ConditionalWeakTable<TKey,TValue>.GetOrCreateValue(TKey) 方法

定義

命名空間:

System.Runtime.CompilerServices

組件:

System.Runtime.dll

來源:

ConditionalWeakTable.cs

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資料表中，則此方法會叫用表示資料表值之類別的無參數建構函式，以建立繫結至指定索引鍵的值。

public:
 TValue GetOrCreateValue(TKey key);

public TValue GetOrCreateValue (TKey key);

member this.GetOrCreateValue : 'Key -> 'Value

Public Function GetOrCreateValue (key As TKey) As TValue

參數

key

TKey

要搜尋的索引鍵。 key 代表要附加屬性的物件。

傳回

TValue

如果 key 已存在於資料表中，則為對應至 key 的值；否則為 TValue 泛型型別參數定義之類別的無參數建構函式所建立新值。

例外狀況

ArgumentNullException

key 為 null。

MissingMethodException

表示資料表值未定義無參數建構函式的類別。

注意：在適用於 Microsoft Store 的 .NET 應用程式或可攜式類別庫中，請改為捕捉基底類別例外狀況 MissingMemberException。

範例

下列範例會定義 類別 MainClass 和 MainInfo 類別，以提供 實例的相關信息 MainClass 。 此範例會GetOrCreateValue呼叫 方法，將 物件及其附加MainInfo物件ConditionalWeakTable<TKey,TValue>加入MainClass至數據表。 此範例也會說明對 和 GetValue 方法的呼叫Add，以將索引鍵/值組新增至數據表，以及TryGetValue擷取現有索引鍵值的方法。

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，方法會加入它，以及藉由呼叫泛型型別參數所定義 TValue 類別的無參數建構函式來具現化的物件。 TValue如果類別沒有無參數建構函式，MissingMethodException則會擲回 。

如果數據表的值類別定義無參數建構函式，則這是擷取現有值或將新值 ConditionalWeakTable<TKey,TValue> 加入數據表的建議方法。 如果未定義無參數建構函式，您可以改為呼叫 GetValue 方法，此方法依賴回呼提供的方法來具現化代表數據表值的物件

如果數據表中找不到索引鍵，則若要擷取現有索引鍵的值，而不新增索引鍵/值組，請呼叫 TryGetValue 方法。

另請參閱

• GetValue(TKey, ConditionalWeakTable<TKey,TValue>.CreateValueCallback)
• TryGetValue(TKey, TValue)