mcs/class/corlib/System.Runtime.CompilerServices/ConditionalWeakTable.cs

   1 // -----------------------------------------------------------------------
   2 // Copyright (c) Microsoft Corporation.  All rights reserved.
   3 // -----------------------------------------------------------------------
   4 //
   5 // This code comes from the Managed Extension Frameworks:
   6 //  http://mef.codeplex.com
   7 //
   8 // And is licensed under the MS-PL license
   9 //
  10         // Glenn said on IRC:
  11         //   "I think our table is weak, but does not do proper compacting"
  12         //
  13
  14 #if NET_4_0 || BOOTSTRAP_NET_4_0 || MOONLIGHT
  15 using System;
  16 using System.Collections;
  17 using System.Collections.Generic;
  18 using System.Linq;
  19
  20 namespace System.Runtime.CompilerServices
  21 {
  22
  23     // This is a broken implementation of ConditionalWeakTable that allows us
  24     // to compile and work on versions of .Net eariler then 4.0. This class is
  25     // broken when there are circular dependencies between keys and values, which
  26     // can only be fixed by using some specific CLR 4.0 features.
  27     // For code samples of the broken behavior see ConditionalWeakTableTests.cs.
  28     public class ConditionalWeakTable<TKey, TValue>
  29         where TKey : class
  30         where TValue : class
  31     {
  32         private readonly Dictionary<object, TValue> _table;
  33         private int _capacity = 4;
  34
  35         public ConditionalWeakTable()
  36         {
  37             this._table = new Dictionary<object, TValue>();
  38         }
  39
  40         public void Add(TKey key, TValue value)
  41         {
  42             CleanupDeadReferences();
  43             this._table.Add(CreateWeakKey(key), value);
  44         }
  45
  46         public bool Remove(TKey key)
  47         {
  48             return this._table.Remove(key);
  49         }
  50
  51         public bool TryGetValue(TKey key, out TValue value)
  52         {
  53             return this._table.TryGetValue(key, out value);
  54         }
  55
  56         private void CleanupDeadReferences()
  57         {
  58             if (this._table.Count < _capacity)
  59             {
  60                 return;
  61             }
  62
  63             ArrayList deadKeys = new ArrayList ();
  64             foreach (var weakRef in _table.Keys){
  65                     if (!((EquivalentWeakReference)weakRef).IsAlive)
  66                             deadKeys.Add (weakRef);
  67             }
  68
  69             foreach (var deadKey in deadKeys)
  70             {
  71                 this._table.Remove(deadKey);
  72             }
  73
  74             if (this._table.Count >= _capacity)
  75             {
  76                 _capacity *= 2;
  77             }
  78         }
  79
  80         private static object CreateWeakKey(TKey key)
  81         {
  82             return new EquivalentWeakReference(key);
  83         }
  84
  85         private class EquivalentWeakReference
  86         {
  87             private readonly WeakReference _weakReference;
  88             private readonly int _hashCode;
  89
  90             public EquivalentWeakReference(object obj)
  91             {
  92                 this._hashCode = obj.GetHashCode();
  93                 this._weakReference = new WeakReference(obj);
  94             }
  95
  96             public bool IsAlive
  97             {
  98                 get
  99                 {
 100                     return this._weakReference.IsAlive;
 101                 }
 102             }
 103
 104             public override bool Equals(object obj)
 105             {
 106                 EquivalentWeakReference weakRef = obj as EquivalentWeakReference;
 107
 108                 if (weakRef != null)
 109                 {
 110                     obj = weakRef._weakReference.Target;
 111                 }
 112
 113                 if (obj == null)
 114                 {
 115                     return base.Equals(weakRef);
 116                 }
 117
 118                 return object.Equals(this._weakReference.Target, obj);
 119             }
 120
 121             public override int GetHashCode()
 122             {
 123                 return this._hashCode;
 124             }
 125         }
 126     }
 127 }
 128 #endif