MSDN Magazine - April 2008 - (Page 37)

it to the control? Have a look at the code in Figure 3. You can use LINQ to SQL to easily load data into an object model that naturally lends itself to containing a hierarchy of data. Note that when you run a query in LINQ to SQL, you actually retrieve only the data you requested explicitly. Put another way, only the first level of the graph is fetched; any related objects are not automatically loaded at the same time. The DataLoadOptions class can be used to modify the default behavior of the LINQ to SQL engine so that data referenced by a specific relationship is loaded immediately. The code in Figure 3 ensures that orders are loaded along with customers and that details are loaded along with orders. The LoadWith method loads data according to the specified relation. The AssociateWith method can then allow for filtering related prefetched objects, as shown here: opt.AssociateWith ( c => c.Orders.Where(o => o.OrderDate.Value.Year == 1997)); when you need to prefetch related data and also when you need to apply a filter. It is entirely up to you to ensure that no cyclical references exist between tables—for instance, when you load orders for a customer and then load the customer for an order, as shown here: DataLoadOptions opt = new DataLoadOptions(); opt.LoadWith (c => c.Orders); opt.LoadWith (o => o.Customer); In this example, only orders issued in the year 1997 are prefetched when customer data is fetched. You use the AssociateWith method Figure 3 Loading the Proper Data Public Class DataCache { public IEnumerable GetCustomers() { NorthwindDataContext db = new NorthwindDataContext(); DataLoadOptions opt = new DataLoadOptions(); opt.LoadWith (c => c.Orders); opt.LoadWith (o => o.Order_Details); db.LoadOptions = opt; var data = from c in db.Customers select new { c.CompanyName, c.Orders }; } return data.ToList(); Now that you have all the data ready, you can start thinking about binding. In this case, a two-level ListView control will do the trick quite well. You bind the top-level ListView to the collection of Customer objects and the innermost ListView to the Orders property of each bound Customer object. The code in Figure 4 shows the markup for a three-level hierarchical view where customers are displayed on the first level and are rendered by the ItemTemplate property of the outermost ListView. The embedded ListView is then bound to orders. And finally, the ItemTemplate of the embedded ListView contains a GridView to list the details of each order. Figure 4 Three-Level Hierarchy <asp:ListView runat=”server” DataSource=’ ’ ItemPlaceholderID=”lvOrdersItemPlaceHolder”> Order # placed on managed by <asp:GridView runat=”server” DataSource=’ ’ SkinID=”OrderDetailsGridSkin” > public int GetCustomersCount() { // Return the number of customers NorthwindDataContext db = new NorthwindDataContext(); return db.Customers.Count(); } public IEnumerable GetCustomers(int maxRows, int startRowIndex) { if (maxRows < 0) return GetCustomers(); NorthwindDataContext db = new NorthwindDataContext(); DataLoadOptions opt = new DataLoadOptions(); opt.LoadWith (c => c.Orders); opt.LoadWith (o => o.Order_Details); db.LoadOptions = opt; var data = (from c in db.Customers select new { c.CompanyName, c.Orders }).Skip(startRowIndex).Take(maxRows); return data.ToList(); } } NorthwindDataContext db = new NorthwindDataContext(); DataLoadOptions opt = new DataLoadOptions(); opt.LoadWith (c => c.Orders); opt.LoadWith (o => o.Order_Details); db.LoadOptions = opt; Cutting Edge april2008 37

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

MSDN Magazine - April 2008 Contents Toolbox CLR Inside Out Basic Instincts Cutting Edge Foundations Test Run Service Station Windows with C++ Going Places { End Bracket }

MSDN Magazine - April 2008

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