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Asked: 2026-06-01T00:58:17+00:00

I am designing a WCF service with callback, but my implementation of the callback

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I am designing a WCF service with callback, but my implementation of the callback function never gets called. And I am confused about the generated service proxy, so please help.

Here is the scenario: in the server side, I defined the service interface IMyService and callback interface IMyServiceCallback, I also implemented the IMyService in the server project. Besides the server project, I surely have another client project, to which I added the service reference in VS. And I implemented the IMyServiceCallback interface on the client side. So here comes the problem: when I am debugging it, the function never goes into my implementation of IMyServiceCallback and of course the desired result never comes out.

And this is I where I got confused: when I added the service reference in the client side, it actually generated three interfaces on the local: IMyService, IMyServiceCallback, and IMyServiceChannel plus the client proxy class. And in my local implementation of IMyServiceCallback, I declared the class to implement the local IMyServiceCallback interface, not the one from service side. Could this be the problem? Why is there two declarations of the interface under different projects(and hence different namespaces)? The reason I implement the client side interface is, if I implemented from the server side interface, it would give the error: “InstanceContext provided to the ChannelFactory contains a UserObject that does not implement the CallbackContractType error” when I tried to call the service. And another confusing part is, on the server side if I declare the callback interface name as IMyCallback, or anything else, instead of IMyServiceCallback, the generated interface on the client side would still be IMyServiceCallback, which is the name of the service interface plus the suffix “Callback”. And in this situation I also got the “InstanceContext provided to the ChannelFactory contains a UserObject that does not implement the CallbackContractType error”.

I guess there is something that I misunderstood about the “add service reference” and how I should implement the interface(which one to implement). Could anyone help me? Thanks!

Updated:

I somehow fixed the problem. Firstly, the two declarations is fine is desired. The local client will need to implement the local interface, which is generated when adding the service reference. And my problem was that I also defined a DataContract but the generated reference file didn’t have it. It could either because I had added the assembly of the service project as reference(somebody said in this case add service reference will not generate the Datacontract) or because I was missing DataMember attribute. But anyway, after I fixed both parts, the function is working now.

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1 Answer

  1. Editorial Team

    When you “Add Service Reference” and generate a proxy, it is totally separate from your service implementation. Remember, you may be consuming a service that you have not written and do not have access to the service source code.

    The client code should use the client generated interfaces. If you change your service, you need to regenerate the proxy.

    If you find this too messy, and you know you will always control both ends, you can share the service interfaces in a common assembly and generate a proxy class at runtime using DuplexChannelFactory.CreateChannel().

    As for your problem, I can only assume you are not registering your callback properly. This is covered here.

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