You are implementing a WCF Data Service to expose data from a database named Northwind.
You create a class named NorthwindDataService that extends DataService. You need to add a service operation to retrieve Order entities for orders shipped to customers in a city given by a parameter.

The service operation method must be an instance method that returns a result set as an IEnumerable or IQueryable. In order to support query options such as sorting, paging, and filtering, service operation methods should return IQueryable. Requests to service operations that include query options are rejected for operations that return only IEnumerable.

The method must be annotated with the [WebGet] or [WebInvoke] attribute. [WebGet] enables the method to be invoked by using a GET request. [WebInvoke] enables the method to be invoked by using a POST request.