LDAP读取扩展属性的问题

[TTLOVEYOU3344]

各位大虾：
我今天在读服务器上LDAP数据的时候遇到了麻烦，我无法读取LDAP上的扩展属性，我的代码如下：

public void ReadLdapUser(string ipAddress, string ldapUsername, string ldapPassword)
        {
            string subpathformat = "LDAP://{0}/o=sdf";
            string subpath = String.Format(subpathformat, ipAddress);
            DirectoryEntry entry = new DirectoryEntry(subpath,ldapUsername,ldapPassword);
            entry.AuthenticationType = AuthenticationTypes.Anonymous;
            DirectorySearcher mySearcher = new DirectorySearcher(entry);
            foreach (System.DirectoryServices.SearchResult resEnt in mySearcher.FindAll())
            {                
                DirectoryEntry de = resEnt.GetDirectoryEntry(); 
                if (de.Properties.Count >= 20)
                {
                    foreach (string elmentName in de.Properties.PropertyNames)
                    {
                        PropertyValueCollection valueCollection = de.Properties[elmentName];//这里一遇到扩展属性的名字一读就报异常，{"未知错误(0x8000500c)"}
                        for (int i = 0; i < valueCollection.Count; i++)
                        {
                            string kkk = valueCollection[i].ToString();
                            MessageBox.Show(kkk);
                        }
                    }                    
                    if (de.SchemaClassName.ToString() == "inetOrgPerson")
                    {
                        string userGuid = Guid.NewGuid().ToString();
                    }
                }
            }

        }

我的目的是要把LDAP上所有的用户数据读出来插入到我的数据库里，同时根据扩展字段职位来给这些用户分配权限