ConditionalWeakTable<TKey,TValue>.CreateValueCallback 委托

参考

定义

命名空间:: System.Runtime.CompilerServices

程序集:: System.Runtime.dll

程序集:: mscorlib.dll

程序集:: netstandard.dll

重要

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表示一种方法，用于创建作为键/值对的一部分添加到 ConditionalWeakTable<TKey,TValue> 对象的非默认值。

public:  where TKey : class where TValue : classdelegate TValue ConditionalWeakTable<TKey, TValue>::CreateValueCallback(TKey key);

public delegate TValue ConditionalWeakTable<TKey,TValue>.CreateValueCallback(TKey key) where TKey : class where TValue : class;

type ConditionalWeakTable<'Key, 'Value (requires 'Key : null and 'Value : null)>.CreateValueCallback = delegate of 'Key -> 'Value

Public Delegate Function ConditionalWeakTable(Of TKey, TValue).CreateValueCallback(key As TKey) As TValue

参数

key: TKey

属于要创建的值的键。

返回值

TValue

一个引用类型的实例，该类型表示要附加到指定键的值。

示例

以下示例定义一个 MainClass 类和一个 MainInfo 类。 MainInfo 提供有关实例的信息 MainClass 。它还在Visual Basic) CreateAttachedValue方法中定义静态 (Shared，该方法可以分配给ConditionalWeakTable<TKey,TValue>.CreateValueCallback委托并传递给GetValue该方法。该示例调用GetValue该方法将对象及其对应的MainInfo对象添加到MainClassConditionalWeakTable<TKey,TValue>表中。该示例还演示了对 Add 向表添加键/值对的调用以及 GetOrCreateValue 检索 TryGetValue 属于现有键的值的方法的调用。

using System;
using System.Reflection;
using System.Runtime.CompilerServices;

public class Example
{
   string Name; 
   
   public Example(string name)
   {
      this.Name = name;
   }
   
   public override string ToString()
   {
      return this.Name;
   }
}

// Define a class to contain information about each Example instance.
public class ExampleInfo
{
   public string Name;
   public int Methods;
   public int Properties;
   
   public override string ToString()
   {
      return String.Format("{0}: {1} Methods, {2} Properties", 
                           this.Name, this.Methods, this.Properties);
   }
}

public class ExampleTest
{
   private static BindingFlags flags = BindingFlags.Public | BindingFlags.Instance;

   public static void Main()
   {
      Example ex1 = new Example("ex1");
      Example ex2 = new Example("ex2");
      Example ex3 = new Example("ex3");
      
      ExampleInfo exInfo1 = new ExampleInfo(); 
      exInfo1.Name = ex1.ToString();
      exInfo1.Methods = ex1.GetType().GetMethods(flags).Length;
      exInfo1.Properties = ex1.GetType().GetProperties(flags).Length;
      
      ExampleInfo exInfo3 = new ExampleInfo(); 
      exInfo3.Name = ex3.ToString();
      exInfo3.Methods = ex3.GetType().GetMethods(flags).Length;
      exInfo3.Properties = ex3.GetType().GetProperties(flags).Length;

      var attached = new ConditionalWeakTable<Example, ExampleInfo>();
      ExampleInfo value = null;

      // Attach a property to ex1 using the Add method, then retrieve it.
      attached.Add(ex1, exInfo1);
      if (attached.TryGetValue(ex1, out value))
         Console.WriteLine("{0}, {1}", ex1, value);
      else
         Console.WriteLine("{0} does not have an attached property.", ex1);

      // Attempt to retrieve the value attached to ex2.
      value = attached.GetValue(ex2, ExampleTest.CreateAttachedValue);      
      if (attached.TryGetValue(ex2, out value))
         Console.WriteLine("{0}, {1}", ex2, value);
      else 
         Console.WriteLine("{0} does not have an attached property.", ex2);
      
      // Attempt to retrieve the value attached to ex3.
      value = attached.GetOrCreateValue(ex3);
      Console.WriteLine("{0}, {1}", ex3, value);
   }

   public static ExampleInfo CreateAttachedValue(Example ex)
   {
      ExampleInfo info = new ExampleInfo();
      info.Name = ex.ToString();
      info.Methods = ex.GetType().GetMethods(flags).Length;
      info.Properties = ex.GetType().GetProperties(flags).Length;
      return info;
   }
}
// The example displays the following output:
//       ex1, ex1: 4 Methods, 0 Properties
//       ex2, ex2: 4 Methods, 0 Properties
//       ex3, : 0 Methods, 0 Properties

