Need help implementing wcf threading with duplex channel.

Question

I am implementing a duplex channel and all works ok. Now I have to implement it in a thread safe manner.I am kind of stuck and wondering if you could point me or suggest anything.

This is just a quick demo code to give you an idea of what I am doing.

How can I make the Buy Function Process asyncronous and ThreadSafe?

        [ServiceContract(CallbackContract = typeof(ICustomerServiceCallback))]
        public interface ICustomerService
       {
           [OperationContract(IsOneWay = true)]
           void Buy(List<Product> productList);
       }


       [ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Reentrant)]
       public class CustomerService : ICustomerService
       {
           private ICustomerServiceCallback _customerServiceCallback;

           //NEED TO CONVERT THIS TO BE WORKING ON A DIFFERENT THREAD AND NOTIFY.
           public void Buy(List<Product> productList)
           {
                _customerServiceCallback = OperationContext.Current.GetCallbackChannel<ICustomerServiceCallback>();

                foreach (var product in productList)
                {
                    //go to the db or do something and notify me after each product is processed.
                    _customerServiceCallback.NotifyAfterEachOrder(true);
                }
           }
        }

       public interface ICustomerServiceCallback
       {
          [OperationContract]void NotifyAfterEachOrder(bool status);
       }

       [DataContract]
       public class Product
       {
           [DataMember]public String Name { get; set; }
       }


       [CallbackBehavior(UseSynchronizationContext = false)]
       public class Client : ICustomerServiceCallback
       {
          public void NotifyAfterEachOrder(bool status)
          {
                //do something now.
          }
       }