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

定义

以原子方式在表中搜索指定键,并返回对应的值。Atomically searches for a specified key in the table and returns the corresponding value. 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。If the key does not exist in the table, the method invokes a callback method to create a value that is bound to the specified key.

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。The key to search for. key 表示该属性附加到的对象。key represents the object to which the property is attached.

createValueCallback
ConditionalWeakTable<TKey,TValue>.CreateValueCallback

可以为给定 key 创建值的方法的委托。A delegate to a method that can create a value for the given key. 它只有一个 TKey 类型的参数,并返回一个 TValue 类型的值。It has a single parameter of type TKey, and returns a value of type TValue.

返回

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。The value attached to key, if key already exists in the table; otherwise, the new value returned by the createValueCallback delegate.

例外

keycreateValueCallbacknullkey or createValueCallback is null.

示例

下面的示例定义了一个 MainClass 类和一个 MainInfo 类,该类提供有关实例的信息 MainClassThe following example defines a MainClass class and a MainInfo class, which provides information about the MainClass instance. 它还定义了 Shared CreateAttachedValue 可分配给 ConditionalWeakTable<TKey,TValue>.CreateValueCallback 委托并传递给方法的 Visual Basic) 方法中的静态 (GetValueIt 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,将 MainClass 对象及其附加 MainInfo 对象添加到 ConditionalWeakTable<TKey,TValue> 表中。The example calls the GetValue method to add a MainClass object and its attached MainInfo object to a ConditionalWeakTable<TKey,TValue> table. 该示例还演示了对和方法的调用, Add GetOrCreateValue 以向表中添加键/值对,并演示了 TryGetValue 方法以检索现有键的值。The example also illustrates calls to the Add and GetOrCreateValue methods to add key/value pairs to the table, and to the TryGetValue method to retrieve the value of 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

如果 key 表中不存在,则 GetValue 调用由参数定义的方法, createValueCallback 并向其传递密钥。If key does not exist in the table, GetValue invokes the method that is defined by the createValueCallback parameter and passes it the key. 新值绑定到表中的键,并返回结果。A new value is bound to the key in the table and returned as a result.

仅当表示表值的类未定义无参数的构造函数时,才使用此方法。Use this method only when the class that represents the table's value does not define a parameterless constructor. 如果它定义了无参数的构造函数,请改用 GetOrCreateValue 方法。If it does define a parameterless constructor, use the GetOrCreateValue method instead. 若要检索现有键的值而不添加键/值对(如果在表中找不到该键),请调用 TryGetValue 方法。To retrieve the value of an existing key without adding the key/value pair if the key is not found in the table, call the TryGetValue method.

如果多个线程尝试创建相同的键,则 createValueCallback 可以用相同的键多次调用。If multiple threads try to create the same key, createValueCallback may be invoked multiple times with the same key. 只有其中一个调用将成功,并且其返回值将添加到表中。Only one of these calls will succeed, and its returned value will be added to the table. 哪个线程成功创建值是不确定的。Which thread succeeds in creating the value is indeterminate. 此规则允许表在 createValueCallback 内部表锁之外调用以防止死锁。This rule permits the table to invoke createValueCallback outside the internal table lock to prevent deadlocks.

适用于

另请参阅