BackgroundWorker ProgressChanged System.InvalidOperationException

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I am using a BackgroundWorker to process data and update my UI with the ProgressChanged event. However, I get a System.InvalidOperationException when I try to change my ToolStripProgressBar.Visible to true, but not sure why. Here's my code for the ProgressChanged event:

C#

toolStripProgressBar1.Visible = true;
toolStripProgressBar1.Value = e.ProgressPercentage;
if (e.UserState != null) toolStripStatusLabel1.Text = e.UserState.ToString();

If I comment out the first line, I don't get the Exception, like so:

C#

//toolStripProgressBar1.Visible = true;
toolStripProgressBar1.Value = e.ProgressPercentage;
if (e.UserState != null) toolStripStatusLabel1.Text = e.UserState.ToString();

Also, the details of the Exception says:

Cross-thread operation not valid: Control '' accessed from a thread other than the thread it was created on.

All the questions that I've searched for seem to have the name of the control, not just empty quote marks, but I'm having a hard time searching for that specifically. I'm not sure why I can access the Value member but not the Visible member of the ToolStripProgressBar.

UPDATE:
I tested a small sample project of what I was trying to do and get the same results. Again, if I comment out the Visible statement, I don't get any exceptions.

C#

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void btnDoWork_Click(object sender, EventArgs e)
    {
        if (bgw.IsBusy == false) bgw.RunWorkerAsync();
    }

    private void bgw_DoWork(object sender, DoWorkEventArgs e)
    {
        ImportData dataImporter = new ImportData();
        dataImporter.ProgChanged += bgw_ProgressChanged;
        dataImporter.Import();
    }

    private void bgw_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        
        toolStripProgressBar1.Visible = true; // Comment line out prevents Exception
        toolStripProgressBar1.Value = e.ProgressPercentage;
        if (e.UserState != null)
            toolStripStatusLabel.Text = e.UserState.ToString();
    }

    private void bgw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        toolStripProgressBar1.Visible = false;
        toolStripStatusLabel.Text = "Finished";
    }        
}

public class ImportData
{
    public ProgressChangedEventHandler ProgChanged;
    private void UpdateProgress(int percent, string state)
    {
        if (ProgChanged != null) ProgChanged(this, new ProgressChangedEventArgs(percent, state));
    }
        
    public void Import()
    {            
        UpdateProgress(100, "Work Completed");
    }
}

Posted 22-Nov-15 8:57am

hpjchobbes

Updated 23-Nov-15 4:10am

v3

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Comments

BillWoodruff 22-Nov-15 18:53pm

We do need to see the code for the BackgroundWorker: are you using the RunWorkerCompleted Event ? Have you looked at RunWorkerAsynch ?

3 solutions

Solution 1

We need to see more of the background worker code and the code subscribed to any of its events.

Posted 22-Nov-15 11:51am

Dave Kreskowiak

Comments

BillWoodruff 22-Nov-15 18:37pm

This is a comment/request to the user, not a solution ! If CP MVP's don't set an example, why should we expect the general quality of QA here to improve from its current poor state ?

Dave Kreskowiak 22-Nov-15 19:52pm

Whoops. I'm playing a mulligan.

Solution 2

It will be great if you can share the DoWork event on your background worker!

Please take a look the MSDN referene on Making Thread-Safe Calls by using BackgroundWorker[^]

Posted 22-Nov-15 14:34pm

Liju Sankar

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Richard Deeming · Accepted Answer · 2015-11-23T04:56:00

The problem is that your DataImporter class is firing its ProgressChanged event from a background thread. As a result, the code in the bgw_ProgressChanged handler cannot access or modify the UI.

You could change the handler to invoke the code on the UI thread:

C#

private void bgw_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    if (InvokeRequired)
    {
        Invoke((ProgressChangedEventHandler)bgw_ProgressChanged, sender, e);
        return;
    }
    
    ...
}

Or you could add an extra layer of indirection:

C#

private void bgw_DoWork(object sender, DoWorkEventArgs e)
{
    ImportData dataImporter = new ImportData();
    dataImporter.ProgChanged += imp_ProgressChanged;
    dataImporter.Import();
}

private void imp_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    bgw.ReportProgress(e.ProgressPercentage, e.UserState);
}

private void bgw_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    // Hooked up to the BackgroundWorker's ProgressChanged event.
    ...
}

Or, you could modify your ImportData class to raise the event on the UI thread:

C#

public class ImportData
{
    private readonly SendOrPostCallback _progressReporter;
    private AsyncOperation _asyncOperation;
    
    public ImportData()
    {
        _progressReporter = arg => OnProgChanged((ProgressChangedEventArgs)arg);
    }
    
    public ProgressChangedEventHandler ProgChanged;
    
    protected virtual void OnProgChanged(ProgressChangedEventArgs e)
    {
        ProgressChangedEventHandler handler = ProgChanged;
        if (handler != null) handler(this, e);
    }
    
    private void UpdateProgress(int percent, string state)
    {
        ProgressChangedEventArgs arg = new ProgressChangedEventArgs(percent, state);
        if (_asyncOperation != null)
        {
            _asyncOperation.Post(_progressReporter, arg);
        }
        else
        {
            OnProgChanged(arg);
        }
    }
        
    public void Import()
    {
        _asyncOperation = AsyncOperationManager.CreateOperation(null);
        UpdateProgress(100, "Work Completed");
    }
}