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    <p>Have you considered using <code>System.Web.Caching</code> instead of having to roll your own ?</p> <p><a href="http://www.hanselman.com/blog/UsingTheASPNETCacheOutsideOfASPNET.aspx" rel="nofollow noreferrer">http://www.hanselman.com/blog/UsingTheASPNETCacheOutsideOfASPNET.aspx</a></p> <p><strong>EDIT</strong></p> <p>Well the above should not add THAT much of an overhead to the system but have a look at this.</p> <p><strong>A few health warnings on the code below.</strong></p> <ul> <li>It's incomplete... see the <code>throw new NotImplementedException()</code>s at the bottom. I'll try and come back to it in a while as it's an interesting puzzle.</li> <li>You may want to cange the way expiration is done &amp; have overrides on the Add Methods to supply different values to the constructed value</li> <li>I've only tested it the bare minimum in a console app. see test code</li> <li>It also needs a bit of work around the TKey &amp; TValue Collections as they'll blindly return the entirety of the inner dictionary's collections without any expiration checking... if you don't need particularly granular expiration. You could add a system.timer to the class which periodically walked the entire collection and removed expired entries.</li> <li>If you look at the Definition for the BCL Dictionary you'll see it implements a hell of a lot of other interfaces to so depending on your requirements you may want to implement these as well. <code>IDictionary&lt;TKey, TValue&gt;, ICollection&lt;KeyValuePair&lt;TKey, TValue&gt;&gt;, IEnumerable&lt;KeyValuePair&lt;TKey, TValue&gt;&gt;, IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback</code></li> </ul> <p><strong>Test Code</strong></p> <pre><code>TimeSpan t = new TimeSpan(0,0,5); //5 Second Expiry ExpiringDictionary&lt;int, string&gt; dictionary = new ExpiringDictionary&lt;int,string&gt;(t); dictionary.Add(1, "Alice"); dictionary.Add(2, "Bob"); dictionary.Add(3, "Charlie"); //dictionary.Add(1, "Alice"); //&lt;&lt;this will throw a exception as normal... System.Threading.Thread.Sleep(6000); dictionary.Add(1, "Alice"); //&lt;&lt; this however should work fine as 6 seconds have passed </code></pre> <p><strong>Implementation</strong></p> <pre><code>public class ExpiringDictionary&lt;TKey, TValue&gt; : IDictionary&lt;TKey, TValue&gt; { private class ExpiringValueHolder&lt;T&gt; { public T Value { get; set; } public DateTime Expiry { get; private set; } public ExpiringValueHolder(T value, TimeSpan expiresAfter) { Value = value; Expiry = DateTime.Now.Add(expiresAfter); } public override string ToString() { return Value.ToString(); } public override int GetHashCode() { return Value.GetHashCode(); } }; private Dictionary&lt;TKey, ExpiringValueHolder&lt;TValue&gt;&gt; innerDictionary; private TimeSpan expiryTimeSpan; private void DestoryExpiredItems(TKey key) { if (innerDictionary.ContainsKey(key)) { var value = innerDictionary[key]; if (value.Expiry &lt; System.DateTime.Now) { //Expired, nuke it in the background and continue innerDictionary.Remove(key); } } } public ExpiringDictionary(TimeSpan expiresAfter) { expiryTimeSpan = expiresAfter; innerDictionary = new Dictionary&lt;TKey, ExpiringValueHolder&lt;TValue&gt;&gt;(); } public void Add(TKey key, TValue value) { DestoryExpiredItems(key); innerDictionary.Add(key, new ExpiringValueHolder&lt;TValue&gt;(value, expiryTimeSpan)); } public bool ContainsKey(TKey key) { DestoryExpiredItems(key); return innerDictionary.ContainsKey(key); } public bool Remove(TKey key) { DestoryExpiredItems(key); return innerDictionary.Remove(key); } public ICollection&lt;TKey&gt; Keys { get { return innerDictionary.Keys; } } public bool TryGetValue(TKey key, out TValue value) { bool returnval = false; DestoryExpiredItems(key); if (innerDictionary.ContainsKey(key)) { value = innerDictionary[key].Value; returnval = true; } else { value = default(TValue);} return returnval; } public ICollection&lt;TValue&gt; Values { get { return innerDictionary.Values.Select(vals =&gt; vals.Value).ToList(); } } public TValue this[TKey key] { get { DestoryExpiredItems(key); return innerDictionary[key].Value; } set { DestoryExpiredItems(key); innerDictionary[key] = new ExpiringValueHolder&lt;TValue&gt;(value, expiryTimeSpan); } } public void Add(KeyValuePair&lt;TKey, TValue&gt; item) { DestoryExpiredItems(item.Key); innerDictionary.Add(item.Key, new ExpiringValueHolder&lt;TValue&gt;(item.Value, expiryTimeSpan)); } public void Clear() { innerDictionary.Clear(); } public int Count { get { return innerDictionary.Count; } } public bool IsReadOnly { get { return false; } } public bool Contains(KeyValuePair&lt;TKey, TValue&gt; item) { throw new NotImplementedException(); } public void CopyTo(KeyValuePair&lt;TKey, TValue&gt;[] array, int arrayIndex) { throw new NotImplementedException(); } public bool Remove(KeyValuePair&lt;TKey, TValue&gt; item) { throw new NotImplementedException(); } public IEnumerator&lt;KeyValuePair&lt;TKey, TValue&gt;&gt; GetEnumerator() { throw new NotImplementedException(); } System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw new NotImplementedException(); } } </code></pre>
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    1. COHe is not doing this in a web application that we know of...
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    2. CO@soandos The link shows how to use it outside of a web application.
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    3. COIt's a kind of `IDS` (Intrusion Detection System) as a stand alone library. I need high performance.
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