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POIs locking on the requested object a bad idea?
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copied!<p>Most advice on thread safety involves some variation of the following pattern:</p> <pre><code>public class Thing { private readonly object padlock = new object(); private IDictionary stuff, andNonsense; public Thing() { this.stuff = new Dictionary(); this.andNonsense = new Dictionary(); } public IDictionary Stuff { get { lock (this.padlock) { if (this.stuff.Count == 0) this.stuff = this.SomeExpensiveOperation(); return this.stuff; } } } public IDictionary AndNonsense { get { lock (this.padlock) { if (this.andNonsense.Count == 0) this.andNonsense = this.AnotherExpensiveOperation(); return this.andNonsense; } } } // Rest of class... } </code></pre> <p>In cases where the get operations are expensive and unrelated, a single locking object is unsuitable because a call to <code>Stuff</code> would block all calls to <code>AndNonsense</code>, degrading performance. And rather than create a lock object for each call, wouldn't it be better to acquire the lock on the member itself (assuming it is not something that implements <code>SyncRoot</code> or somesuch for that purpose? For example:</p> <pre><code>public IDictionary Stuff { get { lock (this.stuff) { if (this.stuff.Count == 0) this.stuff = this.SomeExpensiveOperation(); return this.stuff; } } } </code></pre> <p>Strangely, I have never seen this approach recommended or warned against. Am I missing something obvious?</p> <p><strong>EDIT 24 May 2010</strong></p> <blockquote> <p>I have made some extensive changes because I really borked my example. Serves me right for trying to simplify the example too much for clarity.</p> <p>Summary of edits:</p> <ul> <li>Locking object in first example is no longer static (my original example was from a static class).</li> <li>Fields/properties no longer a string, and initialised in constructor so never null.</li> <li>Moved return statement inside the <code>lock{...}</code> block.</li> </ul> </blockquote>

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