private void button3_Click(object sender, EventArgs e)
        {
            bill b = new bill();

            this.Visible = false;
            b.Visible    = true;
        }
 private void updateStudentToolStripMenuItem_Click(object sender, EventArgs e)
 {
     if (b == null)
     {
         b             = new bill();
         b.MdiParent   = this;
         b.FormClosed += b_FormClosed;
         b.Show();
     }
     else
     {
         b.Activate();
     }
 }
 void b_FormClosed(object sender, FormClosedEventArgs e)
 {
     b = null;
 }