ObservableCollection(Of T) Class
Updated: July 2008
Represents a dynamic data collection that provides notifications when items get added, removed, or when the whole list is refreshed.
Assembly: WindowsBase (in WindowsBase.dll)
XMLNS for XAML: Not mapped to an xmlns.
'Declaration <SerializableAttribute> _ Public Class ObservableCollection(Of T) _ Inherits Collection(Of T) _ Implements INotifyCollectionChanged, INotifyPropertyChanged 'Usage Dim instance As ObservableCollection(Of T)
The type of elements in the collection.
In many cases the data that you work with is a collection of objects. For example, a common scenario in data binding is to use an ItemsControl such as a ListBox, ListView, or TreeView to display a collection of records.
You can enumerate over any collection that implements the IEnumerable interface. However, to set up dynamic bindings so that insertions or deletions in the collection update the UI automatically, the collection must implement the INotifyCollectionChanged interface. This interface exposes the CollectionChanged event, an event that should be raised whenever the underlying collection changes.
WPF provides the class, which is a built-in implementation of a data collection that implements the INotifyCollectionChanged interface.
Before implementing your own collection, consider using or one of the existing collection classes, such as List(Of T), Collection(Of T), and BindingList(Of T), among many others. If you have an advanced scenario and want to implement your own collection, consider using IList, which provides a non-generic collection of objects that can be individually accessed by index. Implementing IList provides the best performance with the data binding engine.
To fully support transferring data values from binding source objects to binding targets, each object in your collection that supports bindable properties must implement an appropriate property changed notification mechanism such as the INotifyPropertyChanged interface.
For more information, see "Binding to Collections" in Data Binding Overview.
Notes on XAML Usage
can be used as a XAML object element in Windows Presentation Foundation (WPF), in versions 3.0 and 3.5. However, the usage has substantial limitations.
must be the root element, because the x:TypeArguments attribute that must be used to specify the constrained type of the generic is only supported on the object element for the root element.
You must declare an x:Class attribute (which entails that the build action for this XAML file must be Page or some other build action that compiles the XAML).
is in a namespace and assembly that are not initially mapped to the default XML namespace. You must map a prefix for the namespace and assembly, and then use that prefix on the object element tag for .
A more straightforward way to use capabilities from XAML in an application is to declare your own non-generic custom collection class that derives from , and constrains it to a specific type. Then map the assembly that contains this class, and reference it as an object element in your XAML.
This example shows how to create and bind to a collection that derives from the class, which is a collection class that provides notifications when items get added or removed.
The following example shows the implementation of a NameList collection:
Public Class NameList Inherits ObservableCollection(Of PersonName) ' Methods Public Sub New() MyBase.Add(New PersonName("Willa", "Cather")) MyBase.Add(New PersonName("Isak", "Dinesen")) MyBase.Add(New PersonName("Victor", "Hugo")) MyBase.Add(New PersonName("Jules", "Verne")) End Sub End Class Public Class PersonName ' Methods Public Sub New(ByVal first As String, ByVal last As String) Me._firstName = first Me._lastName = last End Sub ' Properties Public Property FirstName() As String Get Return Me._firstName End Get Set(ByVal value As String) Me._firstName = value End Set End Property Public Property LastName() As String Get Return Me._lastName End Get Set(ByVal value As String) Me._lastName = value End Set End Property ' Fields Private _firstName As String Private _lastName As String End Class
You can make the collection available for binding the same way you would with other common language runtime (CLR) objects, as described in How to: Make Data Available for Binding in XAML. For example, you can instantiate the collection in XAML and specify the collection as a resource, as shown here:
<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:c="clr-namespace:SDKSample" x:Class="SDKSample.Window1" Width="400" Height="280" Title="MultiBinding Sample"> <Window.Resources> <c:NameList x:Key="NameListData"/> ... </Window.Resources>
You can then bind to the collection:
The definition of NameItemTemplate is not shown here. For the complete sample, see Implementing Parameterized MultiBinding Sample.
The objects in your collection must satisfy the requirements described in the Binding Sources Overview. In particular, if you are using OneWay or TwoWay (for example, you want your UI to update when the source properties change dynamically), you must implement a suitable property changed notification mechanism such as the INotifyPropertyChanged interface.
For more information, see the Binding to Collections section in the Data Binding Overview.
Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.