The INotifyPropertyChanged Interface: Publishing Item Change Notifications in VS .NET

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The INotifyPropertyChanged Interface: Publishing Item Change Notifications
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The INotifyPropertyChanged interface lets an object notify its container any time a property on the object changes This allows the containing collection to raise a ListChanged event when an item in the collection has one or more of its property values changed The BindingList<T> class uses this interface to bubble up ListChanged events when contained objects are edited either through programmatic code or data-bound control This results in consistent synchronization of object values that are bound to multiple controls The IBindingList interface defines a ListChanged event that is designed to notify a collection's client when anything about the list has changed One kind of change it is designed to support is modifications to the data items that are contained in the collection However, there needs to be a way for the list itself to be notified when an item within the collection changes A control can index into a collection and obtain a direct reference to an object The control can use the object in a variety of ways and maintain the reference to it for a long period of time Other controls also bound to that object will want to know when the object has changed so they can refresh the way they are rendered or react to the change The INotifyPropertyChanged interface provides a contract for objects to notify their containers that they have changed, so the container can bubble that information up to any bound controls It is a very simple interface It defines a single event member, PropertyChanged, of type PropertyChangedEventHandler The event signature includes the usual object as the first parameter for the sender and a PropertyChangedEventArgs second parameter The PropertyChangedEventArgs is itself simple: it tells the name of the property that was changed Once you have been notified that a particular property has changed, your consuming code can refresh whatever dependencies you have on that property The main consumer of this interface is the BindingList<T> class, and it uses this to raiseListChanged events to any bound controls, or to the BindingSource component when properties are modified on the collection's items You will see this interface in action in the samples in 9 as well
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The IDataErrorInfo Interface: Providing Error Information
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The IDataErrorInfo interface lets an object store and expose error information that can be used by bound controls to notify the user when errors were encountered while using the object It exposes a top-level error message for the object as a whole, as well as an indexer that can expose per-property error messages Controls such as the DataGridView can use this information to expose in-place error indications and messages to the user When a control is bound to a collection of data items, and the data items can be modified either by the control itself or through other code in the application, things can go wrong Someone could try to stuff a value of an inappropriate type into a business object's loosely typed property Someone could pass in a value that is outside of an allowable validation range An error could occur when the value contained in the object is used to try to persist the data to a database In any of these situations, the bound control might want to know that an error occurred and may be designed to present some information to the user about the error A good example of this is aDataGridView control that is data bound to aDataView If an error occurs in any of the columns of a row in the underlying data table, the DataRow class can store the error information in itsErrors collection When this occurs, it stores not only what the error was, but specifically what column within the row was affected by the error When an error occurs in a data row, it is reflected in the grid with an error icon next to the offending cell When a user's mouse cursor hovers over an error icon, the error message for the problem that occurred displays in that column in that row This all happens when a DataRowView (the row objects within aDataView) implements the IDataErrorInfo interface, and the DataGridView is coded to look for that interface on the data items in any collection it is bound to If the grid sees that the data items it is presenting in the grid's rows implement this interface, the grid will use the interface's properties shown in Table 715 to determine if any of the columns have errors that should be presented or whether the object itself has a general error to be displayed
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