2: Handling Data in C#

Encoder USS Code 39 in C# 2: Handling Data

A more elegant approach is to design your code so that it uses the Model-ViewController (MVC) pattern. The pattern provides natural separation of the various responsibilities involved with editing and changing data through data binding. You should implement custom formatting within the form that is responsible for presenting the data in a certain format, and then associate the validation rules with the data itself, so that the rules can be reused across multiple forms. In the MVC pattern, the data itself is encapsulated in a model object. The view object is the Windows Forms control that the data is bound to. All changes to the model are handled by an intermediary controller object, which is responsible for providing access to the data and for controlling any changes made to the data through the view object. The controller object provides a natural location for validating changes made to the data, and all user interface validation logic should be implemented here. Figure 2.5 depicts the structural relationship between the three objects in the MVC pattern.

Using a controller object in this way has a number of advantages. You can configure a generic controller to provide custom validation rules, which are configurable at run time according to some contextual information (for example, the role of the user). Alternatively, you can provide a number of controller objects, with each controller object implementing specific validation rules, and then select the appropriate object at run time. Either way, because all validation logic is encapsulated in the controller object, the view and model objects do not need to change. In addition to separating data, validation logic, and user interface controls, the MVC model gives you a simple way to automatically update the user interface when the underlying data changes. The controller object is responsible for notifying the user interface when changes to the data have occurred by some other programmatic means. Windows Forms data binding listens for events generated by the objects that are bound to the controls so that the user interface can automatically respond to changes made to the underlying data.

To implement automatic updates of the user interface, you should ensure that the controller implements a change notification event for each property that may change. Events should follow the naming convention <property>Changed, where <property> is the name of the property. For example, if the controller supports a Name property, it should also support a NameChanged event. If the value of the name property changes, this event should be fired so Windows Forms data binding can handle it and update the user interface. The following code example defines a Customer object, which implements a Name property. The CustomerController object handles the validation logic for a Customer object and supports a Name property, which in turn represents the Name property on the underlying Customer object. This controller fires an event whenever the name is changed.

public class Customer { private string _name; public Customer( string name ) { _name = name; } public string Name { get { return _name; } set { _name = value; } } } public class CustomerController { private Customer _customer = null; public event EventHandler NameChanged; public Customer( Customer customer ) { this._customer = customer; } public string Name { get { return _customer.Name; } set { // TODO: Validate new name to make sure it is valid. _customer.Name = value; // Notify bound control of change. if ( NameChanged != null ) NameChanged( this, EventArgs.Empty ); } } }

Note: Customer data source members need to be initialized when they are declared. In the preceding example, the customer.Name member needs to be initialized to an empty string. This is because the .NET Framework does not have a chance to interact with the object and set the default setting of an empty string before the data binding occurs. If the customer data source member is not initialized, the attempt to retrieve a value from an uninitialized variable causes a run-time exception.