ConditionalWeakTable<TKey,TValue>.GetValue 方法

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

Source:: ConditionalWeakTable.cs

Source:: ConditionalWeakTable.cs

Source:: ConditionalWeakTable.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。

以原子方式在表中搜索指定键，并返回对应的值。如果表中不存在该键，此方法将调用一个回调方法来创建绑定到指定键的值。

public:
 TValue GetValue(TKey key, System::Runtime::CompilerServices::ConditionalWeakTable<TKey, TValue>::CreateValueCallback ^ createValueCallback);

public TValue GetValue (TKey key, System.Runtime.CompilerServices.ConditionalWeakTable<TKey,TValue>.CreateValueCallback createValueCallback);

member this.GetValue : 'Key * System.Runtime.CompilerServices.ConditionalWeakTable<'Key, 'Value (requires 'Key : null and 'Value : null)>.CreateValueCallback -> 'Value

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

参数

key: TKey

要搜索的键。 key 表示该属性附加到的对象。

createValueCallback: ConditionalWeakTable<TKey,TValue>.CreateValueCallback

可以为给定 key 创建值的方法的委托。它具有类型的 TKey单个参数，并返回类型的 TValue值。

TValue

如果表中已存在 key，则为附加到 key 的值；否则为 createValueCallback 委托返回的新值。

例外

ArgumentNullException

key 或 createValueCallback 为 null。

示例

以下示例定义一个 MainClass 类和一个 MainInfo 类，它们提供有关实例 MainClass 的信息。它还在 Visual Basic) 方法中定义静态 (Shared ，该静态 (CreateAttachedValue 可以分配给 ConditionalWeakTable<TKey,TValue>.CreateValueCallback 委托并传递给 GetValue 方法。该示例调用 GetValue 方法以将对象及其附加MainInfo对象添加到MainClassConditionalWeakTable<TKey,TValue>表中。该示例还演示了调用 Add 和 GetOrCreateValue 方法以将键/值对添加到表，并调用 TryGetValue 方法以检索现有键的值。

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

如果 key 表中不存在， GetValue 则调用由 createValueCallback 参数定义的方法，并将密钥传递给它。新值绑定到表中的键，并作为结果返回。

仅当表示表值的类未定义无参数构造函数时，才使用此方法。如果它确实定义了无参数构造函数，请改用 GetOrCreateValue 方法。若要在表中找不到键时检索现有键的值，而不添加键/值对，请调用 TryGetValue 方法。

如果多个线程尝试创建同一个密钥， createValueCallback 可以使用同一键多次调用。其中只有一个调用会成功，其返回的值将添加到表中。哪个线程成功创建值是不确定的。此规则允许表在内部表锁外部调用 createValueCallback 以防止死锁。

适用于

通过

ConditionalWeakTable<TKey,TValue>.GetValue 方法

定义

参数

返回

例外

示例

注解

适用于

另请参阅

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