c# - Keep UI thread responsive when running long task in windows forms

Question

I am trying to read a large text file into a TextBox and keep the ui responsive when a file is dragged to the textbox.

Not works as expected, the windows forms is frozen and only seems working the task reading the file and appending the content to the textbox.

A ContextSwitchDeadLock was thrown by the IDE, but not really an error. This is a long running task. I have fixed it changing the behaviour under the exception menu.

Thanks to JSteward, Peter changed the code to this.

How I can keep the ui ( main thread) responsive when running this task ? Thanks.

private SynchronizationContext fcontext;

public Form1()
{      
    InitializeComponent();            
    values.DragDrop += values_DragDrop; //<----------- This is a textbox
    fcontext = WindowsFormsSynchronizationContext.Current;
}

// The async callback 
async void values_DragDrop(object sender, DragEventArgs e)
{
    try
    {
        string dropped = ( (string[]) e.Data.GetData(DataFormats.FileDrop))[0];
        if ( dropped.Contains(".csv") || dropped.Contains(".txt"))
        {
                using ( StreamReader sr = File.OpenText(dropped) )
                {
                    string s = String.Empty;
                    while ( ( s = await sr.ReadLineAsync() ) != null )
                    {                                                                
                       values.AppendText(s.Replace(";",""));
                    }
                }                 
         }
     }
  catch (Exception ex) { }
}

Answer 1

Sometimes it is indeed required to do some asynchronous, background operation on the UI thread (e.g., syntax highlighting, spellcheck-as-you-type, etc). I am not going to question the design issues with your particular (IMO, contrived) example - most likely you should be using the MVVM pattern here - but you can certainly keep the UI thread responsive.

You can do that by sensing for any pending user input and yielding to the main message loop, to give it the processing priority. Here's a complete, cut-paste-and-run example of how to do that in WinForms, based on the task you're trying to solve. Note await InputYield(token) which does just that:

using System;
using System.Runtime.InteropServices;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WinFormsYield
{
    static class Program
    {
        // a long-running operation on the UI thread
        private static async Task LongRunningTaskAsync(Action<string> deliverText, CancellationToken token)
        {
            for (int i = 0; i < 10000; i++)
            {
                token.ThrowIfCancellationRequested();
                await InputYield(token);
                deliverText(await ReadLineAsync(token));
            }
        }

        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            // create some UI

            var form = new Form { Text = "Test", Width = 800, Height = 600 };

            var panel = new FlowLayoutPanel
            {
                Dock = DockStyle.Fill,
                FlowDirection = FlowDirection.TopDown,
                WrapContents = true
            };

            form.Controls.Add(panel);
            var button = new Button { Text = "Start", AutoSize = true };
            panel.Controls.Add(button);

            var inputBox = new TextBox
            {
                Text = "You still can type here while we're loading the file",
                Width = 640
            };
            panel.Controls.Add(inputBox);

            var textBox = new TextBox
            {
                Width = 640,
                Height = 480,
                Multiline = true,
                ReadOnly = false,
                AcceptsReturn = true,
                ScrollBars = ScrollBars.Vertical
            };
            panel.Controls.Add(textBox);

            // handle Button click to "load" some text

            button.Click += async delegate
            {
                button.Enabled = false;
                textBox.Enabled = false;
                inputBox.Focus();
                try
                {
                    await LongRunningTaskAsync(text =>
                        textBox.AppendText(text + Environment.NewLine),
                        CancellationToken.None);
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
                finally
                {
                    button.Enabled = true;
                    textBox.Enabled = true;
                }
            };

            Application.Run(form);
        }

        // simulate TextReader.ReadLineAsync
        private static async Task<string> ReadLineAsync(CancellationToken token)
        {
            return await Task.Run(() =>
            {
                Thread.Sleep(10); // simulate some CPU-bound work
                return "Line " + Environment.TickCount;
            }, token);
        }

        //
        // helpers
        //

        private static async Task TimerYield(int delay, CancellationToken token)
        {
            // yield to the message loop via a low-priority WM_TIMER message (used by System.Windows.Forms.Timer)
            // https://web.archive.org/web/20130627005845/http://support.microsoft.com/kb/96006 

            var tcs = new TaskCompletionSource<bool>();
            using (var timer = new System.Windows.Forms.Timer())
            using (token.Register(() => tcs.TrySetCanceled(), useSynchronizationContext: false))
            {
                timer.Interval = delay;
                timer.Tick += (s, e) => tcs.TrySetResult(true);
                timer.Enabled = true;
                await tcs.Task;
                timer.Enabled = false;
            }
        }

        private static async Task InputYield(CancellationToken token)
        {
            while (AnyInputMessage())
            {
                await TimerYield((int)NativeMethods.USER_TIMER_MINIMUM, token);
            }
        }

        private static bool AnyInputMessage()
        {
            var status = NativeMethods.GetQueueStatus(NativeMethods.QS_INPUT | NativeMethods.QS_POSTMESSAGE);
            // the high-order word of the return value indicates the types of messages currently in the queue. 
            return status >> 16 != 0;
        }

        private static class NativeMethods
        {
            public const uint USER_TIMER_MINIMUM = 0x0000000A;
            public const uint QS_KEY = 0x0001;
            public const uint QS_MOUSEMOVE = 0x0002;
            public const uint QS_MOUSEBUTTON = 0x0004;
            public const uint QS_POSTMESSAGE = 0x0008;
            public const uint QS_TIMER = 0x0010;
            public const uint QS_PAINT = 0x0020;
            public const uint QS_SENDMESSAGE = 0x0040;
            public const uint QS_HOTKEY = 0x0080;
            public const uint QS_ALLPOSTMESSAGE = 0x0100;
            public const uint QS_RAWINPUT = 0x0400;

            public const uint QS_MOUSE = (QS_MOUSEMOVE | QS_MOUSEBUTTON);
            public const uint QS_INPUT = (QS_MOUSE | QS_KEY | QS_RAWINPUT);

            [DllImport("user32.dll")]
            public static extern uint GetQueueStatus(uint flags);
        }
    }
}

Now you should ask yourself what you're going to do if user modifies the content of the editor while it's still being populated with text on the background. Here for simplicity I just disable the button and the editor itself (the rest of the UI is accessible and responsive), but the question remains open. Also, you should look at implementing some cancellation logic, which I leave outside the scope of this sample.