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POConditional thread locking
 

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    <h1>Scenario</h1> <p>I want to download resources. I don't want a resource to be downloaded more that once. If thread <em>a</em> downloads resource <code>1</code> it should be cached, and thread <em>b</em> should wait and use the cached resource <code>1</code> if it attempts to download resource <code>1</code> at the same time. If thread <em>c</em> wants to download resource <code>2</code>, it should not be influenced by thread <em>a</em> and <em>b</em>.</p> <h1>Attempt</h1> <p>I have tried to implement the scenario below:</p> <pre><code>using System; using System.Collections.Generic; using System.Threading; namespace ConsoleApplication1 { class ConditionalThreadLockingProgram { private static readonly object _lockObject = new object(); private static readonly Dictionary&lt;int, string&gt; Locks = new Dictionary&lt;int, string&gt;(); private static readonly Dictionary&lt;int, string&gt; Resources = new Dictionary&lt;int, string&gt;(); public static string GetLock(int resourceId) { lock (_lockObject) { if (Locks.ContainsKey(resourceId)) { return Locks[resourceId]; } return Locks[resourceId] = string.Format( "Lock #{0}", resourceId ); } } public static void FetchResource(object resourceIdObject) { var resourceId = (int)resourceIdObject; var currentLock = GetLock(resourceId); lock (currentLock) { if (Resources.ContainsKey(resourceId)) { Console.WriteLine( "Thread {0} got cached: {1}", Thread.CurrentThread.Name, Resources[resourceId] ); return; } Thread.Sleep(2000); Console.WriteLine( "Thread {0} downloaded: {1}", Thread.CurrentThread.Name, Resources[resourceId] = string.Format( "Resource #{0}", resourceId ) ); } } static void Main(string[] args) { new Thread(FetchResource) { Name = "a" }.Start(1); new Thread(FetchResource) { Name = "b" }.Start(1); new Thread(FetchResource) { Name = "c" }.Start(2); Console.ReadLine(); } } } </code></pre> <h1>Question</h1> <p>Does it work? Any issues?</p>
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    1. CODid you try it?
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    2. COYes, I think it works. But I'm very new to multithreading.
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    3. COI would maybe suggesting using the SyncRoot of the dictionary instead of creating your own lock. GetLock will use (((IDictionary)Locks).SyncRoot) and FetchResouce will use (((IDictionary)Resources).SyncRoot)
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