Imports System.Reflection
Imports System.Runtime.CompilerServices

Public Class Example
   Public Name As String
   
   Public Sub New(name As String)
      Me.Name = name
   End Sub
   
   Public Overrides Function ToString() As String
      Return Me.Name
   End Function
End Class

' Define a class to contain information about each Example instance.
Public Class ExampleInfo
   Public Name As String
   Public Methods As Integer
   Public Properties As Integer
   
   Public Overrides Function ToString() As String
      Return String.Format("{0}: {1} Methods, {2} Properties", _
                           Me.Name, Me.Methods, Me.Properties)
   End Function
End Class

Module TestExample

   Private flags As BindingFlags = BindingFlags.Public Or BindingFlags.Instance

   Public Sub Main()
      Dim ex1 As New Example("ex1")
      Dim ex2 As New Example("ex2")
      Dim ex3 As New Example("ex3")
      
      Dim exInfo1 As New ExampleInfo() 
      exInfo1.Name = ex1.ToString()
      exInfo1.Methods = ex1.GetType().GetMethods(flags).Count
      exInfo1.Properties = ex1.GetType().GetProperties(flags).Count
      
      Dim exInfo3 As New ExampleInfo() 
      exInfo3.Name = ex3.ToString()
      exInfo3.Methods = ex3.GetType().GetMethods(flags).Count
      exInfo3.Properties = ex3.GetType().GetProperties(flags).Count

      Dim attached As New ConditionalWeakTable(Of Example, ExampleInfo)
      Dim value As ExampleInfo = Nothing

      ' Attach a property to ex1 using the Add method, then retrieve it.
      attached.Add(ex1, exInfo1)
      If attached.TryGetValue(ex1, value) Then
         Console.WriteLine("{0}, {1}", ex1, value)
      Else
         Console.WriteLine("{0} does not have an attached property.", ex1)
      End If

      ' Attempt to retrieve the value attached to ex2.
      value = attached.GetValue(ex2, AddressOf TestExample.CreateAttachedValue)      
      If attached.TryGetValue(ex2, value) Then
         Console.WriteLine("{0}, {1}", ex2, value)
      Else 
         Console.WriteLine("{0} does not have an attached property.", ex2)
      End If
      
      ' Atttempt to retrieve the value attached to ex3.
      value = attached.GetOrCreateValue(ex3)
      Console.WriteLine("{0}, {1}", ex3, value)
   End Sub
   
   Public Function CreateAttachedValue(ex As Example) As ExampleInfo
      Dim info As New ExampleInfo()
      info.Name = ex.ToString()
      info.Methods = ex.GetType().GetMethods(flags).Count
      info.Properties = ex.GetType().GetProperties(flags).Count
      Return info
   End Function
End Module
' The example displays the following output:
'       ex1, ex1: 4 Methods, 0 Properties
'       ex2, ex2: 4 Methods, 0 Properties
'       ex3, : 0 Methods, 0 Properties

注解

委托 ConditionalWeakTable<TKey,TValue>.CreateValueCallback 封装在对象中找不到传递给该方法的键时由该方法调用 ConditionalWeakTable<TKey,TValue>.GetValue 的 ConditionalWeakTable<TKey,TValue> 回调方法。该方法 GetValue 将回调方法传递给表示要动态附加属性值的托管对象的键。该方法负责将属性值返回到其调用方，进而将键/值对添加到表中。

该方法 ConditionalWeakTable<TKey,TValue>.CreateValueCallback 用于返回可附加到指定键的引用类型的实例。它允许使用该实例使用非默认值进行初始化。如果未在表中找到该键，该方法 ConditionalWeakTable<TKey,TValue>.GetOrCreateValue 将添加一个键/值对，在该对中，该值使用默认值进行初始化。

扩展方法

GetMethodInfo(Delegate)

获取指示指定委托表示的方法的对象。

适用于

另请参阅

GetValue(TKey, ConditionalWeakTable<TKey,TValue>.CreateValueCallback)

ConditionalWeakTable<TKey,TValue>.CreateValueCallback 委托

定义

参数

返回值

示例

注解

扩展方法

适用于

另请参阅

其他资源