Dispatcher required for changing WPF bound properties?

Question

Hello,
From time to time I read some articles that state that if WPF binds to a property that implements INotifyProperyChanged, such property can be changed from any thread without a dispatcher, because WPF would automatically dispatch the INotifyProperyChanged event to the UI-thread.
On the other hand, all Microsoft samples that I found are still using the dispatcher even for INotifyProperyChanged implementing properties.

To be very clear here, I'm not talking about collections, nor dependency properties.

Some links that talk about such "auto-dispatching" are:
https://stackoverflow.com/questions/8994714/updating-bound-properties-from-a-background-thread
https://learn.microsoft.com/en-us/archive/msdn-magazine/2014/april/mvvm-multithreading-and-dispatching-in-mvvm-applications (2nd paragraph under 'Dispatching in MVVM applications')
https://metashapes.com/blog/not-shooting-foot-wpf-best-practices/ (Item #4)

If this would be true, it would ease programming a lot.
Currently, I'm dispatching everything, and I would only change this if I can be sure that this is an official feature and will still work on .Net 5.

So, any information, especially official sources, are highly appreciated.
Thanks, Sebastian

Answer 1

Hi Sebatian, starting with .NET 4.5 I use NotifyPropertyChanged in non-UI-thread without dispatching to inform the UI for refreshing, only if I don't consume PropertyChanged event in my own code.

Answer 2

Hello PeterFleischer,

I checked your code and found the reason for the InvalidOperationException. It has nothing to do with the PropertyChanged, that you throw from a background thread. The reason for the exception is, that you are trying to modify a UI bound collection from a background thread (see codebehind line 19).

If you want to modify a UI bound collection from a background thread, than you have to use either the UI dispatcher or BindingOperations.EnableCollectionSynchronization.

Here is the modified Window97 class:

Public Class Window97

    Private uiDispatcher As Dispatcher = Dispatcher.CurrentDispatcher

    Public Sub New()

        ' This call is required by the designer.
        InitializeComponent()

        ' Add any initialization after the InitializeComponent() call.
        Dim vm As New Window97VM(AddressOf prot)
        Me.DataContext = vm
    End Sub

    Private Sub prot(msg As String)
        uiDispatcher.Invoke(New Action(Sub()
                                           Me.lbProt.Items.Add(msg)
                                       End Sub))
    End Sub
End Class

Answer 3

Hi Sebastian,

try following demo with and without comments.

XAML:

<Window x:Class="Window97"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="Window97" Height="450" Width="800">
  <StackPanel>
    <ListBox ItemsSource="{Binding View}" Height="200">
      <ListBox.ItemTemplate>
        <DataTemplate>
          <TextBlock>            
            <Hyperlink NavigateUri="{Binding NaviUri}" IsEnabled="{Binding Ready}">
              <Run Text="{Binding LinkName}"/> 
            </Hyperlink>
          </TextBlock>
        </DataTemplate>
      </ListBox.ItemTemplate>
    </ListBox>
    <ListBox x:Name="lbProt" Height="200"/>
  </StackPanel>
</Window>

and code:

Imports System.Collections.ObjectModel
Imports System.ComponentModel
Imports System.Runtime.CompilerServices
Imports System.Threading
Imports System.Windows.Threading

Public Class Window97
  Public Sub New()

    ' This call is required by the designer.
    InitializeComponent()

    ' Add any initialization after the InitializeComponent() call.
    Dim vm As New Window97VM(AddressOf prot)
    Me.DataContext = vm
  End Sub

  Private Sub prot(msg As String)
    Me.lbProt.Items.Add(msg)
  End Sub
End Class

Public Class Window97VM

  Private rnd As New Random
  Public Sub New(p As Action(Of String))
    For i = 1 To 100
      Dim d As New Window97Data With {.LinkName = $"Link {i} noready", .NaviUri = "http://www.microsoft.com", .Ready = False}
      AddHandler d.PropertyChanged, Sub(sender, e)
                                      ' comment this out for test without UI dispatcher
                                      p($"NotifyPropertyChanged in {e.PropertyName}")
                                    End Sub
      col.Add(d)
    Next
    cvs.Source = col
    '
    ThreadPool.SetMaxThreads(3, 3)
    For Each item As Window97Data In col
      ThreadPool.QueueUserWorkItem(Sub(state)
                                     item.Execute(CType(state, Integer))
                                   End Sub, rnd.Next(500, 2000))
    Next
  End Sub

  Private col As New ObservableCollection(Of Window97Data)
  Private cvs As New CollectionViewSource

  Public ReadOnly Property View As ICollectionView
    Get
      Return cvs.View
    End Get
  End Property

End Class

Public Class Window97Data
  Implements INotifyPropertyChanged
  Public Property LinkName As String
  Public Property NaviUri As String
  Public Property Ready As Boolean

  Private uiDispatcher As Dispatcher = Dispatcher.CurrentDispatcher

  Friend Sub Execute(par As Integer)
    Thread.Sleep(par)
    LinkName = LinkName.Replace("noready", "ready")
    Ready = True
    ' comment tis out for test without UI dispatcher
    'uiDispatcher.Invoke(New Action(Sub()
    OnPropChanged(NameOf(LinkName))
    OnPropChanged(NameOf(Ready))
    'End Sub))
  End Sub

  Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
  Private Sub OnPropChanged(<CallerMemberName> Optional propName As String = "")
    RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propName))
  End Sub
End Class