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ConditionalWeakTable<TKey, TValue>.CreateValueCallback Delegate

Represents a method that creates a non-default value to add as part of a key/value pair to a ConditionalWeakTable<TKey, TValue> object.

Namespace:  System.Runtime.CompilerServices
Assembly:  mscorlib (in mscorlib.dll)

public delegate TValue CreateValueCallback(
	TKey key


Type: TKey
The key that belongs to the value to create.

Return Value

Type: TValue
An instance of a reference type that represents the value to attach to the specified key.

The ConditionalWeakTable<TKey, TValue>.CreateValueCallback delegate encapsulates a callback method that is invoked by the ConditionalWeakTable<TKey, TValue>.GetValue method when the key passed to the method is not found in the ConditionalWeakTable<TKey, TValue> object. The GetValue method passes the callback method the key that represents a managed object to which a property value is to be dynamically attached. The method is responsible for returning the property value to its caller, which, in turn, adds the key/value pair to the table.

The ConditionalWeakTable<TKey, TValue>.CreateValueCallback method is used to return an instance of a reference type that can be attached to the specified key. It enables that instance to be initialized using non-default values. If the key is not found in the table, the ConditionalWeakTable<TKey, TValue>.GetOrCreateValue method adds a key/value pair in which the value is initialized using default values.

The following example defines a MainClass class and a MainInfo class. MainInfo provides information about the MainClass instance. It 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. The example calls the GetValue method to add a MainClass object and its corresponding MainInfo object to a ConditionalWeakTable<TKey, TValue> table. The example also illustrates calls to the Add and GetOrCreateValue methods to add key/value pairs to the table, and a call to the TryGetValue method to retrieve the value that belongs to 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);
         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);
         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

.NET Framework

Supported in: 4

.NET Framework Client Profile

Supported in: 4

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.