Change User attribute in Active Directory by web (vb.net)

Hi

I have some problem, i modify attributs in .net by web but nothing
happends, i don;t have any exceptions , everything would be okey but
does'nt

the code :

dim Entry_ldap As New
DirectoryEntry("LDAP://hercena11.d450000.wro.mofnet.gov.pl",
login.text, Password.text)
Dim lista_atr() As String = {"mail"}
Dim Filter As String = ("(&(sAMAccountName=" & login.text &
")(objectclass=" & "user" & "))")
Dim my_ldap_user As DirectorySearcher = New
DirectorySearcher(entry_ldap, Filter, lista_atr)
my_ldap_user.PropertyNamesOnly = True
Dim result As SearchResult
result = my_ldap_user.FindOne()
result.GetDirectoryEntry.Properties("mail").Value = txt_maillst.Text
result.GetDirectoryEntry.CommitChanges()

thanks for all answers
malin