Cross-thread operation not valid:

Question

In my application I had a requirement of notifying the user about the pending applications.

So in the mdiParent I set a BackgroundWorker which keeps querying the database to get any pending application, and if it finds any display it on the Tooltip on the MdiParent

private void button1_Click(object sender, EventArgs e)
{
    backgroundWorker1.RunWorkerAsync(2000);
}

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    fillnotification();
}    

public void fillnotification()
{
    int pending = 0;
    if( Program.USERPK != 0)
    {    
        DataTable dt = nftrans.getnotification();
        pending = dt.Rows.Count;

        String message = "You Have  " + pending + " Applications Pending For Approval";

        // toolTip1.SetToolTip(lblStatus , message);
        toolTip1.Show(message , this, lblStatus.Location);
    }
}

but when I run the solution I am getting an exception:

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

I understand its due to two different threads but cannot sort this out. Can any one suggest a solution? I tried the ideas I read in related questions But cannot find a correct solution

[See my answer here](http://stackoverflow.com/questions/11273562/c-sharp-cross-thread-communication/11274317#11274317) — Grzegorz W, May 11 '13 at 07:11
***Please*** at least *look* at the thousands of other identical questions before you ask the same one again? Please? — Tom W, May 11 '13 at 07:52
I checked the qstns but I cannot find anyone related to tooltip — Sreenath Ganga, May 12 '13 at 13:40

clamchoda · Answer 1 · 2013-05-11T06:23:56.537

Anytime you are accessing a control from an outside thread (the background worker Executes an operation on a separate thread.) you need to do some sort of invoke. This executes a delegate on the thread that owns the underlying window handle.

Something simple like ;

        this.Invoke(new MethodInvoker(delegate()
        {
        //   toolTip1.SetToolTip(lblStatus , message);
        toolTip1.Show(message, this, lblStatus.Location);

        }));

Will probably work for your case, as long as your previous code is not accessing controls, I'm not sure what getnotification() is doing.

score 1 · Answer 2 · answered May 11 '13 at 06:05

1

You are changing the UI (tooltip) on the worker thread.

That is not allowed. Use the UI thread to change the tooltip by calling Invoke on the Window and passing a delegate to a function that changes the the tooltip.

answered May 11 '13 at 06:05

Emond

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score 1 · Answer 3 · answered May 11 '13 at 06:29

Since it appears that the change of tooltip happens at the exact end of your worker thread, you could use the RunWorkerCompleted event - you CAN modify the UI thread from there, and that's what this event was designed for.

Of course Chris Bucklers Invoke solution will also work.

score 0 · Accepted Answer · answered May 11 '13 at 09:07

Change fillnotification() to return your pending value and pass that to "e.Result" in your DoWork() handler. Now wire up the RunWorkerCompleted() event and retrieve the result of your background operation:

    private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
    {
        e.Result = fillnotification();
    }

    public int fillnotification()
    {
        if (Program.USERPK != 0)
        {
            DataTable dt = nftrans.getnotification();
            return dt.Rows.Count;
        }
        return -1;
    }

    private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        int pending = (int)e.Result;
        if (pending != -1)
        {
            String message = "You Have  " + pending.ToString() + " Applications Pending For Approval";
            toolTip1.Show(message, this, lblStatus.Location);
        }
    }

Cross-thread operation not valid:

4 Answers4