void formUserList_FormClosed(object sender, FormClosedEventArgs e)
{
formUserList=null;
}
private void btnUser_Click(object sender, EventArgs e)
{
if (formUserList == null)
{
formUserList = new FormUserList();
formUserList.FormClosed += formUserList_FormClosed;
formUserList.MdiParent = this;
}
formUserList.Show();
formUserList.BringToFront();
}