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default values for objects that must be loaded from the database; and that means the application server must load them. Retrieving an existing object follows the same basic procedure. The UI passes criteria to the application server, which uses the criteria to create a new object and load it with the appropriate data from the database. The populated object is then returned to the UI for use. The UI code might be something like this: Customer result = null; var factory = new ChannelFactory<BusinessService.IBusinessService>("BusinessService"); try { var proxy = factory.CreateChannel(); using (proxy as IDisposable) { result = proxy.GetCustomer(criteria); } } finally { factory.Close(); } Updating an object happens when the UI calls the application server and passes the object to the server. The server can then take the data from the object and store it in the database. Because the update process may result in changes to the object s state, the newly saved and updated object is then returned to the UI. The UI code might be something like this: Customer result = null; var factory = new ChannelFactory<BusinessService.IBusinessService>("BusinessService"); try { var proxy = factory.CreateChannel(); using (proxy as IDisposable) { result = proxy.UpdateCustomer(customer); } } finally { factory.Close(); } Overall, this model is straightforward the application server must simply expose a set of services that can be called from the UI to create, retrieve, update, and delete objects. Each object can simply contain its business logic without the object developer having to worry about application servers or other details. The drawback to this scheme is that the UI code must know about and interact with the application server. If the application server is moved, or some objects come from a different server, the UI code must be changed. Moreover, if a Windows UI is created to use the objects and then later a web UI is created that uses those same objects, you ll end up with duplicated code. Both types of UI will need to include the code in order to find and interact with the application server.
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CHAPTER 2 FRAMEWORK DE SIGN
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The whole thing is complicated further if you consider that the physical configuration of the application should be flexible. It should be possible to switch from using an application server to running the data access code on the client just by changing a configuration file. If there s code scattered throughout the UI that contacts the server any time an object is used, there will be a lot of places where developers might introduce a bug that prevents simple configuration file switching.
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Another option is to move the knowledge of the application server into the objects themselves. The UI can just interact with the objects, allowing them to load defaults, retrieve data, or update themselves. In this model, simply using the new keyword creates a new object: Customer cust = new Customer(); Within the object s constructor, you would then write the code to contact the application server and retrieve default values. It might be something like this: public Customer() { var factory = new ChannelFactory<BusinessService.IBusinessService>("BusinessService"); try { var proxy = factory.CreateChannel(); using (proxy as IDisposable) { var tmp = proxy.GetNewCustomerDefaults(); _field1 = tmp.Field1Default; _field2 = tmp.Field2Default; // load all fields with defaults here } } finally { factory.Close(); } } Notice that the previous code does not take advantage of the built-in support for passing an object by value across the network. In fact, this technique forces the creation of some other class that contains the default values returned from the server. Given that both the UI-in-charge and class-in-charge techniques avoid all this extra coding, let s just abort the discussion of this option and move on.
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Class-in-Charge (Factory Pattern)
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The UI-in-charge approach uses .NET s ability to pass objects by value but requires the UI developer to know about and interact with the application server. The object-in-charge approach enables a very simple set of UI code but makes the object code prohibitively complex by making it virtually impossible to pass the objects by value. The class-in-charge option provides a good compromise by providing reasonably simple UI code that s unaware of application servers while also allowing the use of .NET s ability to pass objects by value, thus reducing the amount of plumbing code needed in each object. Hiding more information
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