MSDN Magazine - March 2008 - (Page 67)

Sponsored Content C#: var q = from order in db.Orders where null == order.ShippedDate && order.RequiredDate < DateTime.Today select order; In the queries above, note the following: • You don’t need to declare a type for the variable q. This variable represents your query, and will be implicitly typed based on the type of entity you select. • db.Orders represents the Orders table from your context. • order is an iterator variable that represents each row within the orders table. • The filter expression in the where clause is built using language expressions, not strings. • The ShippedDate and RequiredDate properties on orders are accessed as strongly typed properties, with IntelliSense and statement completion. • The select statement projects out the entire order that matches the where clause. 4. Execute the query by enumerating the results. The NorthwindEntities class will automatically open a connection to the underlying store using the connection string and mapping information specified when you created the model, generate and execute the query against the store, and retrieve results as strongly-typed objects. VB: For Each order In q Console.WriteLine( _ “OrderID={0} RequiredDate={1}”, _ order.OrderID, _ order.RequiredDate) Next C#: foreach (Orders order in q) Console.WriteLine( “OrderID={0} RequiredDate={1}”, order.OrderID, order.RequiredDate); 5. At this point, you should be able to run your application by pressing Ctrl+F5. Adding Business Logic Although the classes representing your application model are generated through the designer, you can still add business logic through partial classes, together with a new feature in VB and C# called partial methods. Partial classes allow a class definition to be split between two files, in this case one generated by the designer and one hand-written. Partial methods allow you to declare a signature for and make calls to a method in one part of your class (the generated portion). If the method is implemented in another part of the class (the hand-written part) then it is called, otherwise at compile time the call site is completely optimized away. When data classes are generated, two partial methods are declared for each property; one that is called prior to setting the property (named OnPropertyNameChanging) and one that is called after setting the property (named OnPropertyNameChanged). You can insert business logic by implementing these partial methods in partial classes that are compiled together with the generated partial classes. If you later have to regenerated your classes through the designer, your custom logic will not be affected. For example, in our model Order_Details have a Discount property. By implementing the OnDiscountChanged partial method we can add logic to enforce that the discount never exceeded 20%.

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

MSDN Magazine - March 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Office Space Introducing ASP.NET MVC Loosen Up CI Server Performance Office Development Test Run Security Briefs Extreme ASP.NET Foundations .NET Matters {End Bracket}

MSDN Magazine - March 2008

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