Manipulating UI elements from within another thread

Question

I'm trying to have a seperate thread in a WinForms C# application start a background worker which controls a ProgressBar (marquee). The issue is that when i try to set the bar to visible it just does nothing, and i've tried many forms of Invoke but they don't seem to help.

The following method progressBarCycle is called from a separate thread.

    BackgroundWorker backgroundWorker = new BackgroundWorker();

    public void progressBarCycle(int duration)
    {
        backgroundWorker.DoWork += new DoWorkEventHandler(backgroundWorker_DoWork);
        backgroundWorker.ProgressChanged += new ProgressChangedEventHandler(backgroundWorker_ProgressChanged);
        backgroundWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(backgroundWorker_RunWorkerCompleted);
        backgroundWorker.WorkerReportsProgress = true;
        backgroundWorker.WorkerSupportsCancellation = true;
        backgroundWorker.RunWorkerAsync(duration);
    }

    private void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
    {
        BackgroundWorker worker = sender as BackgroundWorker;

        worker.ReportProgress(0);

        DateTime end = DateTime.Now.AddMilliseconds((int)e.Argument);
        while (DateTime.Now <= end)
        {
            System.Threading.Thread.Sleep(1000);
        }
    }

    private void backgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        if (!this.IsHandleCreated)
            this.CreateHandle();
        statusStrip1.Invoke((MethodInvoker)delegate
        {
            progressBar1.Visible = false;
        });
        //    if (!this.IsHandleCreated)
        //    {
        //        this.CreateHandle();
        //        if (InvokeRequired) this.Invoke((MethodInvoker)(() => progressBar1.Visible = false));
        //        else progressBar1.Visible = false;
        //    }
        //    else
        //        if (InvokeRequired) this.Invoke((MethodInvoker)(() => progressBar1.Visible = false));
        //        else progressBar1.Visible = false;
    }

    private void backgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        if (!this.IsHandleCreated)
            this.CreateHandle();
        statusStrip1.Invoke((MethodInvoker)delegate
        {
            progressBar1.Visible = true;
        });
        //    if (!this.IsHandleCreated)
        //    {
        //        this.CreateHandle();
        //        if (InvokeRequired) this.Invoke((MethodInvoker)(() => progressBar1.Visible = true));
        //        else progressBar1.Visible = true;
        //    }
        //    else
        //        if (InvokeRequired) this.Invoke((MethodInvoker)(() => progressBar1.Visible = true));
        //        else progressBar1.Visible = true;
    }

Am I missing something obvious here? The comment sections are other things I've tried.

Answer 1

ProgressChanged is already raised on the UI thread (via the sync-context); your ProgressChanged does not need to do that Invoke - it can manipulate the UI directly (by contrast, DoWork can absolutely not do that). Perhaps the real problem is that you don't do any worker.ReportProgress(...) inside the loop - so it only happens once at the start.

Here's a full example:

using System;
using System.ComponentModel;
using System.Threading;
using System.Windows.Forms;
static class Program
{
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        using (var worker = new BackgroundWorker {
            WorkerReportsProgress = true })
        using (var progBar = new ProgressBar {
            Visible = false, Step = 1, Maximum = 100,
            Dock = DockStyle.Bottom })
        using (var btn = new Button { Dock = DockStyle.Top, Text = "Start" })
        using (var form = new Form { Controls = { btn, progBar } })
        {
            worker.ProgressChanged += (s,a) => {
                progBar.Visible = true;
                progBar.Value = a.ProgressPercentage;
            };
            worker.RunWorkerCompleted += delegate
            {
                progBar.Visible = false;
            };
            worker.DoWork += delegate
            {
                for (int i = 0; i < 100; i++)
                {
                    worker.ReportProgress(i);
                    Thread.Sleep(100);
                }
            };
            btn.Click += delegate
            {
                worker.RunWorkerAsync();
            };
            Application.Run(form);
        }
    }
}

Answer 2

1. Run progressBarCycle from the UI thread. RunWorkerAsync will create the new thread for you.
2. In backgroundWorker_ProgressChanged simply call progressBar1.Visible = true;. There is no need for Invoke.
3. Better also add a progressBar1.Refresh(); .