Notifying Consumers of Changes to the Collection in .NET

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If the collection supports changes, it should also support firing a ListChanged event when the collection changes To indicate that, it should return true from the SupportsChangeNotification property The collection itself should be able to raise ListChanged events when items are added or removed from the collection Ideally, it will also be able to provide ListChanged notifications when existing items in the collection change because their properties have changed However, the collection's properties are changed and what support the contained object types provide ability to do this will be dictated by how the
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As mentioned earlier, if changes are made through a property descriptor's SetValue method, a container can call the AddValueChanged method on the property descriptor and provide a callback delegate so that the container will be notified when the property changes It can then raise the ListChanged event in response to notification from the property descriptor that the property changed This is exactly what an implementation of the IRaiseItemChangedEvents interface, discussed in a later section, is expected to do However, if the property is changed directly through its property setter through a reference to the object, there is no way for the collection to know about the change unless the object itself notifies the collection The support for that comes from the INotifyPropertyChanged interface Another form of change that a collection can support is a dynamically changing schema, where new properties are added to the collection items at runtime or design time The ListChanged event also supports notification of this type of change through its event arguments The ListChanged event is of type ListChangedEventHandler, which carries an event argument of typeListChangedEventArgs along with it This event argument's properties, listed in Table 711, give you more information on the changes to the list
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Table 711 ListChangedEventArgs Properties
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Name ListChangedType Type ListChangedType Description Indicates what was changed about the list The values of the ListChangedType enumeration are shown inTable 712 This is a read-only property NewIndex Integer Indicates the index of the item in the list that was affected by the change The default value is 1 if not applicable to the change type This is a read-only property Gives the old index of the item that was changed in case the change type is a move The default value is 1 if not applicable to the change type This is a read-only property Gives the property's property descriptor that was affected by the change This is a read-only property
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OldIndex
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PropertyDescriptor
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PropertyDescriptor
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Table 712 ListChangedType Enumeration Values
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Value Reset Description Indicates a major change in the list and is a signal to bound controls that they should completely rebind to the collection Indicates a new item was added to the collection The NewIndex property of the event argument indicates the location of the new item in the collection ItemDeleted Indicates an item was removed from the collection Strangely, the NewIndex property of the event argument indicates the index of the item that was removed (OldIndex would seem more appropriate for this case, but is not used) ItemMoved Indicates that an item was relocated to a new index position in the collection The NewIndex property of the event argument contains the new index position and the OldIndex property shows where it was before ItemChanged Indicates that an item in the collection was modified directly This requires that the data item object type supports notifying the collection of the change so that it can raise the ListChanged event PropertyDescriptorAdded Indicates that the schema of the items in the collection changed by adding a property (or column) to the data items If this is the ListChangedType, then the PropertyDescriptor property of the event arguments will contain the property information about the new property PropertyDescriptorDeleted Indicates that the schema of the items in the collection changed by deleting a property (or column) from the data items If this is the ListChangedType, then the PropertyDescriptor property of the event arguments will contain the property information about the property that was deleted PropertyDescriptorChanged Indicates that something changed about a property in the schema The PropertyDescriptor will contain the new property information after the change
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