ConditionalWeakTable<TKey,TValue>.CreateValueCallback 委托
定义
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MainClass
ConditionalWeakTable<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。 |