C#

class Test
{
   AutoResetEvent evt = new AutoResetEvent(false);
   void Thread1()
   {
     // do operation 1
     evt.WaitOne();
     // do operation 2
   }
   void Thread2()
   {
      // do some other operation
      evt.Set(); // signal other thread to resume
      // do something else
   }

class MainForm
{
   Form childForm;

   public void CreateForm()
   {
      if (childForm == null)
         childForm = new ChildForm(); // create child
      else
         childForm.Focus();
   }
}

private frmMcStock mcStock;

private void tscmdMcStock_Click(object sender, EventArgs e)
{
    //when it's closed, it can't be re-opened!
    if (mcStock == null)
    {
        mcStock = new frmMcStock();
        mcStock.MdiParent = this;
        mcStock.Show();
        //MessageBox.Show("a new form has been created");
    }
    else
    {
        mcStock.Focus();
        //MessageBox.Show("an existing form has been focused");
    }
}