MSDN Magazine - July 2008 - (Page 74)

Figure 7 Using ObjectContext using (ObjectContext context = new ObjectContext("name=travelEntities")) { // create a query for customers ObjectQuery personQuery = context.CreateQuery ( @"select value c from travelEntitiesGeneral.People as c where c.PersonID == 1"); // by enumerating the query will be implicitly executed // against the store and you can now work with an // IEnumerable foreach (Person c in personQuery) { // dereference anything you like from Customer Console.WriteLine(c.PersonID + ": " + c.Name); c.Name = "New Name"; } try { context.SaveChanges(); } catch (OptimisticConcurrencyException opt) { // catching this exception allows you to // refresh travelEntities with either store/client wins // project the travelEntities into this failed travelEntities. var failedEntities = from e3 in opt.StateEntries select new { e3.Entity }; // Note: in future you should be able to just pass // the opt.StateEntities // in to refresh. context.Refresh(RefreshMode.ClientWins, failedEntities.ToList()); context.SaveChanges(); dynamic tool or application that consumes EDM models. When using Visual Studio as the development environment, however, developers see the added benefit of a strongly typed ObjectContext, adding properties and methods to surface capabilities that may be specific to the EDM being targeted. Figure 8 shows a query built using a strongly typed ObjectContext. This example demonstrates the use of LINQ as the query language. Using a strongly typed ObjectContext exposes properties for each EntitySet, making them more discoverable; for example, travelEntities.BlogPosts instead of travelEntities.CreateQuery (“travelEntitiesGeneral.BlogPosts”). LINQ to Entities can be seen as a fairly thin layer over Object Services, providing query facilities directly within the programming language (see Figure 8) as opposed to a string-based query. In this case, the ObjectQuery class implements IQueryable, allowing it to take a LINQ expression tree, pushing the query through the Entity Framework as a CCT query expression in the same manner as Object Services would pass an ESQL query to the EntityClient provider. N-Tier Development with the Entity Framework } } While it is not my primary goal to fully address n-tier development here, as one of the more interesting scenarios for development with the Entity Framework it should be touched on. In version 1.0, the Entity Framework supports n-tier development in a couple of Figure 8 Query Built Using a Strongly Typed ObjectContext using (MyTravelPostEntities travelEntities = new MyTravelPostEntities()) { // get the latest blog post, with the comments and the people // I'm querying for all the blog posts that are related to this blog. // I want to include the comments and the people who wrote the // comments. // I also want only the most recent posting. // Note: Since we use the EntityKey that is put on the EntityReference // we can either do a tracking query or use span. BlogPost post = (from bp in travelEntities.BlogPosts .Include("Comments.Person") .Include("Blog") where bp.Blog.BlogID == requestedBlog.BlogID orderby bp.BlogDate descending select bp).First(); return post; } Framework that is responsible for initially attracting many developers to the technology when looking at the available ORM technologies in the marketplace. As you saw in Figure 1, the high-level function of the Object Services layer is to take in either ESQL or LINQ queries from the application, pass a query expression to the EntityClient below, and return an IEnumerable . To look a bit deeper, however, you see that at the heart of the Object Services layer is the ObjectContext, representing the session of interaction between the application and the underlying data store. The ObjectContext is the primary construct that the developer will work with to query, add, and delete instances of his entities and to save new state back to the database. Creation and use of ObjectContext to query, manipulate, and SaveChanges to an Entity is shown in Figure 7. This example uses ESQL as the query language. The process of tracking changes as they are made to the objects in memory and the process of saving those changes back to the database is simplified for the developer through the use of Object Services. Object Services makes use of the ObjectStateManager to track not only the current state of the instances in memory but also the initial state of each instance as it was retrieved from the store, allowing the Entity Framework to apply optimistic concurrency as data is pushed back to the database. Tracked changes are easily saved and pushed back to the data store with invocation of the SaveChanges method on the ObjectContext. Up to this point, I have been speaking about the ObjectContext in general, and my examples have shown the use of a base ObjectContext, which is often used in the scenario where you have a 74 msdn magazine Figure 9 ADO.NET Data Services in N-Tier Apps static Uri baseService = new Uri("http://localhost:17338/MyTravelPostService.svc"); MyPeople2Entities context = new MyPeople2Entities(baseService); // get the comment that is being marked for deletion // and get the view state blog post. BlogPost post = (BlogPost)ViewState["BlogPost"]; // move the comment to the deleted comment selection. Comment deletedComment = post.Comments[e.RowIndex]; // call the DeleteComment service context.AttachTo("Comments", deletedComment); context.DeleteObject(deletedComment); DataServiceResponse r = context.SaveChanges(); // reload page so that F5, refresh doesn't update all this data. ReloadPage(); Entity Framework

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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