How to expose service contract interfaces with multiple inheritance in WCF service on single endpoint

Question

I have only simple data types in method signature of service (such as int, string). My service class implements single ServiceContract interface say IMathService, and this interface in turn inherits from some other base interface say IAdderService. I want to expose the MathService using interface contract IAdderService as a service on a single endpoint. However some of the clinet's which know about IMathService should be able to access the extra services provided by IMathService on that single endpoint i.e. by just typecasting IAdderService to IMathService.

//Interfaces and classes at server side
[ServiceContract]
public interface IAdderService
{
[OperationContract]
int Add(int num1, int num2);
}

[ServiceContract]
public interface IMathService : IAdderService
{
[OperationContract]
int Substract(int num1, int num2);
}

public class MathService : IMathService
{
#region IMathService Members
public int Substract(int num1, int num2)
{
    return num1 - num2;
}
#endregion

#region IAdderService Members
public int Add(int num1, int num2)
{
    return num1 + num2;
}
#endregion
}

//Run WCF service as a singleton instace
MathService mathService = new MathService();
ServiceHost host = new ServiceHost(mathService);
host.Open();

Server side Configuration:

 <configuration>
   <system.serviceModel>
     <services>
            <service name="IAdderService"
         behaviorConfiguration="AdderServiceServiceBehavior"> 
        <endpoint address="net.pipe://localhost/AdderService"
      binding="netNamedPipeBinding"
      bindingConfiguration="Binding1"
      contract="TestApp.IAdderService" />
         <endpoint address="mex"
      binding="mexNamedPipeBinding"
      contract="IMetadataExchange" />
        <host>
           <baseAddresses>
            <add baseAddress="net.pipe://localhost/AdderService"/>
           </baseAddresses>
        </host>
           </service>
          </services>
       <bindings>
        <netNamedPipeBinding>
      <binding name="Binding1" >
          <security mode = "None">
           </security>
           </binding >
        </netNamedPipeBinding>
       </bindings>

      <behaviors>
        <serviceBehaviors>
     <behavior name="AdderServiceServiceBehavior">
            <serviceMetadata />
       <serviceDebug includeExceptionDetailInFaults="True" />
     </behavior>
        </serviceBehaviors>
      </behaviors>
     </system.serviceModel>
    </configuration>

Client Side imeplementation:

IAdderService adderService = new
ChannelFactory<IAdderService>("AdderService").CreateChannel();
int result = adderService.Add(10, 11);

IMathService mathService = adderService as IMathService; 
result = mathService.Substract(100, 9);

Client side configuration:

    <configuration>
       <system.serviceModel>
          <client>
            <endpoint name="AdderService" address="net.pipe://localhost/AdderService" binding="netNamedPipeBinding" bindingConfiguration="Binding1" contract="TestApp.IAdderService" />
          </client>

          <bindings>
            <netNamedPipeBinding>
         <binding name="Binding1" maxBufferSize="65536" maxConnections="10">
          <security mode = "None">
          </security>
         </binding >
            </netNamedPipeBinding>
          </bindings>
        </system.serviceModel>
      </configuration>

Using above code and configuration I am not able to typecast IAdderService instnace to IMathService, it fails and I get null instance of IMathService at client side.

My observation is if server exposes IMathService to client then client can safely typecast to IAdderService and vice versa is also possible. However if server exposes IAdderService then the typecast fails.

Is there any solution to this? or am I doing it in a wrong way.

Answer 1

Just as you observed, you have to expose the derived service (MathService) instead of the AdderService.

WCF has no idea that AdderService happens to actually be a MathService implementing a MathService interface. It is regarded as AdderService - so there is no way to cast it to a derived class.

Answer 2

I replicated your situation exactly except I used an IMathService endpoint rather than an IAdderService endpoint.

IAdderService adderService = new ChannelFactory<IMathService>("MathService").CreateChannel();
int result = adderService.Add(10, 11);

IMathService mathService = adderService as IMathService;
result = mathService.Substract(100, 9);

This works for me. You can't cast a base type to a derived type. You can do the reverse however.