ConditionalWeakTable<TKey,TValue>.GetValue 方法

參考

定義

命名空間:: System.Runtime.CompilerServices

組件:: System.Runtime.dll

組件:: mscorlib.dll

組件:: netstandard.dll

來源:: ConditionalWeakTable.cs

來源:: ConditionalWeakTable.cs

來源:: ConditionalWeakTable.cs

重要

部分資訊涉及發行前產品，在發行之前可能會有大幅修改。 Microsoft 對此處提供的資訊，不做任何明確或隱含的瑕疵擔保。

以不可分割方式搜尋表格中的指定索引鍵並傳回對應值。如果索引鍵不存在於表格中，則此方法會叫用回呼方法來建立繫結至指定索引鍵的值。

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

參數

key: TKey

要搜尋的索引鍵。 key 代表要附加屬性的物件。

createValueCallback: ConditionalWeakTable<TKey,TValue>.CreateValueCallback

方法的委派，可針對指定的 key 建立值。它具有類型的 TKey單一參數，並傳回類型的 TValue值。

傳回

TValue

如果 key 已存在於表格中，則為附加至 key 的值，否則為 createValueCallback 委派所傳回的新值。

例外狀況

ArgumentNullException

key 或 createValueCallback 為 null。

範例

下列範例會 MainClass 定義類別和 MainInfo 類別，以提供實例的相關信息 MainClass 。它也會在Visual Basic) 方法中定義靜態 (SharedCreateAttachedValue ，這個方法可以指派給 ConditionalWeakTable<TKey,TValue>.CreateValueCallback 委派並傳遞至 GetValue 方法。此範例會GetValue呼叫方法，將物件及其附加MainInfo物件ConditionalWeakTable<TKey,TValue>加入MainClass數據表。此範例也會說明呼叫 Add 和 GetOrCreateValue 方法，以將索引鍵/值組新增至數據表，以及 TryGetValue 擷取現有索引鍵的值的方法。

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

備註

如果 key 數據表中不存在， GetValue 請叫用參數所 createValueCallback 定義的方法，並傳遞索引鍵。新的值會系結至數據表中的索引鍵，並傳回作為結果。

只有當表示數據表值的類別未定義無參數建構函式時，才使用這個方法。如果它確實定義無參數建構函式，請改用 GetOrCreateValue 方法。若要在數據表中找不到索引鍵/值組的情況下擷取現有索引鍵的值，請呼叫 TryGetValue 方法。

如果多個線程嘗試建立相同的密鑰， createValueCallback 可能會使用相同的索引鍵多次叫用。只有其中一個呼叫成功，而且其傳回的值將會新增至數據表。哪個線程成功建立值是不確定的。此規則允許數據表在內部數據表鎖定之外叫 createValueCallback 用，以防止死結。

適用於

共用方式為

ConditionalWeakTable<TKey,TValue>.GetValue 方法

定義

參數

傳回

例外狀況

範例

備註

適用於

另請參閱

其他資源