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    <p>In this case, your cache shouldn't cache the <em>actual</em> object, but rather, a wrapper around the object being cached that also contains information about the state of the object in the cache.</p> <p>For example, you could have a simple class like so, representing the items in your cache:</p> <pre><code>public class CacheItem&lt;T&gt; { // Since the cache is the only thing // that should be making CacheItems, // make this internal to the assembly // that the cache is implemented in. // This constructor is called before // an add. internal CacheItem(T item) { // Set the property values. Item = item; } // Poor-man's immutability. public T Item { get; private set; } // The backing field for IsCached, it // is volatile so that it can serialize // access for single reads/writes, which is // what the property does. // Assume it is being added when constructed. private volatile bool _isCached = true; // Only able to be set by the cache. // The setter is set to false when the item // is stale (not in the cache any longer). public bool IsCached { get { return _isCached; } set { _isCached = value; } } } </code></pre> <p>The idea here is simple:</p> <ul> <li><p>When your cache is about to enter a new instance of an item into the cache, then it calls the constructor (the constructor should <em>only</em> be available to the cache, if necessary, you can make <code>CacheItem</code> a nested class with a private constructor instead of internal) which sets the <code>IsCached</code> property value to true. The item is then placed in the cache.</p></li> <li><p>When the item expires from the cache, then the cache sets the <code>IsCached</code> property to false (again, this property should only be accessible by the cache).</p></li> <li><p>The responsibility is moved to the <em>client</em> to check the <code>CacheItem&lt;T&gt;</code> to see if it is stale.</p></li> </ul> <p>Note that this is a pull operation. If you want, you could add <em>push</em> operation support by adding an event to the <code>CacheItem&lt;T&gt;</code> like so:</p> <pre><code>public class CacheItem&lt;T&gt; { // Since the cache is the only thing // that should be making CacheItems, // make this internal to the assembly // that the cache is implemented in. // This constructor is called before // an add. internal CacheItem(T item) { // Set the property values. Item = item; } // Poor-man's immutability. public T Item { get; private set; } // The lock for the event registrations. // Since everything else is immutable, this needs // to be as well. private readonly object _eventLock = new object(); // The backing field for the Expired // event. Since everything else is immutable // this needs to be as well. private readonly EventHandler _expiredHandlers; // The expires event. public event EventHandler Expired { add { lock (_eventLock) _expiredHandlers += value; } remove { lock (_eventLock) _expiredHandlers -= value; } } // The backing field for IsCached, it // is volatile so that it can serialize // access for single reads/writes, which is // what the property does. // Assume it is being added when constructed. private volatile bool _isCached = true; // The setter is set to false by the // Expire method (called by the cached) // when the item is stale // (not in the cache any longer). public bool IsCached { get { return _isCached; } } // Called internally by the cache. internal void Expire() { // Set _isCached to false. _isCached = false; // Get the event handlers and fire // the event. Getting the handlers // needs to be synchronized. EventHandler handlers; // Synchronize. lock (_eventLock) handlers = _expiredHandlers; // Fire if there are handlers. if (handlers != null) handlers(this, EventArgs.Empty); } } </code></pre> <p>Now, you can have your clients subscribe to the <code>Expired</code> event, which would indicate to clients when the cache item is invalidated. This is fired by the cache by calling the internal <code>Expire</code> event on the item when it is removed from the cache (and it will set the <code>IsCached</code> property to false as well).</p>
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    1. POHow not to delete cache items that are referenced by other objects
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    1. COThank you! A item cache for encapsulate is very useful, at any rate, I'm sorry if I've understood correctly but, the ItemCache will dispose or finalize when the cache expires it. Why set a property? I put the item in cache using a CacheItemPolicy timeout. When time is completed, cache items are finalizing.
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    2. CO@enagra No, that is not correct. When the cache is done with an object, it releases the reference, but as you've indicated that `MyBusinessObject` is still holding onto the reference, so garbage collection won't work here. Finalization happens when GC takes place, not when you release items from the cache. Note, if you want a pull operation, you can use a `WeakReference` to do the same thing as the first iteration, but if you want to be notified when the cache changes, then the second option is what you need.
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    3. COI understand. I thought "bo" should continue to refer to the "actual" obtect, instead of referencing to CacheItem. Really, you are suggesting a particular pattern implementation of "Proxy" which I think may help.
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