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Generator PDF417 in Visual C#.NET The INotifyCollectionChanged Interface

The INotifyCollectionChanged Interface
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In .NET 3.0, Microsoft introduced a new option for building lists for data binding. This new option only works with WPF and Silverlight and is not supported by Windows Forms or Web Forms. The System.ComponentModel.INotifyCollectionChanged interface defines a CollectionChanged event that is raised by any list implementing the interface. The simplest way to do this is to have the collection classes inherit from System.ComponentModel.ObservableCollection<T>. This generic class implements the interface and related behaviors. When implementing a list or a collection you must choose to use either IBindingList or INotifyCollectionChanged. If you implement both, data binding in WPF will become confused, as it honors both interfaces and will always get duplicate events for any change to the list. You should only choose to implement INotifyCollectionChanged or use ObservableCollection<T> if you are absolutely certain your application will only need to support WPF or Silverlight and never Windows Forms. Because CSLA .NET supports Windows Forms and Web Forms along with WPF, the list and collection types defined in the framework implement IBindingList by subclassing BindingList<T>.
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Events and Serialization
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The events that are raised by business collections and business objects are all valuable. Events support the data binding infrastructure and enable utilization of its full potential. Unfortunately, there s a conflict between the idea of objects raising events and the use of .NET serialization via the Serializable attribute. When an object is marked as Serializable, the .NET Framework is told that it can pass the object across the network by value. As part of this process, the object will be automatically converted into a byte stream by the .NET runtime. It also means that any other objects referenced by the object
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will be serialized into the same byte stream, unless the field representing it is marked with the NonSerialized attribute. What may not be immediately obvious is that events create an object reference behind the scenes. When an object declares and raises an event, that event is delivered to any object that has a handler for the event. WPF forms and Windows Forms often handle events from objects, as illustrated in Figure 2-4.
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Figure 2-4. A Windows form referencing a business object How does the event get delivered to the handling object It turns out that behind every event is a delegate a strongly typed reference that points back to the handling object. This means that any object that raises events can end up with bidirectional references between the object and the other object/entity that is handling those events, as shown in Figure 2-5.
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Figure 2-5. Handling an event on an object causes a back reference to the form. Even though this back reference isn t visible to developers, it s completely visible to the .NET serialization infrastructure. When serializing an object, the serialization mechanism will trace this reference and attempt to serialize any objects (including forms) that are handling the events. Obviously, this is rarely desirable. In fact, if the handling object is a form, this will fail outright with a runtime error because forms aren t serializable.
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If any nonserializable object handles events that are raised by a serializable object, you ll be unable to serialize the object because the .NET runtime serialization process will error out.
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Solving this means marking the events as NonSerialized. It turns out that this requires a bit of special syntax when dealing with events. Specifically, a more explicit block structure must be used to declare the event. This approach allows manual declaration of the delegate field so it is possible to mark that field as NonSerialized. The BindingList<T> class already declares its event in this manner, so this issue only pertains to the implementation of INotifyPropertyChanged and INotifyPropertyChanging (or any custom events you choose to declare in your business classes).
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