Form_GS_Analysis form_GS_Analysis;

private void GSA_Open_Form()
{
//Form wird initialisiert
form_GS_Analysis = new Form_GS_Analysis();
form_GS_Analysis.FormClosed += new FormClosedEventHandler(form_GS_Analysis_FormClosed);

 //Form AssistCA wird angezeigt
form_GS_Analysis.Show();
}

private void form_GS_Analysis_FormClosed(object sender, FormClosedEventArgs e)
 /*
      tritt ein, wenn AssistCA geschlossen wird
 */
 {
            //Handler wird beendet
            form_GS_Analysis.FormClosed -= form_GS_Analysis_FormClosed;
            gBx_GSA.Visible = false;

            //Marker wird deaktiviert
            Is_GSAform_open = false;
}

....
form_GS_Analysis.cmd_Close_with_Chart.Click += new System.EventHandler(form_GS_Analysis_GetChart_Click);
....

public void form_GS_Analysis_GetChart_Click(object sender, EventArgs e)
{
   MessageBox.Show("Test");
   form_GS_Analysis.cmd_Close_with_Chart.Click -= form_GS_Analysis_GetChart_Click;
}