ConditionalWeakTable<TKey,TValue>.CreateValueCallback 委托

定义

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。Represents a method that creates a non-default value to add as part of a key/value pair to a ConditionalWeakTable<TKey,TValue> object.

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 

类型参数

TKey
TValue

参数

key

属于要创建的值的键。The key that belongs to the value to create.

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。An instance of a reference type that represents the value to attach to the specified key.

继承
ConditionalWeakTable<TKey,TValue>.CreateValueCallback

示例

下面的示例定义了MainClass一个类和MainInfo一个类。The following example defines a MainClass class and a MainInfo class. MainInfo提供有关实例的MainClass信息。MainInfo provides information about the MainClass instance. 它还定义了一个静态Shared (在 Visual Basic CreateAttachedValue ) 方法, 该方法ConditionalWeakTable<TKey,TValue>.CreateValueCallback可分配给委托并传递GetValue给方法。It also defines a static (Shared in Visual Basic) CreateAttachedValue method that can be assigned to the ConditionalWeakTable<TKey,TValue>.CreateValueCallback delegate and passed to the GetValue method. 该示例调用GetValue方法, MainClass将对象及其相应MainInfo的对象添加到ConditionalWeakTable<TKey,TValue>表中。The example calls the GetValue method to add a MainClass object and its corresponding MainInfo object to a ConditionalWeakTable<TKey,TValue> table. 该示例还演示了对和Add GetOrCreateValue方法的调用, 以便向表中添加键/值对, TryGetValue并对方法的调用以检索属于现有键的值。The example also illustrates calls to the Add and GetOrCreateValue methods to add key/value pairs to the table, and a call to the TryGetValue method to retrieve the value that belongs to an existing key.

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

委托封装一个回调方法, 当在ConditionalWeakTable<TKey,TValue>对象中找ConditionalWeakTable<TKey,TValue>.GetValue不到传递给方法的键时, 该方法将调用此方法。 ConditionalWeakTable<TKey,TValue>.CreateValueCallbackThe ConditionalWeakTable<TKey,TValue>.CreateValueCallback delegate encapsulates a callback method that is invoked by the ConditionalWeakTable<TKey,TValue>.GetValue method when the key passed to the method is not found in the ConditionalWeakTable<TKey,TValue> object. GetValue方法传递回调方法密钥, 该密钥表示要向其动态附加属性值的托管对象。The GetValue method passes the callback method the key that represents a managed object to which a property value is to be dynamically attached. 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。The method is responsible for returning the property value to its caller, which, in turn, adds the key/value pair to the table.

ConditionalWeakTable<TKey,TValue>.CreateValueCallback方法用于返回可附加到指定键的引用类型的实例。The ConditionalWeakTable<TKey,TValue>.CreateValueCallback method is used to return an instance of a reference type that can be attached to the specified key. 它允许使用非默认值初始化该实例。It enables that instance to be initialized using non-default values. 如果在表中找不到该键, 则ConditionalWeakTable<TKey,TValue>.GetOrCreateValue方法添加一个键/值对, 其中使用默认值初始化值。If the key is not found in the table, the ConditionalWeakTable<TKey,TValue>.GetOrCreateValue method adds a key/value pair in which the value is initialized using default values.

扩展方法

GetMethodInfo(Delegate)

获取指示指定委托表示的方法的对象。Gets an object that represents the method represented by the specified delegate.

适用于

另请参阅