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;
}