IDisposable and Scoped values

Object pooling is a popular way to reduce memory allocations, using IDisposable wrappers to return objects to the pool. All cache classes in BitFaster.Caching own the lifetime of cached values, and will automatically dispose values when they are evicted. To safely store IDisposable pooled objects within the cache, use IScopedCache.

To avoid races using objects after they have been disposed by the cache, use IScopedCache which wraps values in Scoped<T>. The call to ScopedGetOrAdd creates a Lifetime that guarantees the scoped object will not be disposed until the lifetime is disposed. Scoped cache is thread safe, and guarantees correct disposal for concurrent lifetimes.

var lru = new ConcurrentLruBuilder<int, SomeDisposable>()
    .WithCapacity(128)
    .AsScopedCache()
    .Build();
var valueFactory = new SomeDisposableValueFactory();

using (var lifetime = lru.ScopedGetOrAdd(1, valueFactory.Create))
{
    // lifetime.Value is guaranteed to be alive until the lifetime is disposed
}

class SomeDisposableValueFactory
{
   public Scoped<SomeDisposable>> Create(int key)
   {
      return new Scoped<SomeDisposable>(new SomeDisposable(key));
   }
}

In the following sequence of operations, Threads A and B both lookup an IDisposable object from the cache and hold a lifetime. Thread C then deletes the object from the cache while it is in use. Each thread's lifetime instance ensures that the object is alive until all threads have disposed their lifetime.

sequenceDiagram
    autonumber
    participant Thread A
    participant Thread B
    participant Thread C
    participant Cache
    participant Scope
    participant Lifetime Cache    
    participant Lifetime A
    participant Lifetime B
    Thread A->>Cache: A calls ScopedGetOrAdd
    Cache->>Scope: create scope
    activate Scope 
    Scope-->>Object: create object
    activate Object
    Cache-->Lifetime Cache: cache holds lifetime
    activate Lifetime Cache
    Cache-->Lifetime A: creates A's lifetime
    activate Lifetime A
    Lifetime A-->> Thread A: A holds lifetime
    Thread B->>Cache: B calls ScopedGetOrAdd
    Cache-->Lifetime B: creates B's lifetime
    activate Lifetime B
    Lifetime B-->> Thread B: B holds lifetime
    Thread C->>Cache: C calls TryRemove
    Cache--xLifetime Cache: Cache removes the object and disposes lifetime
    deactivate Lifetime Cache
    Thread A->>Object: Thread A uses the object
    Lifetime A->>Thread A: A disposes lifetime
    Lifetime A--x Scope: lifetime de-refs scope
    deactivate Lifetime A
    Thread B->>Object: Thread B uses the object
    Lifetime B->>Thread B: B disposes lifetime
    Lifetime B--x Scope: B de-refs scope
    deactivate Lifetime B
    Scope--xObject: dispose object
    deactivate Object
    deactivate Scope

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