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    <p>After the first receiver exits, its channel is still saved in the receiverClients collection in the service, so the next time a transmit occurs, the server will attempt to callback to a non-existent client. You must remove the receiver from the collection when it ends, perhaps with an unsubscribe contract operation. There are a number of ways to implement this. Here is one example. (These are the changes, only - please let me know if you would like the entire project.)</p> <p>In IContactManagerUserService.cs:</p> <pre><code>// receivers must identify themselves [OperationContract(IsOneWay = true)] void SubscribeAsReceiver(string receiverID); // new operation [OperationContract(IsOneWay = true)] void UnSubscribeAsReceiver(string receiverID); </code></pre> <p>In ContactManagerUserService.cs</p> <pre><code>// replace synchronized collection with thread-safe dictionary using System.Collections.Concurrent; ConcurrentDictionary&lt;string, IContactManagerCallBackService&gt; receiverClients; public ContactManagerUserService() { receiverClients = new ConcurrentDictionary&lt;string, IContactManagerCallBackService&gt;(); } public void SubscribeAsReceiver(string receiverID) { IContactManagerCallBackService client = OperationContext.Current.GetCallbackChannel&lt;IContactManagerCallBackService&gt;(); receiverClients.TryAdd(receiverID, client); } public void UnSubscribeAsReceiver(string receiverID) { IContactManagerCallBackService client; receiverClients.TryRemove(receiverID, out client); } public void TransmitUserData(UserInfo info) { foreach (IContactManagerCallBackService receiverClient in receiverClients.Values) { receiverClient.PublishUserData(info); } } </code></pre> <p>In ReceiverClient.Program.cs (don't forget to update ServiceReferences since the contract has changed):</p> <pre><code>// one way to uniquely identify this receiver string rcvID = Guid.NewGuid().ToString(); // .... // receivingClient.SubscribeAsReceiver(rcvID); // .... // Console.ReadLine(); // Important - this line must be executed, or service will again throw error. // So when debugging, don't just close window, press enter to execute this line. // In more robust setting, this would probably go in finally{} block, or in Dispose() receivingClient.UnSubscribeAsReceiver(rcvID); </code></pre> <p><strong>Debugging Tip</strong> You may want to have a version of the service you can run locally, so you can step into it and debug it. Also, I have found logging messages to the service log to be very helpful when you can't run the service locally.</p> <p>You can search WCF Publisher/Subscriber for more info. </p>
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    1. COYes it would be nice if you can send me the code at `nirajdoshi1985 [at] gmail [dot] com`
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    2. COHi Alex, Thanks for the code. It runs perfectly for the first time, but when I run it second time the Transmitter causes the problem (which we never subscribe or unsubscribe). Please see the edit
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    3. COuser1467261: One more step ahead, if I close receiver window, transmitter throws the above aborted exception. Somehow the channel is getting aborted.
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