从Active Directory获取所有ou

是否有直接的方法来获取活动目录下的所有ou ?我已经尝试了以下代码，我在

处得到Com异常(无效操作)

ouSearch.FindAll()

我的代码如下所示:

public static List<string> GetAllOus(string ldapServer, string ldapUserName, string ldapPassWord)
    {
        List<string> orgUnits = new List<string>();
        string defaultNamingContext;
        DirectoryEntry rootDSE = new DirectoryEntry(ldapServer + "/dc=server-dc,dc=com", ldapUserName, ldapPassWord, AuthenticationTypes.Anonymous);
        //defaultNamingContext = rootDSE.Properties["defaultNamingContext"].Value.ToString();
        //DirectoryEntry defaultentry = new DirectoryEntry("LDAP://" + defaultNamingContext);
        DirectorySearcher ouSearch = new DirectorySearcher(rootDSE,
                                             "(objectClass=organizational-Unit)",
                                             null, SearchScope.Subtree);
        foreach (SearchResult resEnt in ouSearch.FindAll())
        {
            string OUName = resEnt.GetDirectoryEntry().Name;
            orgUnits.Add(OUName);
        }
        return orgUnits;
    }

请帮我解决这个问题。

Thanks in advance