BindingSource Component Architecture

Artikel
04/23/2007

With the BindingSource component, you can universally bind all Windows Forms controls to data sources.

The BindingSource component simplifies the process of binding controls to a data source and provides the following advantages over traditional data binding:

Enables design-time binding to business objects.
Encapsulates CurrencyManager functionality and exposes CurrencyManager events at design time.
Simplifies creating a list that supports the IBindingList interface by providing list change notification for data sources that do not natively support list change notification.
Provides an extensibility point for the System.ComponentModel.IBindingList.AddNew method.
Provides a level of indirection between the data source and the control. This indirection is important when the data source may change at run time.
Interoperates with other data-related Windows Forms controls, specifically the BindingNavigator and the DataGridView controls.

For these reasons, the BindingSource component is the preferred way to bind your Windows Forms controls to data sources.

BindingSource Features

The BindingSource component provides several features for binding controls to data. With these features, you can implement most data-binding scenarios with almost no coding on your part.

The BindingSource component accomplishes this by providing a consistent interface for accessing many different kinds of data sources. This means that you use the same procedure for binding to any type. For example, you can attach the DataSource property to a DataSet or to a business object and in both cases you use the same set of properties, methods, and events to manipulate the data source.

The consistent interface provided by the BindingSource component greatly simplifies the process of binding data to controls. For data-source types that provide change notification, the BindingSource component automatically communicates changes between the control and the data source. For data-source types that do not provide change notification, events are provided that let you raise change notifications. The following list shows the features supported by the BindingSource component:

Indirection.
Currency management.
Data source as a list.
BindingSource as an IBindingList.
Custom item creation.
Transactional item creation.
IEnumerable support.
Design-time support.
Static ListBindingHelper methods.
Sorting and filtering with the IBindingListView interface.
Integration with BindingNavigator.

Indirection

The BindingSource component provides a level of indirection between a control and a data source. Instead of binding a control directly to a data source, you bind the control to a BindingSource, and you attach the data source to the BindingSource component's DataSource property.

With this level of indirection, you can change the data source without resetting the control binding. This gives you the following capabilities:

You can attach the BindingSource to different data sources while retaining the current control bindings.
You can change items in the data source and notify bound controls. For more information, see How to: Reflect Data Source Updates in a Windows Forms Control with the BindingSource.
You can bind to a Type instead of an object in memory. For more information, see How to: Bind a Windows Forms Control to a Type. You can then bind to an object at run time.

Currency Management

The BindingSource component implements the ICurrencyManagerProvider interface to handle currency management for you. With the ICurrencyManagerProvider interface, you can also access to the currency manager for a BindingSource, in addition to the currency manager for another BindingSource bound to the same DataMember.

The BindingSource component encapsulates CurrencyManager functionality and exposes the most common CurrencyManager properties and events. The following table describes some of the members related to currency management.

CurrencyManager property
Gets the currency manager associated with the BindingSource.

GetRelatedCurrencyManager method
If there is another BindingSource bound to the specified data member, gets its currency manager.

Current property
Gets the current item of the data source.

Position property
Gets or sets the current position in the underlying list.

EndEdit method
Applies pending changes to the underlying data source.

CancelEdit method
Cancels the current edit operation.

Data Source as a List

The BindingSource component implements the IBindingListView and ITypedList interfaces. With this implementation, you can use the BindingSource component itself as a data source, without any external storage.

When the BindingSource component is attached to a data source, it exposes the data source as a list.

The DataSource property can be set to several data sources. These include types, objects, and lists of types. The resulting data source will be exposed as a list. The following table shows some of the common data sources and the resulting list evaluation.

DataSource property	List results
A null reference (Nothing in Visual Basic)	An empty IBindingList of objects. Adding an item sets the list to the type of the added item.
A null reference (Nothing in Visual Basic) with DataMember set	Not supported; raises ArgumentException.
Non-list type or object of type "T"	An empty IBindingList of type "T".
Array instance	An IBindingList containing the array elements.
IEnumerable instance	An IBindingList containing the IEnumerable items
List instance containing type "T"	An IBindingList instance containing type "T".

