How do I write a WCF function with a callback?

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Hi
I'm writing WCF Service functions that will run on Azure. This is my interface, class and enum on the server site:

C#

[ServiceContract]
public interface ICertificateService
{
    [OperationContract]
    Task<Certificate> GetCertificateAsync(long serialNumber);

    [OperationContract]
    void GetCertificateWithCallback(long serialNumber, Action<Certificate> callback);
}

C#

[DataContract]
public class Certificate
{
    [DataMember]
    public long serialNumber { get; set; }

    DataMember]
    public byte[] Document { get; set; }

    [DataMember]
    public ECertificateType CertificateType { get; set; }
}

C#

[DataContract]
public enum ECertificateType
{
    [EnumMember]
    NONE = 0,
    [EnumMember]
    Single = 1,
    [EnumMember]
    Combined = 2
}

If I comment out the following

C#

//The result of this code is that I cannot see any functions at all on client side
//[OperationContract]
//void GetCertificateWithCallback(long serialNumber, Action<Certificate> callback);

then I can see and use my other function GetCertificateAsync(...) on the client side after adding a Service Reference. But if I do not comment out the code above then I cannot see any of the two functions on the client side even though I have no compile errors on server side.

I think the problem is the callback

C#

Action<Certificate> callback

So how do I write the WCF function GetCertificateWithCallback(...) below? What should be done in the interface to use Action<Certificate> callback?

C#

[OperationContract]
void GetCertificateWithCallback(long serialNumber, Action<Certificate> callback);

Posted 25-Jan-16 21:08pm

Henrik R L

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Richard Deeming · Accepted Answer · 2016-01-26T03:28:00

You can't pass a delegate to a WCF service. If you want the server to call back into the client code, you need to create a Duplex Service[^] instead.

Something like this should work:

C#

[ServiceContract(CallbackContract = typeof(ICertificateServiceCallback))]
public interface ICertificateService
{
    [OperationContract(IsOneWay = true)]
    void GetCertificateWithCallback(long serialNumber);
}

public interface ICertificateServiceCallback
{
    [OperationContract(IsOneWay = true)]
    void GetCertificateCallback(Certificate certificate);
}

sealed class CertificateService : ICertificateService
{
    private ICertificateServiceCallback Callback
    {
        get { return OperationContext.Current.GetCallbackChannel<ICertificateServiceCallback>(); }
    }
    
    public void GetCertificateWithCallback(long serialNumber)
    {
        Action<Certificate> callback = Callback.GetCertificateCallback;
        ...
    }
}

Have a look at this article for more information:
Duplex Service in WCF[^]