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I am currently developing a C# Windows Form Application that I intend to let it interact with a server. The server will receive posting from a mobile application that I have developed and whenever a posting is received, my Windows Form Application should be notified and give me a notification.

E.g. My mobile application sends an posting over to my server. Once my server receives the message, my windows form application should display a new notification showing the content of the message received and updates the UI accordingly.

In this type of scenario, it is better to use duplex WCF service or the just the regular WCF service?

If duplex, mind explaining why do I need to use duplex service? Thanks!

Thomas
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2 Answers2

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A duplex service is a service where two channels are created.

  • The first channel is the ordinary client -> server channel using your service contract. This is what you'll find in every WCF service, and is how your client can send a request to the service and it can respond.
  • The second channel is a server -> client channel using a different service contract that you define. This second channel is how the server can send messages to the client without the client requesting them.

In your scenario, you seem to indicate that an event taking place on the server should send a message to your client. If this is the case, then yes, you need a duplex service so that the second channel exists, which allows the server to notify your client without the client initiating a request.

Adam Robinson
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  • hi. thank you for your reply. i understand the usage of the second channel. but for the first channel, how is it being used in my scenario? As in why does my client needs to send a request to the service in the first place? thanks! – Thomas Nov 16 '11 at 05:32
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    @Thomas if for nothing else, the 'first' channel is required so that the server knows to open the second channel. The WinForm application needs to initiate the communication to a) notify the server that it's running & ready to receive messages, and b) give its address to the server. – Kirk Broadhurst Nov 16 '11 at 05:46
  • oh. so is there a need for me to implement a service contract for the first channel? or I can just leave it blank and just implement the service call back channel – Thomas Nov 16 '11 at 06:32
  • @Thomas: You need a contract with at least one operation on the first channel. It's within this operation on the service side that you'll actually retrieve a reference to the client as the callback contract; otherwise how would you envision the service knowing how to talk to the client? – Adam Robinson Nov 16 '11 at 12:23
  • oh. I have somewhat additional queries regarding this topic. if possible would you mind taking a look at my another question and guiding me? http://stackoverflow.com/questions/8151415/how-does-the-wcf-service-interact-with-my-winform-app – Thomas Nov 16 '11 at 12:35
  • @Thomas: Sure, I'll take a look, but if this has answered this particular question, please be sure to click the checkmark to accept it. – Adam Robinson Nov 16 '11 at 13:04
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Working on the assumption that your Windows app and your server are one the same domain I would suggest you use a publish/subscribe pattern for this type of interaction. You could use something along the lines of the IDesign sample which is available on their website. Essentially your Windows app is subscribing to events which are generated by your mobile application sending a posting to your server. Your publisher will then push the event to your Windows application.

To accomplish this your connection to the server/publisher needs to be always open. This is best achieved with tcpBinding as it is bi-directional and allows you to set high timeouts (effectively infinity).

If you cannot use TCP then your job has become a little harder. Using a duplex channel is a little problematic because you have to monitor the channel as well, because neither side will notifiy the other if the channel closes. You will only find out when you try to use it. This can still happen with the TCP connection of course but its a bit more stable that using http.

The other alternative is to use MSMQ binding. This will guarantee the delivery of your message because you are interacting via MSMQ rather than a communication channel such as http or tcp. In this instance you wouldn't even need the pub/sub framework, you could just have the service that receives your mobile posting send on a message to the queue, which your Windows application is monitoring. The upside to using the framework is that you can have multiple applications listening for the same event.

HTH.

stephenl
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