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    <p>After lots of head-scratching I was finally able to find a solution for my original question. First off, let me say I got a lot of great responses and I tested all of them (<em>commenting each with the results</em>). The main problems were that all the proposed solutions led to either dead-locks (<em>leading to 100% timeout scenario's</em>) or made an otherwise Asyncronous process syncronous. I don't like answering my own question (<em>first time ever</em>), but in this case I took the advice of the StackOverflow FAQ since I've truely learned my own lesson and wanted to share it with the community.</p> <p>In the end, I combined the proposed solutions with the invocation of delagates into alternate AppDomains. It's a bit more code and it's a little more expensive, but this avoids the dead-locks and allows fully asyncronous invocations which is what I required. Here are the bits...</p> <p><strong>First</strong> I needed something to invoke a delegate in another AppDomain</p> <pre><code>/// &lt;summary&gt; /// Invokes actions in alternate AppDomains /// &lt;/summary&gt; public static class DomainInvoker { /// &lt;summary&gt; /// Invokes the supplied delegate in a new AppDomain and then unloads when it is complete /// &lt;/summary&gt; public static T ExecuteInNewDomain&lt;T&gt;(Delegate delegateToInvoke, params object[] args) { AppDomain invocationDomain = AppDomain.CreateDomain("DomainInvoker_" + delegateToInvoke.GetHashCode()); T returnValue = default(T); try { var context = new InvocationContext(delegateToInvoke, args); invocationDomain.DoCallBack(new CrossAppDomainDelegate(context.Invoke)); returnValue = (T)invocationDomain.GetData("InvocationResult_" + invocationDomain.FriendlyName); } finally { AppDomain.Unload(invocationDomain); } return returnValue; } [Serializable] internal sealed class InvocationContext { private Delegate _delegateToInvoke; private object[] _arguments; public InvocationContext(Delegate delegateToInvoke, object[] args) { _delegateToInvoke = delegateToInvoke; _arguments = args; } public void Invoke() { if (_delegateToInvoke != null) AppDomain.CurrentDomain.SetData("InvocationResult_" + AppDomain.CurrentDomain.FriendlyName, _delegateToInvoke.DynamicInvoke(_arguments)); } } } </code></pre> <p><strong>Second</strong> I needed something to orchestrate collection of the required parameters and collect/resolve the results. This will also define the timeout and worker processes which will be called asyncronously in an alternate AppDomain</p> <p><em>Note: In my tests, I extended the dispatch worker method to take random amounts of time to observe that everything worked as expected in both timeout and non-timeout cases</em> </p> <pre><code>public delegate IResponse DispatchMessageWithTimeoutDelegate(IRequest request, int timeout = MessageDispatcher.DefaultTimeoutMs); [Serializable] public sealed class MessageDispatcher { public const int DefaultTimeoutMs = 500; /// &lt;summary&gt; /// Public method called on one more many threads to send a request with a timeout /// &lt;/summary&gt; public IResponse SendRequest(IRequest request, int timeout) { var dispatcher = new DispatchMessageWithTimeoutDelegate(SendRequestWithTimeout); return DomainInvoker.ExecuteInNewDomain&lt;Response&gt;(dispatcher, request, timeout); } /// &lt;summary&gt; /// Worker method invoked by the &lt;see cref="DomainInvoker.ExecuteInNewDomain&lt;&gt;"/&gt; process /// &lt;/summary&gt; private IResponse SendRequestWithTimeout(IRequest request, int timeout) { IResponse response = null; var dispatcher = new DispatchMessageDelegate(DispatchMessage); //Request Dispatch var asyncResult = dispatcher.BeginInvoke(request, null, null); //Wait for dispatch to complete or short-circuit if it takes too long if (!asyncResult.AsyncWaitHandle.WaitOne(timeout, false)) { /* Timeout action */ response = null; } else { /* Invoked call ended within the timeout period */ response = dispatcher.EndInvoke(asyncResult); } return response; } /// &lt;summary&gt; /// Worker method to do the actual dispatch work while being monitored for timeout /// &lt;/summary&gt; private IResponse DispatchMessage(IRequest request) { /* Do real dispatch work here */ return new Response(); } } </code></pre> <p><strong>Third</strong> I need something to stand-in for the actual thing that is asyncronously triggering the dispatches</p> <p><em>Note: This is just to demonstrate the asyncronous behaviours I required. In reality, the <strong>First</strong> and <strong>Second</strong> items above demonstrate the isolation of timeout behaviours on alternate threads. This just demonstrates how the above resources are used</em></p> <pre><code>public delegate IResponse DispatchMessageDelegate(IRequest request); class Program { static int _responsesReceived; static void Main() { const int max = 500; for (int i = 0; i &lt; max; i++) { SendRequest(new Request()); } while (_responsesReceived &lt; max) { Thread.Sleep(5); } } static void SendRequest(IRequest request, int timeout = MessageDispatcher.DefaultTimeoutMs) { var dispatcher = new DispatchMessageWithTimeoutDelegate(SendRequestWithTimeout); dispatcher.BeginInvoke(request, timeout, SendMessageCallback, request); } static IResponse SendRequestWithTimeout(IRequest request, int timeout = MessageDispatcher.DefaultTimeoutMs) { var dispatcher = new MessageDispatcher(); return dispatcher.SendRequest(request, timeout); } static void SendMessageCallback(IAsyncResult ar) { var result = (AsyncResult)ar; var caller = (DispatchMessageWithTimeoutDelegate)result.AsyncDelegate; Response response; try { response = (Response)caller.EndInvoke(ar); } catch (Exception) { response = null; } Interlocked.Increment(ref _responsesReceived); } } </code></pre> <p>In retrospect, this approach has some unintended consequences. Since the worker method occurs in an alternate AppDomain, this adds addition protections for exceptions (<em>although it can also hide them</em>), allows you to load and unload other managed assemblies (<em>if required</em>), and allows you to define highly constrained or specialized security contexts. This requires a bit more productionization but provided the framework to answer my original question. Hope this helps someone.</p>
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