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ConditionalWeakTable<TKey, TValue> Class

Enables compilers to dynamically attach object fields to managed objects.

System.Object
  System.Runtime.CompilerServices.ConditionalWeakTable<TKey, TValue>

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

[ComVisibleAttribute(false)]
public sealed class ConditionalWeakTable<TKey, TValue>
where TKey : class 
where TValue : class

Type Parameters

TKey

The reference type to which the field is attached.

TValue

The field's type. This must be a reference type.

The ConditionalWeakTable<TKey, TValue> type exposes the following members.

  NameDescription
Public methodSupported by Portable Class LibraryConditionalWeakTable<TKey, TValue>Initializes a new instance of the ConditionalWeakTable<TKey, TValue> class.
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  NameDescription
Public methodSupported by Portable Class LibraryAddAdds a key to the table.
Public methodSupported by Portable Class LibraryEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodFinalizeEnsures that resources are freed and other cleanup operations are performed when the garbage collector reclaims the ConditionalWeakTable<TKey, TValue> object. (Overrides Object.Finalize().)
Public methodSupported by Portable Class LibraryGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodSupported by Portable Class LibraryGetOrCreateValueAtomically searches for a specified key in the table and returns the corresponding value. If the key does not exist in the table, the method invokes the default constructor of the class that represents the table's value to create a value that is bound to the specified key.
Public methodSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodSupported by Portable Class LibraryGetValueAtomically searches for a specified key in the table and returns the corresponding value. If the key does not exist in the table, the method invokes a callback method to create a value that is bound to the specified key.
Public methodSupported by Portable Class LibraryRemoveRemoves a key and its value from the table.
Public methodSupported by Portable Class LibraryToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodSupported by Portable Class LibraryTryGetValueGets the value of the specified key.
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The ConditionalWeakTable<TKey, TValue> class enables language compilers to attach arbitrary properties to managed objects at run time. A ConditionalWeakTable<TKey, TValue> object is a dictionary that binds a managed object, which is represented by a key, to its attached property, which is represented by a value. The object's keys are the individual instances of the TKey class to which the property is attached, and its values are the property values that are assigned to the corresponding objects.

Keys must be unique; in other words, the ConditionalWeakTable<TKey, TValue> class supports one attached value per managed object. Two keys are equal if passing them to the Object.ReferenceEquals method returns true.

NoteNote

You cannot control equality comparisons by overriding Object.GetHashCode to explicitly set the hash code for a key. The ConditionalWeakTable<TKey, TValue> class does not use the Object.GetHashCode method to compute hash codes, and therefore does not invoke Object.GetHashCode overrides.

Although the ConditionalWeakTable<TKey, TValue> class holds a collection of key/value pairs, it is best thought of as a table rather than a dictionary object. The ConditionalWeakTable<TKey, TValue> class differs from a dictionary in several ways:

  • It does not persist keys. That is, a key is not kept alive only because it is a member of the collection.

  • It does not include all the methods (such as GetEnumerator or Contains) that a dictionary typically has.

  • It does not implement the IDictionary<TKey, TValue> interface.

The ConditionalWeakTable<TKey, TValue> class differs from other collection objects in its management of the object lifetime of keys stored in the collection. Ordinarily, when an object is stored in a collection, its lifetime lasts until it is removed (and there are no additional references to the object) or until the collection object itself is destroyed. However, in the ConditionalWeakTable<TKey, TValue> class, adding a key/value pair to the table does not ensure that the key will persist, even if it can be reached directly from a value stored in the table (for example, if the table contains one key, A, with a value V1, and a second key, B, with a value P2 that contains a reference to A). Instead, ConditionalWeakTable<TKey, TValue> automatically removes the key/value entry as soon as no other references to a key exist outside the table. The example provides an illustration.

The following example illustrates that a key stored in the ConditionalWeakTable<TKey, TValue> table does not persist after references to it outside the table are destroyed. The example defines two classes: ManagedClass, which represents the key in the table, and AttachedProperty, which represents the key's value. The example instantiates three objects of each type. It also instantiates a WeakReference object that represents the second ManagedClass, and then destroys the second ManagedClass instance. The attempt to retrieve the second ManagedClass object from the Target property indicates that no references to the object remain.

using System;
using System.Runtime.CompilerServices;

public class Example
{
   public static void Main()
   {
      var mc1 = new ManagedClass();
      var mc2 = new ManagedClass();
      var mc3 = new ManagedClass();

      var cwt = new ConditionalWeakTable<ManagedClass, ClassData>();
      cwt.Add(mc1, new ClassData());          
      cwt.Add(mc2, new ClassData());
      cwt.Add(mc3, new ClassData());

      var wr2 = new WeakReference(mc2);
      mc2 = null;

      GC.Collect();

      ClassData data = null; 

      if (wr2.Target == null)
          Console.WriteLine("No strong reference to mc2 exists.");   
      else if (cwt.TryGetValue(mc2, out data))
          Console.WriteLine("Data created at {0}", data.CreationTime);      
      else
          Console.WriteLine("mc2 not found in the table.");
   }
}

public class ManagedClass
{ 
}

public class ClassData
{
   public DateTime CreationTime;
   public object Data;   

   public ClassData()
   {
      CreationTime = DateTime.Now;
      this.Data  = new object();     
   }
}
// The example displays the following output: 
//       No strong reference to mc2 exists.

.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

Instances of the ConditionalWeakTable<TKey, TValue> class are thread safe. They do not require callers to do any additional locking.

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