Question

How would I switch the implementation of the service contract at runtime?

Say I have:

[ServiceContract]
public interface IService {
    [OperationContract]
    DoWork();
}

public class ServiceA : IService {
    public string DoWork() {
        // ....
    }
}

public class ServiceB : IService {
    public string DoWork() {
        // ....
    }
}

I’d like to be able to switch the implementation that is being used from say a config file or a value in a database between the two. Would it also be possible to do this while the WCF service is hot?

Answer 1

You need to write a servicebehavior by implementing IServiceBehavior, and initialize the service instance using an instance provider. The following initializes a new service instance, you may implement a different logic:

public class XInstanceProviderServiceBehavior : Attribute, IServiceBehavior
{        

    public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
    {
        foreach (var item in serviceHostBase.ChannelDispatchers)
        {
            var dispatcher = item as ChannelDispatcher;
            if (dispatcher != null) 
            {
                dispatcher.Endpoints.ToList().ForEach(endpoint =>
                {
                    endpoint.DispatchRuntime.InstanceProvider = new XInstanceProvider(serviceDescription.ServiceType);
                });
            }
        }
    }

    public void Validate(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
    {
    }

    public void AddBindingParameters(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase, Collection<ServiceEndpoint> endpoints, BindingParameterCollection bindingParameters)
    {
    }
}

And have your instance provider class implement IInstanceProvider and return related instance in GetInstance method.

public XInstanceProvider :IInstanceProvider
{
    ...

    public object GetInstance(InstanceContext instanceContext, System.ServiceModel.Channels.Message message)
    {
        return new ServiceX();
    }
}

Then all you need is to add servicebehaviour to service; something like

[XInstanceProviderServiceBehavior()]    
    public class MyService : IMyService