PriorityBinding Class

Describes a collection of Binding objects that is attached to a single binding target property, which receives its value from the first binding in the collection that produces a value successfully.

Namespace: System.Windows.Data
Assembly: PresentationFramework (in presentationframework.dll)
XML Namespace:  http://schemas.microsoft.com/winfx/2006/xaml/presentation

'Declaration
<ContentPropertyAttribute("Bindings")> _
Public Class PriorityBinding
	Inherits BindingBase
	Implements IAddChild
'Usage
Dim instance As PriorityBinding

/** @attribute ContentPropertyAttribute("Bindings") */ 
public class PriorityBinding extends BindingBase implements IAddChild
ContentPropertyAttribute("Bindings") 
public class PriorityBinding extends BindingBase implements IAddChild
<PriorityBinding>
  Bindings
</PriorityBinding>

PriorityBinding lets you associate a binding target (target) property with a list of bindings. The first binding that returns a value successfully becomes the active binding.

A binding returns a value successfully if:

  1. The path to the binding source resolves successfully.

  2. The value converter, if any, is able to convert the resulting value.

  3. The resulting value is valid for the target property.

The value DependencyProperty.UnsetValue is not considered a successful return value.

The priority of the bindings is determined by their order in the list in descending order, with the binding that appears first in the list with the highest priority.

The binding engine starts with the first binding in the list and verifies whether that binding returns a value successfully; if it does, the value from that binding is used. If the first binding does not return a value successfully, the binding engine examines the second binding to determine whether it returns a value successfully; if it does, the value from the second binding becomes the active value. This verification process continues to the end of the list of bindings; if none of the bindings returns a value successfully, the binding uses the FallbackValue.

The binding engine continues to listen for changes on all bindings. If at any point one of the bindings that has a higher priority returns a value successfully, the value for that binding becomes the active value over the current value that has a lower priority.

PriorityBinding in Windows Presentation Foundation (WPF) works by specifying a list of bindings. The list of bindings is ordered from highest priority to lowest priority. If the highest priority binding returns a value successfully when it is processed then there is never a need to process the other bindings in the list. It could be the case that the highest priority binding takes a long time to be evaluated, the next highest priority that returns a value successfully will be used until a binding of a higher priority returns a value successfully.

To demonstrate how PriorityBinding works, the AsyncDataSource object has been created with the following three properties: FastDP, SlowerDP, and SlowestDP.

The get accessor of FastDP returns the value of the _fastDP data member.

The get accessor of SlowerDP waits for 3 seconds before returning the value of the _slowerDP data member.

The get accessor of SlowestDP waits for 5 seconds before returning the value of the _slowestDP data member.

NoteNote:

This example is for demonstration purposes only. The Microsoft .NET guidelines recommend against defining properties that are orders of magnitude slower than a field set would be. For more information, see Choosing Between Properties and Methods.

Public Class AsyncDataSource
    ' Properties
    Public Property FastDP As String
        Get
            Return Me._fastDP
        End Get
        Set(ByVal value As String)
            Me._fastDP = value
        End Set
    End Property

    Public Property SlowerDP As String
        Get
            Thread.Sleep(3000)
            Return Me._slowerDP
        End Get
        Set(ByVal value As String)
            Me._slowerDP = value
        End Set
    End Property

    Public Property SlowestDP As String
        Get
            Thread.Sleep(5000)
            Return Me._slowestDP
        End Get
        Set(ByVal value As String)
            Me._slowestDP = value
        End Set
    End Property


    ' Fields
    Private _fastDP As String
    Private _slowerDP As String
    Private _slowestDP As String
End Class

The Text property binds to the above AsyncDS using PriorityBinding:

<Window.Resources>
  <c:AsyncDataSource SlowestDP="Slowest Value" SlowerDP="Slower Value"
                     FastDP="Fast Value" x:Key="AsyncDS" />
</Window.Resources>
    
<StackPanel HorizontalAlignment="Center" VerticalAlignment="Center"
  DataContext="{Binding Source={StaticResource AsyncDS}}">
  <TextBlock FontSize="18" FontWeight="Bold" Margin="10"
    HorizontalAlignment="Center">Priority Binding</TextBlock>
  <TextBlock Background="Honeydew" Width="100" HorizontalAlignment="Center">
    <TextBlock.Text>
      <PriorityBinding FallbackValue="defaultvalue">
        <Binding Path="SlowestDP" IsAsync="True"/>
        <Binding Path="SlowerDP" IsAsync="True"/>
        <Binding Path="FastDP" />
      </PriorityBinding>
    </TextBlock.Text>
  </TextBlock>    
</StackPanel>

When the binding engine processes the Binding objects, it starts with the first Binding, which is bound to the SlowestDP property. When this Binding is processed, it does not return a value successfully because it is sleeping for 5 seconds, so the next Binding element is processed. The next Binding does not return a value successfully because it is sleeping for 3 seconds. The binding engine then moves onto the next Binding element, which is bound to the FastDP property. This Binding returns the value "Fast Value". The TextBlock now displays the value "Fast Value".

After 3 seconds elapses, the SlowerDP property returns the value "Slower Value". The TextBlock then displays the value "Slower Value".

After 5 seconds elapses, the SlowestDP property returns the value "Slowest Value". That binding has the highest priority because it is listed first. The TextBlock now displays the value "Slowest Value".

See PriorityBinding for information about what is considered a successful return value from a binding.

For the complete sample, see Multiple Binding Using Priority Sample.

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0

Community Additions

ADD
Show: