The IListSource Interface: Exposing Collections of Collections in .NET framework

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The IListSource Interface: Exposing Collections of Collections
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The IListSource interface allows a type to indicate whether it contains a collection of collections and to expose a default collection for use in data binding Data-binding code can use the IListSource interface implementation on an object to obtain a list to bind against when the data source itself is not a list, or to obtain a list of collections that the object contains Frequently you will need to work with collections of data that are related to one another The most common case of this is working with data sets in NET A data set can be thought of as a collection of collections because it can contain multiple data tables EachDataTable is a collection of data items, which are the rows of data (of type DataRow) You might also implement your own custom container type that contains other collections, such as a list of lists Or perhaps your container type only contains a single list, but you want to let consumers use your object as their data source and obtain the list from your object for the purposes of data binding For example, suppose you had a business class called InventoryManager that lets you manage your inventory items and gain access to them for various use cases You might want to allow applications to data bind to your inventory manager and have the manager control what gets exposed for data binding based on some other criteria that the class exposes: public class InventoryManager : IListSource { public string InventoryFilter { get { } set { } } public IList GetList() { // Lazy load inventory items based on current filter criteria } public bool ContainsListCollection { return false; } }
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You might also have a container with multiple collections, such as an inventory manager class that contains lists of in-stock inventory items, on-order items, and suppliers In that case you might want to provide an easy way for consumers to get a list of the lists that your object contains The IListSource interface supports these scenarios through its two members (described in Table 76) It lets you return a default collection (specifically, an object that implements IList) from a call to GetList That list could either be a simple list of objects for data binding or a list of lists contained by the object If you return true from the ContainsListCollection property, then it is assumed that the list you return from GetList is itself a list of lists
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Table 76 IListSource Interface Members
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Member IList GetList() Description This method returns an object that implements the IList interface that represents the default collection of data managed by a collection of collections container class bool ContainsListCollection This property is expected to return true if the object implementing the interface contains collections itself You would return false if your object only contains a single list collection that is accessed through the GetList method
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Property Descriptors: Allowing Dynamic Data Item Information Discovery
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An important class for allowing data-binding code to work against any collection and data item type is the PropertyDescriptor class This class is defined in the SystemComponentModel namespace and provides dynamic discovery of type information about the individual properties of an object You can obtain a property descriptor in a number of ways As discussed in the following sections, some of the data-binding interfaces have methods or properties that will return property descriptors You can also obtain them for an object any time you have a reference The TypeDescriptor class, also in the SystemComponentModel namespace, lets you request the property descriptors for an object's properties with the GetProperties method In fact, the TypeDescriptor class has a bunch of other methods that let you dynamically discover almost anything there is to know about an object, including its methods, properties, events, attributes, and so on As their name implies, property descriptors are used to describe information about an object's property The PropertyDescriptor class derives from MemberDescriptor, which provides many of the properties that you can use to discover information about the described property on the object You can find out the name of the property, its type, what its type converter is, and what attributes have been applied to its declaration in the type that it belongs to For the most part, when it comes to data binding, you will mainly be interested in four properties exposed on the PropertyDescriptor class: Name, PropertyType, ComponentType, and Converter Each of these is a read-only property
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The Name property tells you the name of the property as it was declared in the data object type definition You can use this for display purposes, such as setting the column headers in a grid, or you can use its reflection to get or set the value of the property The PropertyType gives you the type of the property itself The ComponentType gives you the type of the component (or data item type) on which the property is defined The Converter property gives you back a reference to aTypeConverter object for the property type, if there is one As discussed in 4, the Binding class uses type converters to perform automatic formatting of bound-data values to corresponding types Other complex data-bound controls, such as the DataGridView, can use type converters to do the same thing internally when rendering the values of data members within a data source
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Additionally, you can use the PropertyDescriptor to access the value of the property for instances of your data objects The GetValue method takes an object reference to the data item for which you want to obtain the value of the property described by the property descriptor It returns an object reference that contains the value of that property on the data object's instance that you pass in See Listing 71 later in this chapter for an example of usingGetValue to obtain the values of a data item's properties TheSetValue method works in a similar fashion, allowing you to set the value of an object's specific property without having any compile-time type information about that object Property descriptors also support providing change notifications for changes to property values, if those changes are made through the SetValue method exposed by the property descriptor When data-bound controls let the user edit a bound data item through the control, they do so with the SetValue method on that item's property descriptors without being coupled to the particular object type If some other piece of code needs to be notified when property values are changed in this way, you can use the AddValueChanged method to provide a callback object and method (through a delegate) This will be called whenever that property's value is changed by the property descriptor's SetValue method The BindingSource component uses this capability to be notified any time a property changes on one of the data items in the collection it contains It then raises the ListChanged event on its IBindingList implementation, enabling it to keep multiple controls that are bound to the same data source synchronized when the values are edited through those controls
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