Additionally, DataSource can be set to other list types, such as IListSource and ITypedList, and the BindingSource will handle them appropriately. In this case, the type that is contained in the list should have a default constructor.

BindingSource as an IBindingList

The BindingSource component provides members for accessing and manipulating the underlying data as an IBindingList. The following table describes some of these members.

Member	Description
List property	Gets the list that results from the evaluation of the DataSource or DataMember properties.
AddNew method	Adds a new item to the underlying list. Applies to data sources that implement the IBindingList interface and allow adding items (that is, the AllowNew property is set to true).

Custom Item Creation

You can handle the AddingNew event to provide your own item-creation logic. The AddingNew event occurs before a new object is added to the BindingSource. This event is raised after the AddNew method is called, but before the new item is added to the underlying list. By handling this event, you can provide custom item creation behavior without deriving from the BindingSource class. For more information, see How to: Customize Item Addition with the Windows Forms BindingSource.

Transactional Item Creation

The BindingSource component implements the ICancelAddNew interface, which enables transactional item creation. After a new item is provisionally created by using a call to AddNew, the addition may be committed or rolled back in the following ways:

The EndNew method will explicitly commit the pending addition.
Performing another collection operation, such as an insertion, removal, or move, will implicitly commit the pending addition.
The CancelNew method will roll back the pending addition if the method has not already been committed.

IEnumerable Support

The BindingSource component enables binding controls to IEnumerable data sources. With this component, you can bind to a data source such as a System.Data.SqlClient.SqlDataReader.

When an IEnumerable data source is assigned to the BindingSource component, the BindingSource creates an IBindingList and adds the contents of the IEnumerable data source to the list.

Design-Time Support

Some object types cannot be created at design time, such as objects created from a factory class, or objects returned by a Web service. You may sometimes have to bind your controls to these types at design time, even though there is no object in memory to which your controls can bind. You may, for example, need to label the column headers of a DataGridView control with the names of your custom type's public properties.

To support this scenario, the BindingSource component supports binding to a Type. When you assign a Type to the DataSource property, the BindingSource component creates an empty BindingList of Type items. Any controls you subsequently bind to the BindingSource component will be alerted to the presence of the properties or schema of your type at design time, or at run time. For more information, see How to: Bind a Windows Forms Control to a Type.

Static ListBindingHelper Methods

The System.Windows.Forms.BindingContext, System.Windows.Forms.CurrencyManager, and BindingSource types all share common logic to generate a list from a DataSource/DataMember pair. Additionally, this common logic is publicly exposed for use by control authors and other third parties in the following static methods:

Sorting and Filtering with the IBindingListView Interface

The BindingSource component implements the IBindingListView interface, which extends the IBindingList interface. The IBindingList offers single column sorting and the IBindingListView offers advanced sorting and filtering. With IBindingListView, you can sort and filter items in the data source, if the data source also implements one of these interfaces. The BindingSource component does not provide a reference implementation of these members. Instead, calls are forwarded to the underlying list.

The following table describes the properties you use for sorting and filtering.

Member	Description
Filter property	If the data source is an IBindingListView, gets or sets the expression used to filter which rows are viewed.
Sort property	If the data source is an IBindingList, gets or sets a column name used for sorting and sort order information. -or- If the data source is an IBindingListView and supports advanced sorting, gets multiple column names used for sorting and sort order

Filter property

If the data source is an IBindingListView, gets or sets the expression used to filter which rows are viewed.

Sort property

If the data source is an IBindingList, gets or sets a column name used for sorting and sort order information.

-or-

If the data source is an IBindingListView and supports advanced sorting, gets multiple column names used for sorting and sort order

Integration with BindingNavigator

You can use the BindingSource component to bind any Windows Forms control to a data source, but the BindingNavigator control is designed specifically to work with the BindingSource component. The BindingNavigator control provides a user interface for controlling the BindingSource component's current item. By default, the BindingNavigator control provides buttons that correspond to the navigation methods on the BindingSource component. For more information, see How to: Navigate Data with the Windows Forms BindingNavigator Control.

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