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POHow Do I Detect When a Client Thread Exits?
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copied!<p>Here’s an interesting library writer’s dilemma. In my library (in my case EasyNetQ) I’m assigning thread local resources. So when a client creates a new thread and then calls certain methods on my library new resources get created. In the case of EasyNetQ a new channel to the RabbitMQ server is created when the client calls ‘Publish’ on a new thread. I want to be able to detect when the client thread exits so that I can clean up the resources (channels).</p> <p>The only way of doing this I’ve come up with is to create a new ‘watcher’ thread that simply blocks on a Join call to the client thread. Here a simple demonstration:</p> <p>First my ‘library’. It grabs the client thread and then creates a new thread which blocks on ‘Join’:</p> <pre><code>public class Library { public void StartSomething() { Console.WriteLine("Library says: StartSomething called"); var clientThread = Thread.CurrentThread; var exitMonitorThread = new Thread(() => { clientThread.Join(); Console.WriteLine("Libaray says: Client thread existed"); }); exitMonitorThread.Start(); } } </code></pre> <p>Here’s a client that uses my library. It creates a new thread and then calls my library’s StartSomething method:</p> <pre><code>public class Client { private readonly Library library; public Client(Library library) { this.library = library; } public void DoWorkInAThread() { var thread = new Thread(() => { library.StartSomething(); Thread.Sleep(10); Console.WriteLine("Client thread says: I'm done"); }); thread.Start(); } } </code></pre> <p>When I run the client like this:</p> <pre><code>var client = new Client(new Library()); client.DoWorkInAThread(); // give the client thread time to complete Thread.Sleep(100); </code></pre> <p>I get this output:</p> <pre><code>Library says: StartSomething called Client thread says: I'm done Libaray says: Client thread existed </code></pre> <p>So it works, but it's ugly. I really don’t like the idea of all these blocked watcher threads hanging around. Is there a better way of doing this?</p> <p><strong>First alternative.</strong> </p> <p>Provide a method that returns a worker that implements IDisposable and make it clear in the documentation that you should not share workers between threads. Here's the modified library:</p> <pre><code>public class Library { public LibraryWorker GetLibraryWorker() { return new LibraryWorker(); } } public class LibraryWorker : IDisposable { public void StartSomething() { Console.WriteLine("Library says: StartSomething called"); } public void Dispose() { Console.WriteLine("Library says: I can clean up"); } } </code></pre> <p>The client is now a little more complicated:</p> <pre><code>public class Client { private readonly Library library; public Client(Library library) { this.library = library; } public void DoWorkInAThread() { var thread = new Thread(() => { using(var worker = library.GetLibraryWorker()) { worker.StartSomething(); Console.WriteLine("Client thread says: I'm done"); } }); thread.Start(); } } </code></pre> <p>The main problem with this change is that it's a breaking change for the API. Existing clients will have to be re-written. Now that's not such a bad thing, it would mean revisiting them and making sure they are cleaning up correctly. </p> <p><strong>Non-breaking second alternative</strong>. The API provides a way for the client to declare 'work scope'. Once the scope completes, the library can clean up. The library provides a WorkScope that implements IDisposable, but unlike the first alternative above, the StartSomething method stays on the Library class:</p> <pre><code>public class Library { public WorkScope GetWorkScope() { return new WorkScope(); } public void StartSomething() { Console.WriteLine("Library says: StartSomething called"); } } public class WorkScope : IDisposable { public void Dispose() { Console.WriteLine("Library says: I can clean up"); } } </code></pre> <p>The client simply puts the StartSomething call in a WorkScope...</p> <pre><code>public class Client { private readonly Library library; public Client(Library library) { this.library = library; } public void DoWorkInAThread() { var thread = new Thread(() => { using(library.GetWorkScope()) { library.StartSomething(); Console.WriteLine("Client thread says: I'm done"); } }); thread.Start(); } } </code></pre> <p>I like this less than the first alternative because it doesn't force the library user to think about scope.</p>

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