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How to keep your WPF UI responsive and report progress during a long running task. C# and asynchronous multithreading.

    The problem Have you ever had to execute a bit of code that took an extremely long time to complete?  If so, I'm sure you've noticed that your user interface becomes unresponsive.  You can't use any controls on the form until the process completes.  This is extremely problematic and makes for a poor user experience. The cause This problem is because you have executed a synchronous block of code on the same thread as the user interface.  The user interface will not be able to respond to any user action until the synchronous method has completed. Let's look at an example that illustrates the execution of a synchronous method executing on the user interface thread. The example   This example is very simple.  It's a WPF application with two buttons.   One of the buttons will start a long running for loop and the other button opens a MessageBox and displays a message to let you know that the UI is active. When you click the button to start the long running process, try to click the other button.   You will notice that you are unable to click the button to show the message box.   Here's the MainWindow.xaml markup <Window x:Class="SynchronousExecution.MainWindow" xmlns="" xmlns:x="" Title="MainWindow" Height="350" Width="525"> <Grid Width="111" Height="118"> <Button Content="Start Process" Height="23" HorizontalAlignment="Left" Margin="10,10,0,0" Name="btnStartProcess" VerticalAlignment="Top" Width="89" Click="btnStartProcess_Click" /> <Button Content="Try to click me" Height="23" HorizontalAlignment="Left" Margin="10,43,0,0" Name="btnTryMe" VerticalAlignment="Top" Width="89" Click="btnTryMe_Click" /> </Grid> </Window>   And here is the code behind MainWindow.xaml.cs using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Windows; using System.Windows.Controls; using System.Windows.Data; using System.Windows.Documents; using System.Windows.Input; using System.Windows.Media; using System.Windows.Media.Imaging; using System.Windows.Navigation; using System.Windows.Shapes; using System.Threading; namespace SynchronousExecution { /// <summary> /// Interaction logic for MainWindow.xaml /// </summary> public partial class MainWindow : Window { public MainWindow() { InitializeComponent(); } private void btnStartProcess_Click(object sender, RoutedEventArgs e) { MessageBox.Show("Starting the long process. Try to click the button below."); for (var loopCounter = 0; loopCounter < 500; loopCounter++) { Thread.Sleep(100); } MessageBox.Show("The process has ended."); } private void btnTryMe_Click(object sender, RoutedEventArgs e) { MessageBox.Show("User interface is responding!"); } } } Download (52.38 kb) The solution The solution to this problem is to spawn a new thread to execute the long running task.  By placing the execution on a seperate thread, this leaves the main UI thread free to interact with the user's input. Reporting progress  If you execute a long running task, you usually want to let your users know about the progress of the task.  A control to assist with this is the progressbar control.   The WPF progress bar control inherits from the RangeBase class.  The control contains the following properties to control how the progress is displayed to the user. Minimum The minimum value of the Value property. Maximum The maximum value of the Value property. SmallChange The value in which the Value property is incremented. Value The current value position between the Minimum and Maximum property. Multithreading concerns Multithreading is a complex topic and is beyond the scope of this article.  However, there are a few concerns that you must be aware of if you plan to access values on the UI thread from the worker thread. The System.Windows.Threading.Dispatcher object is used to execute code on the UI thread from the executing worker thread.  The Dispatcher object has the BeginInvoke method which will execute an asynchronous delegate on the thread in which the dispatcher object was created.  