namespace WFormsAppOneSTest1
{
    partial class Form1
    {

        private System.ComponentModel.BackgroundWorker bgrWorker1;

        private void InitializeComponent()
        {
            this.bgrWorker1 = new System.ComponentModel.BackgroundWorker();

            this.bgrWorker1.WorkerReportsProgress = true;
            this.bgrWorker1.DoWork += new System.ComponentModel.DoWorkEventHandler(this.bgrWorker1_DoWork);
            this.bgrWorker1.RunWorkerCompleted += new 
System.ComponentModel.RunWorkerCompletedEventHandler(this.bgrWorker1_RunWorkerCompleted);
        }
    }
}

this.bgrWorker1.RunWorkerAsync();

private void bgrWorker1_DoWork(object sender, DoWorkEventArgs e)

void bgrWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)

void bgrWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)

this.bgrWorker1.IsBusy

        private void button1_Click(object sender, EventArgs e)
        {
            // Cancel the asynchronous operation.
            this.bgrWorker1.CancelAsync();
            LoadThread.Abort();

            while (this.bgrWorker1.IsBusy)
            {
                Thread.Sleep(10);  // пусть немного поспит, но только до того, как фоновый поток возвратится.
                Application.DoEvents();
            }

            MessageBox.Show("Поток закрылся");
        }