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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
)

Parameters

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);
      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

.NET Framework

Supported in: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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