Nullable<T> 結構

定義

表示可以指派 null 的實值類型。Represents a value type that can be assigned 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)

類型參數

T

Nullable<T> 泛型型別的基礎實值型別。The underlying value type of the Nullable<T> generic type.

繼承
Nullable<T>
屬性

範例

下列程式碼範例會在 Microsoft Pubs 範例資料庫中,定義資料表的三個數據列。The following code example defines three rows of a table in the Microsoft Pubs sample database. 資料表包含兩個不可為 null 的資料行,以及兩個可為 null 的資料行。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.

備註

如果您可以指派值或指派值給類型,這表示類型可以是可為 null 的類型 null ,這表示類型沒有任何值。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. 依預設,所有的參考型別(例如 String )都可以是可為 null,但所有實值型別(例如 Int32 )都不是。By default, all reference types, such as String, are nullable, but all value types, such as Int32, are not.

在 c # 和 Visual Basic 中,您可以使用 ? 值型別後面的標記法,將實值型別標示為可為 null。In C# and Visual Basic, you mark a value type as nullable by using the ? notation after the value type. 例如, int? 在 c # 或 Integer? 中 Visual Basic 會宣告可指派的整數值型別 nullFor example, int? in C# or Integer? in Visual Basic declares an integer value type that can be assigned null.

Nullable<T>結構僅支援使用實值型別作為可為 null 的型別,因為參考型別可依設計為 null。The Nullable<T> structure supports using only a value type as a nullable type because reference types are nullable by design.

Nullable類別會為結構提供互補的支援 Nullable<T>The Nullable class provides complementary support for the Nullable<T> structure. Nullable類別支援取得可為 null 型別的基礎型別,以及其基礎實值型別不支援泛型比較和等號比較運算的可為 null 類型配對的比較和等號比較運算。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.

基本屬性Fundamental Properties

結構的兩個基本成員為 Nullable<T> HasValueValue 屬性。The two fundamental members of the Nullable<T> structure are the HasValue and Value properties. 如果 HasValue 物件的屬性 Nullable<T> 是,就 true 可以使用屬性來存取物件的值 ValueIf the HasValue property for a Nullable<T> object is true, the value of the object can be accessed with the Value property. 如果 HasValue 屬性為 false ,則物件的值為未定義,而且嘗試存取屬性時,會擲回 Value InvalidOperationExceptionIf the HasValue property is false, the value of the object is undefined and an attempt to access the Value property throws an InvalidOperationException.

Boxing 和 UnboxingBoxing and Unboxing

當有可為 null 的型別時,common language runtime 會自動將物件的基礎值 Nullable<T> (而不是物件本身)作為方塊 Nullable<T>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. 也就是說,如果 HasValue 屬性是 true ,就會將屬性的內容 Value 裝箱。That is, if the HasValue property is true, the contents of the Value property is boxed. 當可為 null 類型的基礎值為取消裝箱時,common language runtime 會建立 Nullable<T> 初始化為基礎值的新結構。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.

如果 HasValue 可為 null 之類型的屬性為 false ,則會將裝箱作業的結果設為 nullIf the HasValue property of a nullable type is false, the result of a boxing operation is null. 因此,如果將可為 null 的可為 null 型別傳遞至需要物件引數的方法,則該方法必須準備好處理引數為的情況 nullConsequently, 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. 當取消 null 裝箱成為可為 null 的型別時,common language runtime 會建立新的 Nullable<T> 結構,並 HasValue 將其屬性初始化為 falseWhen 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 4.5.1 和 Windows 執行階段元件and Windows Runtime Components

從開始 .NET Framework 4.5.1.NET Framework 4.5.1 ,您可以將型別包含 Nullable<T> 在 WinMD 程式庫中匯出之結構的成員。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. 先前並不支援這種情況。Previously, this was not supported.

建構函式

Nullable<T>(T)

Nullable<T> 結構的新執行個體初始化為指定值。Initializes a new instance of the Nullable<T> structure to the specified value.

屬性

HasValue

取得值,指出目前的 Nullable<T> 物件是否具有其基礎類型的有效值。Gets a value indicating whether the current Nullable<T> object has a valid value of its underlying type.

Value

取得目前 Nullable<T> 物件的值,如果這個物件已有指派的有效基礎值。Gets the value of the current Nullable<T> object if it has been assigned a valid underlying value.

方法

Equals(Object)

指示目前的 Nullable<T> 物件是否等於指定的物件。Indicates whether the current Nullable<T> object is equal to a specified object.

GetHashCode()

擷取 Value 屬性所傳回之物件的雜湊碼。Retrieves the hash code of the object returned by the Value property.

GetValueOrDefault()

擷取目前 Nullable<T> 物件的值,或底層型別的預設值。Retrieves the value of the current Nullable<T> object, or the default value of the underlying type.

GetValueOrDefault(T)

擷取目前 Nullable<T> 物件的值,或指定的預設值。Retrieves the value of the current Nullable<T> object, or the specified default value.

ToString()

傳回目前 Nullable<T> 物件值的文字表示。Returns the text representation of the value of the current Nullable<T> object.

運算子

Explicit(Nullable<T> to T)

定義 Nullable<T> 執行個體到其基礎值的明確轉換。Defines an explicit conversion of a Nullable<T> instance to its underlying value.

Implicit(T to Nullable<T>)

建立新 Nullable<T> 已經初始化為指定值的物件。Creates a new Nullable<T> object initialized to a specified value.

適用於

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