So by using the dispatcher object from the user interface code you are able to access the user interface controls from your worker thread. We'll take a look at another example that will bring all of these concepts together to illustrate executing a long running task on a seperate thread with a progress bar that updates the user interface via the Dispatcher object. The example The example is a WPF application.  The main window contains the following controls; lstRandomValues A ListBox control that will hold random values that are generated during the long running process. The pupose of this control is to illustrate the responsiveness of the user interface during the long running process. prgProgress A ProgressBar control that will illustrate the progress of the long running task. The ProgressBar is hidden by default. It's shown when the long running process begins and is hidden again when the process has ended or is canceled. txtProgress A TextBox control that displays the long running process percentage of completion. btnCancel A Button control that will cancel the long running process and in turn hide the progress bar and progress textbox. btnInteract A Button control that will launch a MessageBox to illustrate the responsiveness of the UI while the long running process is executing. btnBegin A Button control that will begin the long running process. And here is the code  Window1.xaml <Window x:Class="RandomNameSpace.ViewModels.Window1" xmlns="" xmlns:x="" Title="Window1" Height="400" Width="400"> <!-- Parent Grid --> <Grid> <Grid.RowDefinitions> <RowDefinition /> <RowDefinition /> </Grid.RowDefinitions> <Grid.ColumnDefinitions> <ColumnDefinition /> <ColumnDefinition /> </Grid.ColumnDefinitions> <ListBox Grid.Row="0" Grid.Column="0" Name="lstRandomValues" Grid.ColumnSpan="2" /> <Grid Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2"> <Grid.RowDefinitions> <RowDefinition /> <RowDefinition /> </Grid.RowDefinitions> <Grid.ColumnDefinitions> <ColumnDefinition /> <ColumnDefinition /> </Grid.ColumnDefinitions> <Border Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="2" x:Name="bdrProgress" Visibility="Hidden" Background="Azure" CornerRadius="10" BorderBrush="DarkGray" BorderThickness="1" HorizontalAlignment="Center" VerticalAlignment="Center"> <StackPanel> <Grid Margin="5"> <ProgressBar x:Name="prgProgress" Width="200" Height="20" Minimum="0" Maximum="1" /> <TextBlock x:Name="txtProgress" HorizontalAlignment="Center" VerticalAlignment="Center" /> </Grid> <Button x:Name="btnCancel" Content="Cancel" HorizontalAlignment="Right" Margin="3" Click="btnCancel_Click" /> </StackPanel> </Border> <Button Grid.Row="1" Grid.Column="1" x:Name="btnInteract" HorizontalAlignment="Center" Content="Interact with UI" Click="btnInteract_Click" Height="23.96" /> <Button Grid.Row="1" Grid.Column="0" x:Name="btnBegin" HorizontalAlignment="Left" Content="Start Long Process" Click="btnBegin_Click" Height="23.96" Margin="35,33.27,0,33.27" /> </Grid> </Grid> </Window> Window1.xaml.cs using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Windows; using System.Windows.Controls; using System.Windows.Data; using System.Windows.Documents; using System.Windows.Input; using System.Windows.Media; using System.Windows.Media.Imaging; using System.Windows.Navigation; using System.Windows.Shapes; using System.Threading; using System.Windows.Threading; namespace RandomNameSpace.ViewModels { /// <summary> /// Interaction logic for Window1.xaml /// </summary> public partial class Window1 : Window { private bool IsCanceled = false; private RandomViewModel _viewModel; public RandomViewModel ViewModel { get { return _viewModel; } set { _viewModel = value; } } public Window1() { InitializeComponent(); PrepareViewModel(); } private void PrepareViewModel() { ViewModel = new RandomViewModel(); this.lstRandomValues.DisplayMemberPath = "Value"; this.lstRandomValues.DataContext = ViewModel; this.lstRandomValues.ItemsSource = ViewModel; } private void btnBegin_Click(object sender, RoutedEventArgs e) { BeginProcess(); } private void btnInteract_Click(object sender, RoutedEventArgs e) { MessageBox.Show("Interaction!"); } private