c# net qr code generator Calling the client from the server in C#

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Calling the client from the server
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Our service is going to need to maintain a list of connected clients so that it can notify every client when it receives each note. We can store the list as private data in our service class, but since that one list needs to be available across all sessions, we need to tell WCF that we only ever want it to create one instance of that class. WCF offers several different modes for creating instances of your service class. It can create one per client session that s useful when you want per-session state. But in our case, all notes get sent to everyone, so the only interesting state is global. Since our application state is global, we don t have much use for per-client instances here. WCF can also create a new instance of your service class for every single request if you don t hold any state in the service class itself this is a reasonable thing to do. But in our case, we want one instance for the lifetime of the service. We can indicate this like so:
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[ServiceBehavior( InstanceContextMode=InstanceContextMode.Single, ConcurrencyMode=ConcurrencyMode.Reentrant)] public class ChatService : IChatService {
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We added a ServiceBehavior attribute to the code to specify this single-instance behavior. Notice that we also asked for a ConcurrencyMode of Reentrant. This tells WCF to have our service work on requests for only one session at a time if requests from multiple clients come in simultaneously, WCF will service them one after another. This is convenient as it means that as long as any single client does only one thing at a time, we don t need to write any code to ensure the thread safety of our state handling.
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An alternative to the single-instance context mode would have been to store our state in a static field. This would share the data across all clients, which is what we need. But then we d be on our own for thread safety. The ConcurrencyMode property applies only to any particular instance of the service, so if you don t choose the single-instance mode, WCF will let different instances of your service execute simultaneously. In practice, real applications are likely to need to do their own thread synchronization. Here we re relying on clients making only one call at a time, which might work in a small, controlled example but is a risky thing to do if you don t completely trust your client machines. (Even with only one session at a time, a single client session could invoke multiple operations simultaneously.) You may be wondering why we didn t use ConcurrencyMode.Single, which enforces a completely strict one-at-a-time model. Unfortunately, that turns out to prevent you from calling back into clients while you re in the middle of handling a call from a client a blocking outbound call from a nonreentrant singlethreaded context presents an opportunity for deadlocks, so WCF forbids it.
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Next, we ll add a field to hold the state a collection of currently connected clients:
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private Dictionary<IChatClient, string> clientsAndNames = new Dictionary<IChatClient, string>();
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This is a dictionary where the key type is the client callback interface we defined earlier. The value is the client s name. To see how this gets used, here s the Connect implementation:
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public bool Connect(string name) { if (clientsAndNames.ContainsValue(name)) { // Name already in use, so refuse connection return false; } IChatClient clientCallback = OperationContext.Current.GetCallbackChannel<IChatClient>(); // clientsAndNames is shared state, but we're not locking // here, because we're relying on ConcurrentMode.Reentrant // to give us messages one at a time. clientsAndNames.Add(clientCallback, name); Console.WriteLine(name + " connected"); } return true;
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The first thing we do is check that the username is unique. Now that we re maintaining a list of connected clients, we re in a position to prevent multiple users from picking the same name. If a new user is trying to sign up with a duplicate name, we return false. (A return code here makes more sense than an exception because this isn t really an exceptional condition.) If the name looks OK, we retrieve the client callback interface with the following expression:
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OperationContext.Current.GetCallbackChannel<IChatClient>()
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