private void payrollCalculatorToolStripMenuItem1_Click(object sender, EventArgs e)
        {
            if (payrollCalculatorForm == null)
            {
                payrollCalculatorForm             = new PayrollCalculatorForm();
                payrollCalculatorForm.MdiParent   = this;
                payrollCalculatorForm.FormClosed += PayrollCalculatorForm_FormClosed;
                payrollCalculatorForm.Show();
            }

            else
            {
                payrollCalculatorForm.Activate();
            }
        }
 private void PayrollCalculatorForm_FormClosed(object sender, FormClosedEventArgs e)
 {
     payrollCalculatorForm = null;
 }