private void GetSystemList()
        {
            SystemList.Clear();

            foreach (ConfigParams cparams in ConfigParamsList)
            {
                if (!SystemList.Contains(cparams.Systemname))   // Сравнивает DDLSystem.Text с cparams.Systemname
                {
                    SystemList.Add(cparams.Systemname);
                }
            }

            DDLSystem.DataSource = SystemList;
        }

private void GetServerList()
        {
            ServerList.Clear();

            foreach (ConfigParams cparams in ConfigParamsList)
            {
                if (!ServerList.Contains(cparams.Servername))
                {
                    if (CheckSystem(cparams.Systemname))  // Сравнивает DDLSystem.Text с cparams.Systemname
                    {
                        ServerList.Add(cparams.Servername);
                    }
                }
            }

            DDLServer.DataSource = ServerList;
        }

 private void GetDataBasesList()
        {
            DBList.Clear();

            foreach (ConfigParams cparams in ConfigParamsList)
            {
                if (!DBList.Contains(cparams.DBname))
                {
                    if (CheckSystem(cparams.Systemname))
                    {
                        DBList.Add(cparams.DBname);
                    }
                }
            }

            DDLdb.DataSource = DBList;
        }

private void DDLSystem_SelectedIndexChanged(object sender, EventArgs e)
        {
            GetServerList();
            GetDataBasesList();
        }

DDLSystem_SelectedIndexChanged

private void DDLSystem_SelectedIndexChanged(object sender, EventArgs e)
        {
            StringToFile(DDLSystem.Text);
            GetServerList();
            GetDataBasesList();
        }

 private bool CheckSystem(string system)
        {
            string SystemName = DDLSystem.Text;

            if (String.Compare(SystemName,system) == 0)
            {
                return true;
            }
            else 
            {
                return false;
            }
        }

ServerList.Clear();