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copied!<p>Use .net 4.0's <code>ConcurrentDictionary<A, R></code> without the unnecessary <code>Lazy<R></code>.<br> The key is <code>GetOrAdd(A, Func<A, R>)</code> which renders into a beautifully trivial lambda.</p> <pre><code>public static Func<A, R> Memoize<A, R>(this Func<A, R> f) { var cache = new ConcurrentDictionary<A, R>(); return a => cache.GetOrAdd(a, f); }; </code></pre> <p><strong>Update</strong> The above solution does allow multiple simultaneous readers & writers with the minimum of overhead. But, it doesn't prevent <code>f(a)</code> from being executed more than once for the same value (during the period while it is being calculated).</p> <p>If that is vital to you, you could wrap the value in <code>Lazy<R></code> but you incur a cost for every read.</p> <pre><code>public static Func<A, R> Memoize<A, R>(this Func<A, R> f) { var cache = new ConcurrentDictionary<A, Lazy<R>>(); return a => cache.GetOrAdd(a, new Lazy<R>(() => f(a))).Value; } </code></pre> <p><strong>Update</strong> Timing tests for a million reads of a pre-populated 1000-item cache show <strong>19ms</strong> for <code>ConcurrentDictionary</code> -- same as regular <code>Dictionary</code> -- but <strong>720ms</strong> for the <code>Lazy</code> version.</p> <p>If that sounds too steep, you can get the best of both worlds with a more complex solution.</p> <pre><code>public static Func<A, R> Memoize<A, R>(this Func<A, R> f) { var cache = new ConcurrentDictionary<A, R>(); var syncMap = new ConcurrentDictionary<A, object>(); return a => { R r; if (!cache.TryGetValue(a, out r)) { var sync = syncMap.GetOrAdd(a, new object()); lock (sync) { r = cache.GetOrAdd(a, f); } syncMap.TryRemove(a, out sync); } return r; }; } </code></pre>

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