I have the following code XAML:
MainWindow.xaml:
<Window x:Class="MultiThreadListBox.MainWindow" 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:MultiThreadListBox" mc:Ignorable="d" Title="MainWindow" Width="250" Height="450"> <Grid> <Grid.RowDefinitions> <RowDefinition Height="*"/> <RowDefinition Height="Auto"/> <RowDefinition Height="Auto"/> </Grid.RowDefinitions> <ListBox x:Name="listBox" Grid.Row="0" ItemsSource="{Binding ItemList}" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="10,10" Width="Auto" > <ListBox.ItemTemplate> <DataTemplate> <StackPanel Orientation="Horizontal"> <TextBlock Text="{Binding Comment}"/> <TextBlock Text="{Binding Id}"/> </StackPanel> </DataTemplate> </ListBox.ItemTemplate> </ListBox> <Grid Grid.Row="1"> <Grid.ColumnDefinitions> <ColumnDefinition Width="*"/> <ColumnDefinition Width="*"/> <ColumnDefinition Width="*"/> </Grid.ColumnDefinitions> <Button x:Name="sameThreadButton" Grid.Column="0" Click="sameThreadButton_Click" Content="Same Thread" Width="75" Height="30"/> <Button x:Name="newThreadButton" Grid.Column="1" Click="newThreadButton_Click" Content="New Thread" Height="30" Width="75"/> <Button x:Name="autoMultiThreadButton" Grid.Column="2" Click="newThreadButton_Click" Content="New Thread" Height="30" Width="75"/> </Grid> <StatusBar Grid.Row="2" Background="Gray" HorizontalAlignment="Stretch" Margin="0,10, 0, 0" Height="Auto" VerticalAlignment="Bottom"> <Label Foreground="White" FontSize="11" Content="{Binding StatusMessage}"/> </StatusBar> </Grid> </Window> This works with the following code behind perfectly:
MainWindow.xaml.cs:
public partial class MainWindow : Window, INotifyPropertyChanged { private ObservableCollection<JoeyObject> itemList; private static object _locker = new object(); private string statusMessage; public MainWindow() { InitializeComponent(); InitializeData(); BindingOperations.EnableCollectionSynchronization(ItemList, _locker); DataContext = this; } private void InitializeData() { ItemList = new ObservableCollection<JoeyObject>(); ItemList.Add(new JoeyObject("hello ", 1234)); ItemList.Add(new JoeyObject("listbox ", 4567)); StatusMessage = $"Initialized with {ItemList.Count} items"; } private void sameThreadButton_Click(object sender, RoutedEventArgs e) { int count = ItemList.Count; ItemList.Add(new JoeyObject("more ", 7890)); ItemList.Add(new JoeyObject("items ", 9876)); ItemList.Add(new JoeyObject("here ", 7890)); StatusMessage = $"Added {ItemList.Count - count} items on the UI thread"; } private async void newThreadButton_Click(object sender, RoutedEventArgs e) { await Task.Run(() => { int count = ItemList.Count; ItemList.Add(new JoeyObject("background thread-pool thread added 1", 7890)); ItemList.Add(new JoeyObject("background thread-pool thread added 2", 9876)); ItemList.Add(new JoeyObject("background thread-pool thread added 3", 7890)); // StatusMessage = $"Added {ItemList.Count - count} items on a background thread"; <- THIS WORKS. }); // Back on UI thread, do major UI work here. StatusMessage = $"Added {ItemList.Count - count} items on a background thread"; // using async/await and leveraging .NET4.5+. This essentially sets up a // Task continuation _for you_ and is syntactic sugar for doing the following: //Task.Factory.StartNew(() => //{ // int count = ItemList.Count; // ItemList.Add(new JoeyObject("background thread-pool thread added 1", 7890)); // ItemList.Add(new JoeyObject("background thread-pool thread added 2", 9876)); // ItemList.Add(new JoeyObject("background thread-pool thread added 3", 7890)); // StatusMessage = $"Added {ItemList.Count - count} items on a background thread"; //}).ContinueWith((ant) => //{ // // Back on UI thread, do major UI work here. // StatusMessage = $"Added {ItemList.Count - count} items on a background thread"; //}, TaskScheduler.FromCurrentSynchronizationContext()); } public event PropertyChangedEventHandler PropertyChanged; protected void OnPropertyChanged(string propertyName) { PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName)); } public string StatusMessage { get { return statusMessage; } set { statusMessage = value; OnPropertyChanged("StatusMessage"); } } public ObservableCollection<JoeyObject> ItemList { get { return itemList; } private set { itemList = value; OnPropertyChanged("ItemList"); } } } I am writing this as one example of how to update a collection from a background thread-pool thread. My question is concerning the ability to do
await Task.Run(() => { StatusMessage = "Some message"; }); This is updating the UI via a background thread, my question is:
- Why does this work without throwing
InvalidOperationException[as we are updating the UI, via background thread]?
Of course I will also be showing how these updates to the collection and UI can be done using IProgress<T> and a thread safe collection.
Thanks for your time.
