AD Usre lastLogon Time

Problem with getting lastlogon for account - C#.NET / AD

Hi guys!
I have a code:

1. StringLastlogon(string username,string domain)
2. {
3. try
4. {
5. {
6. DirectoryContext context =newDirectoryContext(DirectoryContextType.Domain,domain);
7. DateTime latestLogon =DateTime.MinValue;
8. string servername =null;
9. DomainControllerCollection dcc =DomainController.FindAll(context);
10. foreach(DomainController dc in dcc)
11. {
12. DirectorySearcher ds;
13. using(dc)
14. using(ds = dc.GetDirectorySearcher())
15. {
16. ds.Filter=String.Format("(sAMAccountName={0})",username);
17. ds.PropertiesToLoad.Add("lastLogon");
18. ds.SizeLimit=1;
19. SearchResult sr = ds.FindOne();
20. if(sr !=null)
21. {
22. DateTimelastLogon=DateTime.MinValue;
23. if(sr.Properties.Contains("lastLogon"))
24. {
25. lastLogon=DateTime.FromFileTime((long)sr.Properties["lastLogon"][0]);
26. }
27. if(DateTime.Compare(lastLogon, latestLogon)>0)
28. {
29. latestLogon =lastLogon;
30. servername = dc.Name;
31. }
32. }
33. }
34. }
35. return latestLogon.ToString();
36. }
37. }
38. catch(Exception)
39. {
40. err =true;
41. returnnull;
42. }
43. }