ditentukan. |
ToString() |
Mengembalikan representasi teks dari nilai objek saat ini Nullable<T> . |
Operator
Explicit(Nullable<T> to T) |
Mendefinisikan konversi Nullable<T> eksplisit instans ke nilai yang mendasarnya. |
Implicit(T to Nullable<T>) |
Membuat objek baru Nullable<T> yang diinisialisasi ke nilai tertentu. |