ConditionalWeakTable<TKey,TValue>.GetOrCreateValue(TKey) 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,此方法将调用表示表值的类的无参数构造函数,以创建绑定到指定键的值。If the key does not exist in the table, the method invokes the parameterless constructor of the class that represents the table's value to create a value that is bound to the specified key.

public:
 TValue GetOrCreateValue(TKey key);
public TValue GetOrCreateValue (TKey key);
member this.GetOrCreateValue : 'Key -> 'Value
Public Function GetOrCreateValue (key As TKey) 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.

返回

TValue

如果表中已存在 key,则为对应于 key 的值;否则为 TValue 范型类型参数定义的类的无参数构造函数创建的新值。The value that corresponds to key, if key already exists in the table; otherwise, a new value created by the parameterless constructor of the class defined by the TValue generic type parameter.

例外

keynullkey is null.

表示表中值的类未定义无参数构造函数。The class that represents the table's value does not define a parameterless constructor.

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. 该示例调用 GetOrCreateValue 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 GetOrCreateValue 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 GetValue 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 GetValue 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,则该方法将添加它以及通过调用泛型类型参数定义的类的无参数构造函数进行实例化的对象 TValueIf key does not exist in the table, the method adds it, along with the object that is instantiated by calling the parameterless constructor of the class defined by the TValue generic type parameter. 如果 TValue 类没有无参数构造函数, MissingMethodException 则将引发。If the TValue class has no parameterless constructor, a MissingMethodException is thrown.

ConditionalWeakTable<TKey,TValue>如果表值的类定义无参数的构造函数,则建议使用此方法检索现有值或向表中添加新值。This is the recommended method of retrieving an existing value or adding a new value to the ConditionalWeakTable<TKey,TValue> table if the class of the table's value defines a parameterless constructor. 如果它未定义无参数构造函数,则可以改为调用 GetValue 方法,该方法依赖于回调提供的方法来实例化表示表值的对象If it does not define a parameterless constructor, you can instead call the GetValue method, which relies on a callback-provided method to instantiate the object representing the table's value

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.

适用于

另请参阅