MSDN Magazine - July 2008 - (Page 30)

that eager loads the Order Details for Note though that for this type of each Order, each Order entity returned app, the ObjectContext should not be from the middle tier will contain a held but created/destroyed as required. List accessible through Due to identity resolution, holding on its navigation property. to the same object context will eventuWhile the serialized entities can be ally lead to inconsistent, stale data and passed between the presenters and (as more and more data gets tracked) the service layer via WCF, the Objectdegraded performance when doing Context is neither serialized nor passed identity resolution, and it can lead to the presenter. This means the entito problems with updates in a multities can be used in the UI layers but threaded environment. the ObjectContext is left in the lower The following code shows the tiers where it will be able to access the Customer Manager class’s FindEDM and the full resources of the EnCustomerList method in the business tity Framework. layer. This method declares a LINQ to Leaving the ObjectContext behind Entities query that accesses the context means that it can’t be used to retrieve Figure 5 Editing Customers to request a list of Customer entities or modify entities in the UI layer directly, and it can’t be used to that start with the criteria. When this query is executed, it evalumanage change tracking in the UI layer. These roles are ideally left ates the mappings from the conceptual layer to the storage layer to the lower layers anyway. But when the entities are passed back and generates an appropriate SELECT command: public List FindCustomerList(string companyName) down to the lower layers, the application has to synchronize with { the ObjectContext so it can persist any changes in the entities. var q = from c in context.Customers where c.CompanyName.StartsWith(companyName) When a user clicks the Search button shown in Figure 4, the select c; presenter calls the service layer, which then calls the business layreturn q.ToList(); } er (found in the NWBusinessManagers project) to retrieve the List . This layer has two major roles. The first role is If you would like, you can view the queries as they execute using to get or put any data to or from the EDM. The second role is to the SQL Server® Profiler. handle any business logic that may exist. The CustomerManager uses the ObjectContext to handle the Persisting Changes interaction with the EDM, so it defines a local field called context Now that I have walked through the application using a simple and creates an instance of it in its constructor. The ObjectContext retrieval, it is time to examine how modifications to data can be could be created and destroyed in each method. However, it is op- persisted. When a user edits a customer, the CustomerView view timized to open and close database connection resources as need- appears bound to the appropriate Customer entity instance (see Figed. Also, by making the ObjectContext accessible throughout the ure 5). The CustomerView raises an event to the presenter, who in class, it can maintain change tracking without having to be passed turn requests the Customer entity instance from the lower layers. around a series of private methods within the class: When a user makes a modification to the customer and saves public CustomerManager() it, the entity is passed to the lower layers from the presenter using { the code shown in Figure 6. This code evaluates whether the user context = new NWEntities(); } was adding or modifying a customer and then calls the appropriate service layer method, passing the entity along for the ride. Figure 6 SaveCustomer in the Presenter The service layer will then pass control on to the business layer, which will then save the customer entity to the database. Since the public virtual void view_SaveCustomer() { customer entity is no longer part of an ObjectContext, it must first Customer customer = view.CurrentCustomer; be reunited with one by using the ObjectContext’s Attach methvar svc = new NWServiceClient(); switch (view.Mode) od, as shown in the code shown below. Once the entity has been { attached to the context, the entity’s properties must be marked case ViewMode.EditMode: svc.UpdateCustomer(customer); as modified. This can be done by using the context’s ObjectState break; Manager and invoking the SetModified method for each property. case ViewMode.AddMode: svc.AddCustomer(customer); Now that the context knows the entity is modified, the SaveChanges break; method can be issued, which will then generate a SQL UPDATE default: break; command and execute it against the database: } view.CurrentCustomer = FindCustomer(); } public void UpdateCustomer(Customer customer) { context.Attach(customer); customer.SetAllModified(context); 30 msdn magazine Data Points

Table of Contents Feed for the Digital Edition of MSDN Magazine - July 2008

MSDN Magazine - July 2008 Contents Toolbox CLR Inside Out Flex Your Data Data Points Advanced Basics Office Space Cutting Edge Data Services ADO.NET Data and WPF Transactions WCF P2P Test Run Security Briefs Foundations .NET Matters {End Bracket}

MSDN Magazine - July 2008

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