void btnCancel_Click(object sender, RoutedEventArgs e) { CancelProcess(); } public void BeginProcess() { bdrProgress.Visibility = System.Windows.Visibility.Visible; DisableBeginButton(); Action StartLoop; StartLoop = () => DoLongRunningProcess(); Thread t; t = new Thread(StartLoop.Invoke); t.Start(); } public void CancelProcess() { this.IsCanceled = true; Thread.Sleep(1500); ClearListBox(); EnableBeginButton(); } private void ClearListBox() { Dispatcher.BeginInvoke(DispatcherPriority.Background, (SendOrPostCallback)delegate { this.ViewModel.Clear(); }, null); } private void DoLongRunningProcess() { for (var loopCounter = 0.0; loopCounter < 1.0; loopCounter = loopCounter + .1) { if (!IsCanceled) { //add a random number to the viewmodel collection to be bound to the listview Dispatcher.BeginInvoke(DispatcherPriority.Background, (SendOrPostCallback)delegate { this.ViewModel.AddValue(1564); }, null); UpdateProgressBar(loopCounter); Thread.Sleep(2000); } else { break; } } IsCanceled = false; ClearListBox(); MessageBox.Show("Completed"); EnableBeginButton(); HideProgressBar(); } private void EnableBeginButton() { Dispatcher.BeginInvoke(DispatcherPriority.Background, (SendOrPostCallback)delegate { this.btnBegin.SetValue(Button.IsEnabledProperty, true); }, null); } private void DisableBeginButton() { Dispatcher.BeginInvoke(DispatcherPriority.Background, (SendOrPostCallback)delegate { this.btnBegin.SetValue(Button.IsEnabledProperty, false); }, null); } private void UpdateProgressBar(double value) { Dispatcher.BeginInvoke(DispatcherPriority.Background, (SendOrPostCallback)delegate { this.prgProgress.SetValue(ProgressBar.ValueProperty, value); }, null); Dispatcher.BeginInvoke(DispatcherPriority.Background, (SendOrPostCallback)delegate { this.txtProgress.SetValue(TextBlock.TextProperty, (value * 100).ToString()); }, null); } private void HideProgressBar() { Dispatcher.BeginInvoke(DispatcherPriority.Background, (SendOrPostCallback)delegate { this.bdrProgress.SetValue(Border.VisibilityProperty, Visibility.Collapsed); }, null); } } }   RandomValue.cs using System; using System.Collections.Generic; using System.ComponentModel; using System.Linq; using System.Text; namespace RandomNameSpace.ViewModels { public class RandomValue : INotifyPropertyChanged { private int _value; public int Value { get { return _value; } set { if (_value != value) _value = value; OnPropertyChanged("Value"); } } public event PropertyChangedEventHandler PropertyChanged; public void OnPropertyChanged(string propertyName) { var handler = PropertyChanged; if (handler != null) { handler(this, new PropertyChangedEventArgs(propertyName)); } } } } RandomViewModel.cs using System; using System.Collections.Generic; using System.Collections.ObjectModel; using System.Linq; using System.Text; using System.Threading; using System.Windows; using System.Windows.Controls; using System.Windows.Threading; namespace RandomNameSpace.ViewModels { public class RandomViewModel : ObservableCollection<RandomValue> { public RandomViewModel() : base() {} public void AddValue(int value) { Random rnd = new Random(); var calculatedValue = (value * rnd.Next(3000)); this.Add(new RandomValue { Value = (calculatedValue) }); } } } Here is the entire solution for download. (77.39 kb)   This concludes the article.  Thank you for taking the time to read it! Until next time... ~/Buddy James kick it on    

About the author

My name is Buddy James.  I'm a Microsoft Certified Solutions Developer from the Nashville, TN area.  I'm a Software Engineer, an author, a blogger (, a mentor, a thought leader, a technologist, a data scientist, and a husband.  I enjoy working with design patterns, data mining, c#, WPF, Silverlight, WinRT, XAML, ASP.NET, python, CouchDB, RavenDB, Hadoop, Android(MonoDroid), iOS (MonoTouch), and Machine Learning. I love technology and I love to develop software, collect data, analyze the data, and learn from the data.  When I'm not coding,  I'm determined to make a difference in the world by using data and machine learning techniques. (follow me at @budbjames).  

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