c# – 如何列出所有计算机以及他们最后一次登录AD？

我正在尝试检索计算机名称列表以及它们上次从Active Directory登录的日期,并将它们返回到数据表中.
获取名称很容易但是当我尝试添加“lastLogon”或“lastLogonTimestamp”时,如下所示,我获得的lastLogonTimestamp的唯一值是“System._ComObject”

public DataTable GetListOfComputers(string domainName)
{
  DirectoryEntry entry = new DirectoryEntry("LDAP://DC=" + domainName + ",DC=com");
  DirectorySearcher search = new DirectorySearcher(entry);
  string query = "(objectclass=computer)";
  search.Filter = query;

  search.PropertiesToLoad.Add("name");
  search.PropertiesToLoad.Add("lastLogonTimestamp");

  SearchResultCollection mySearchResultColl = search.FindAll();

  DataTable results = new DataTable();
  results.Columns.Add("name");
  results.Columns.Add("lastLogonTimestamp");

  foreach (SearchResult sr in mySearchResultColl)
  {
    DataRow dr = results.NewRow();
    DirectoryEntry de = sr.GetDirectoryEntry();
    dr["name"] = de.Properties["Name"].Value;
    dr["lastLogonTimestamp"] = de.Properties["lastLogonTimestamp"].Value;
    results.Rows.Add(dr);
    de.Close();
  }

  return results;
}

如果我使用像LDP这样的工具查询AD,我可以看到该属性存在并填充了数据.
我怎样才能获得这些信息？

PrincipalContext pc = new PrincipalContext(ContextType.Domain,domainName);
PrincipalSearcher ps = new PrincipalSearcher(new ComputerPrincipal(pc));
PrincipalSearchResult<Principal> psr = ps.FindAll();
foreach (ComputerPrincipal cp in psr)
{
    DataRow dr = results.NewRow();
    dr["name"] = cp.Name;
    dr["lastLogonTimestamp"] = cp.LastLogon;    
    results.Rows.Add(dr);
}