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POThread-safe memoization
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copied!<p>Let's take <a href="http://blogs.msdn.com/wesdyer/archive/2007/01/26/function-memoization.aspx" rel="noreferrer">Wes Dyer's</a> approach to function memoization as the starting point:</p> <pre><code>public static Func<A, R> Memoize<A, R>(this Func<A, R> f) { var map = new Dictionary<A, R>(); return a => { R value; if (map.TryGetValue(a, out value)) return value; value = f(a); map.Add(a, value); return value; }; } </code></pre> <p>The problem is, when using it from multiple threads, we can get into trouble:</p> <pre><code>Func<int, int> f = ... var f1 = f.Memoize(); ... in thread 1: var y1 = f1(1); in thread 2: var y2 = f1(1); // We may be recalculating f(1) here! </code></pre> <p>Let's try to avoid this. Locking on <code>map</code>:</p> <pre><code>public static Func<A, R> Memoize<A, R>(this Func<A, R> f) { var map = new Dictionary<A, R>(); return a => { R value; lock(map) { if (map.TryGetValue(a, out value)) return value; value = f(a); map.Add(a, value); } return value; }; } </code></pre> <p>is clearly a horrible idea, because it prevents us from calculating <code>f1</code> on many <em>different</em> arguments at once. Locking on <code>a</code> won't work if <code>a</code> has a value type (and at any rate is a bad idea, since we don't control <code>a</code> and outside code may lock on it, too). </p> <p>Here are two options I can think of:</p> <p>Assuming a <code>Lazy<T></code> class for lazy evaluation (see <a href="http://msdn.microsoft.com/en-us/vcsharp/bb870976.aspx" rel="noreferrer">here</a>):</p> <pre><code>public static Func<A, R> Memoize<A, R>(this Func<A, R> f) { var map = new Dictionary<A, Lazy<R>>(); return a => { Lazy<R> result; lock(map) { if (!map.TryGetValue(a, out result)) { result = () => f(a); map.Add(a, result); } } return result.Value; }; } </code></pre> <p>Or keeping an additional dictionary of objects for synchronization:</p> <pre><code>public static Func<A, R> Memoize<A, R>(this Func<A, R> f) { var map = new Dictionary<A, R>(); var mapSync = new Dictionary<A, object>(); return a => { R value; object sync; lock(mapSync) { if (!mapSync.TryGetValue(a, out sync)) { sync = new object(); mapSync[a] = sync; } } lock(map) { if (map.TryGetValue(a, out value)) return value; } lock(sync) { value = f(a); lock(map) { map[a] = value; } return value; } }; } </code></pre> <p>Any better options?</p>

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