判断一个窗体是否已经打开

Form frm1;
private void btnOK_Click(object sender, EventArgs e)
{

if (frm1 == null || frm1.IsDisposed)
            {
                frm1 = new KHXXFrm();
                frm1.Show();
            }
            else
            {
                frm1.Activate();
                frm1.WindowState = FormWindowState.Normal;
            }
}