Nullable<T> 结构

参考

定义

命名空间:: System

程序集:: System.Runtime.dll

程序集:: mscorlib.dll

程序集:: netstandard.dll

Source:: Nullable.cs

Source:: Nullable.cs

Source:: Nullable.cs

重要

一些信息与预发行产品相关，相应产品在发行之前可能会进行重大修改。对于此处提供的信息，Microsoft 不作任何明示或暗示的担保。

表示可分配有 null 的值类型。

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)

类型参数

T

Nullable<T> 泛型类型的基础值类型。

继承: Object

ValueType
Nullable<T>

属性: SerializableAttribute

示例

下面的代码示例定义 Microsoft Pubs 示例数据库中表的三行。该表包含两个不可为 null 的列和两个可以为 null 的列。

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.

注解

有关此 API 的详细信息，请参阅可为 Null T<> 的补充 API 备注。

构造函数

Nullable<T>(T)

将 Nullable<T> 结构的新实例初始化为指定值。

属性

HasValue	获取一个值，该值指示 Nullable<T> 对象是否具有基础类型的有效值。
Value	获取当前 Nullable<T> 对象的值，如果它已被分配了有效的基础值。

方法

Equals(Object)	指示当前 Nullable<T> 对象是否与指定的对象相等。
GetHashCode()	检索由 Value 属性返回的对象的哈希代码。
GetValueOrDefault()	检索当前 Nullable<T> 对象的值，或基础类型的默认值。
GetValueOrDefault(T)	检索当前 Nullable<T> 对象的值或指定的默认值。
ToString()	返回当前 Nullable<T> 对象的值的文本表示形式。

运算符

Explicit(Nullable<T> to T)	定义 Nullable<T> 实例到其基础值的显式转换。
Implicit(T to Nullable<T>)	创建一个新的 Nullable<T> 对象，并将其初始化为指定的值。

适用于

通过

Nullable<T> 结构

定义

类型参数

示例

注解

构造函数

属性

方法

运算符

适用于

另请参阅

其他资源