private string hellow="hello";
    private void button1_Click(object sender, EventArgs e)
    {
        bool createAndRunWorkResult = CreateAndRunWork();
        if (createAndRunWorkResult)
        {
            //Do something that need wait RunBackGroundWorkerCompleted execute here.
            //MessageBox.Show(hello);
        }
    }

    private bool CreateAndRunWork()
    {
        BackgroundWorker worker = new BackgroundWorker();
        worker.DoWork += worker_DoWork;
        worker.RunWorkerCompleted += worker_RunWorkerCompleted;
        worker.RunWorkerAsync();
        return true;
    }

    private void worker_DoWork(object sender, DoWorkEventArgs e)
    {
        //Nothing here;
    }

    private void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        hello="aloha";
    }

private string hellow="hello";
private void button1_Click(object sender, EventArgs e)
{
    bool createAndRunWorkResult = CreateAndRunWork();
}

private bool CreateAndRunWork()
{
    BackgroundWorker worker = new BackgroundWorker();
    worker.DoWork += worker_DoWork;
    worker.RunWorkerCompleted += worker_RunWorkerCompleted;
    worker.RunWorkerAsync();
    return true;
}

private void worker_DoWork(object sender, DoWorkEventArgs e)
{
    //Nothing here;
}

private void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    hellow="aloha";
        //Do something that need wait RunBackGroundWorkerCompleted execute here.
        //MessageBox.Show(hellow);
}