Nullable<T> Nullable<T> Nullable<T> Nullable<T> Struct

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
[System.Serializable]
public struct Nullable<T> where T : struct
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>Nullable<T>Nullable<T>Nullable<T>
屬性

範例

下列程式碼範例會定義 Microsoft Pubs 範例資料庫中資料表的三個資料列。The following code example defines three rows of a table in the Microsoft Pubs sample database. 此資料表包含不可以是 null 的兩個資料行和兩個資料行可為 null。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

型別即為可為 null,如果它可以指派值,或可以指派null,這表示型別沒有任何值。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. 根據預設,所有必須參考型別,例如String,可為 null,但所有實值類型,例如Int32,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,您將標示為可為 null 的實值型別使用?實值型別後的標記法。In C# and Visual Basic, you mark a value type as nullable by using the ? notation after the value type. 例如, int? C# 中或Integer?Visual Basic 中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,支援使用只有實值類型為 null 的型別,因為參考型別可以是 null 的設計。The Nullable<T> structure supports using only a value type as a nullable type because reference types are nullable by design.

Nullable類別會提供補充支援Nullable<T>結構。The Nullable class provides complementary support for the Nullable<T> structure. Nullable類別支援取得基礎型別可為 null 的型別,並對其基礎實值型別可為 null 類型的比較和等號比較作業不支援泛型的比較和等號比較作業。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.

基本內容Fundamental Properties

兩個基本成員Nullable<T>結構是否HasValueValue屬性。The two fundamental members of the Nullable<T> structure are the HasValue and Value properties. 如果HasValue屬性Nullable<T>物件true,可以使用存取物件的值Value屬性。If 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屬性是false,物件的值會是未定義並嘗試存取Value屬性會擲回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

當可為 null 的型別為 boxed 時,common language runtime 自動方塊的基礎值Nullable<T>物件,不Nullable<T>物件本身。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,內容Value屬性會進行 boxed 處理。That is, if the HasValue property is true, the contents of the Value property is boxed. Unboxed 實的基礎值為 null 的類型時,通用語言執行平台建立新Nullable<T>結構初始化為基礎的值。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屬性為 null 的類型是false,boxing 作業的結果是nullIf the HasValue property of a nullable type is false, the result of a boxing operation is null. 因此,如果經過 boxing 處理 null 的型別傳遞至預期的物件引數的方法,該方法必須準備好處理的情況,其中的引數是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. null是 unbox 處理成可為 null 的型別,通用語言執行平台建立新Nullable<T>結構,並初始化其HasValue屬性設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

開頭.NET Framework 4.5.1.NET Framework 4.5.1,您可以包含Nullable<T>WinMD 程式庫中匯出的結構成員的型別。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,這不支援。Previously, this was not supported.

建構函式

Nullable<T>(T) Nullable<T>(T) Nullable<T>(T) 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 HasValue HasValue 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 Value Value 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) Equals(Object) Equals(Object) 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() GetHashCode() GetHashCode() 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() GetValueOrDefault() GetValueOrDefault() 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) GetValueOrDefault(T) GetValueOrDefault(T) 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() ToString() ToString() 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) Explicit(Nullable<T> to T) Explicit(Nullable<T> to T) 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>) Implicit(T to Nullable<T>) Implicit(T to Nullable<T>) 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

另請參閱