A thread-safe dictionary implementation that stores values with weekly referenced keys. See WeakReference<T>.
Once a key is not strong-referenced anymore the entry will be removed automatically from WeakDictionary<TKey,TValue>.
This collection can be used for similar purposes as ConditionalWeakTable<TKey,TValue> except that WeakDictionary<TKey,TValue> can accept custom IEqualityComparer<T> instance for key comparison. ConditionalWeakTable<TKey,TValue> can only compare keys by reference no matter if GetHashCode() and Equals() methods are overridden or not.
Internally WeakDictionary<TKey,TValue> uses a combination of Dictionary<TKey,TValue> and ConditionalWeakTable<TKey,TValue> types to manage entries.
Use the same way as a generic Dictionary<TKey,TValue>.
var dictionary = new WeakDictionary<MyReferenceType, AnyType>(optionalEqualityComparer);Note that WeakDictionary<TKey,TValue> implements only IDictionary<TKey,TValue>, IEnumerable<T> and IEnumerable interfaces.
Note that TKey must be reference type, but TValue can be anything you want.
Also note that TValue by default is not stored as a WeakReference<T>. Therefore if key and value reference the same object GC can not collect the instance!
Consider this if you plan to use WeakDictionary<TKey,TValue> as a HashSet<T> and avoid using the same instance for key and value.
The single use case of this library is when you need to use a custom IEqualityComparer<T> for the weakly referenced keys. If this is not a requirement use the ConditionalWeakTable<TKey,Tvalue> instead.
All public and protected members of WeakDictionary<TKey,TValue> are thread-safe and may be used concurrently from multiple threads. However, members accessed through one of the interfaces the WeakDictionary<TKey,TValue> implements, including extension methods, are not guaranteed to be thread safe and may need to be synchronized by the caller.
The below results show that WeakDictionary<TKey,TValue> has some performance cost compared to the other data structures probably because of all the locks surrounding the logic of the internal data structure mutations.
| Method | N | Mean | Error | StdDev | Ratio | RatioSD |
|---|---|---|---|---|---|---|
| WeakDictionary_Add | 100000 | 217.97 ms | 117.891 ms | 6.462 ms | 4.56 | 1.12 |
| ConcurrentDictionary_Add | 100000 | 49.66 ms | 9.680 ms | 0.531 ms | 1.04 | 0.24 |
| Dictionary_Add | 100000 | 18.18 ms | 47.396 ms | 2.598 ms | 0.37 | 0.07 |
| ConditionalWeakTable_Add | 100000 | 49.44 ms | 177.786 ms | 9.745 ms | 1.00 | 0.00 |
| Method | N | Mean | Error | StdDev | Ratio | RatioSD |
|---|---|---|---|---|---|---|
| WeakDictionary_TryGetValue | 100000 | 71.26 ms | 48.43 ms | 2.655 ms | 2.09 | 0.27 |
| ConcurrentDictionary_TryGetValue | 100000 | 12.10 ms | 36.76 ms | 2.015 ms | 0.35 | 0.04 |
| Dictionary_TryGetValue | 100000 | 20.75 ms | 31.57 ms | 1.730 ms | 0.61 | 0.09 |
| ConditionalWeakTable_TryGetValue | 100000 | 34.57 ms | 89.00 ms | 4.879 ms | 1.00 | 0.00 |
| Method | N | Mean | Error | StdDev | Ratio | RatioSD |
|---|---|---|---|---|---|---|
| WeakDictionary_Set | 100000 | 378.94 ms | 746.94 ms | 40.942 ms | 18.87 | 0.79 |
| ConcurrentDictionary_Set | 100000 | 26.72 ms | 32.10 ms | 1.759 ms | 1.33 | 0.04 |
| Dictionary_Set | 100000 | 22.34 ms | 17.56 ms | 0.963 ms | 1.12 | 0.14 |
| ConditionalWeakTable_Set | 100000 | 20.07 ms | 33.44 ms | 1.833 ms | 1.00 | 0.00 |
| Method | N | Mean | Error | StdDev | Ratio | RatioSD |
|---|---|---|---|---|---|---|
| WeakDictionary_Remove | 100000 | 130.70 ms | 262.21 ms | 14.373 ms | 8.85 | 3.03 |
| ConcurrentDictionary_Remove | 100000 | 23.11 ms | 38.31 ms | 2.100 ms | 1.53 | 0.26 |
| Dictionary_Remove | 100000 | 14.89 ms | 39.01 ms | 2.138 ms | 0.99 | 0.22 |
| ConditionalWeakTable_Remove | 100000 | 15.49 ms | 59.07 ms | 3.238 ms | 1.00 | 0.00 |