How to close messagebox in backgroundworker C#, winform

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C#

Hi everybody!
I develop sample project about backgroundworker and progressbar control.
Current, I meet a problem,
How to closed "MessageBox(aaaacascscascsacascsa this is tesst)" in BackgroundWorker?

If i cancellation thread backgroundworker then messagebox still display.
This is all my code:

public partial class FrmMain : Form
    {
        public FrmMain()
        {
            InitializeComponent();
        }
        int _max = 1000;
        private void btnStart_Click(object sender, EventArgs e)
        {
            //Execute th background worker doWork()
            this.bgWorker.RunWorkerAsync();
            this.pgbar.Maximum = this._max;
        }

        private void btnCancel_Click(object sender, EventArgs e)
        {
            //request cancellation of bgWorker
            this.bgWorker.CancelAsync();
        }

        //Display
        public void display(string text)
        {
            MessageBox.Show(text);
        }

        private void bgWorker_DoWork(object sender, DoWorkEventArgs e)
        {
            //There should be no GUI component method
            for (int i = 0; i <= _max; i++)
            {
                //set cancellation to pending. Execute cancellation
                if (this.bgWorker.CancellationPending)
                {
                    e.Cancel = true;
                }
                else
                {
                    MessageBox.Show("aaaacascscascsacascsa this is tesst");
                    this.simulateHeavyWork();
                    this.bgWorker.ReportProgress(i);
                }
            }
        }

        private void bgWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            //this is updated from doWork. Its where GUI components are updated
            //revices updates after 100 ms
            this.pgbar.Value = e.ProgressPercentage;
            this.label1.Text = e.ProgressPercentage.ToString() + "%";
        }

        private void bgWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            //called when the heavy operation in bg in ower.Can also accept GUI component
            if (e.Cancelled)
            {
                display("Work cancelled!");
            }
            else
            {
                display("Work completed successfully!");
            }
        }

        //simulate complex calculations- time let refresh
        private void simulateHeavyWork()
        {
            Thread.Sleep(100);
        }
    }

I using C#, winform.
Any ideas, I am very grateful!
thank you very much!

What I have tried:

URL reference:
http://stackoverflow.com/questions/10498555/calling-showdialog-in-backgroundworker
http://stackoverflow.com/questions/10283881/messagebox-on-worker-thread