Active Directory query doesn't work...

Hello, everybody.

I'd like to do this: For a big program (a web service) I need information
about the usergroups an active-directory-user is member of. To be more
precise, I need to know if a particular user is in a particular group or not.
This is my first Active-Directory-query in a C#-program, so it might look
crude or primitive...well, it doesn't work anyway...

The interesting part of the code is this:

public bool GetADUserGroups(string userName, string gruppe)
{
bool ergebnis = false;

DirectoryEntry ebr = new
DirectoryEntry("LDAP://DOMAINE.DO","DOMAIN_USER","PASSWORD";);

/*(do I need a domain admin for this or is a standard domain user
sufficient)*/

DirectorySearcher search = new DirectorySearcher(ebr);

/*(these are the many filter variants I tried. Except for the last one that
is not a comment, all terminated with errors)*/

//search.Filter = String.Format("(cn={0})", userName);

//search.Filter =
String.Format("&(objectClass=user)(userprincipalname={0})", userName);

//search.Filter = "&(objectClass=user)(userprincipalname=" +
userName + ")";

search.Filter = "(objectClass=user)";

/*(the username has the format "firstname.lastname", just like the login
name)*/

search.PropertiesToLoad.Add("memberOf");
search.PropertiesToLoad.Add("samAccountName");

foreach (SearchResult table in search.FindAll())
{
int groupCount = table.Properties["memberOf"].Count;

logger.LogInfo(table.Properties["samAccountName"].ToString());

if (table.Properties["samAccountName"].ToString() == userName)
{

for (int i = 0; i < groupCount; i++)
{

logger.LogInfo(table.Properties["memberOf"][i].ToString());

if (table.Properties["mebmerOf"][i].ToString() ==
gruppe)
{
ergebnis = true;
}
}
}
}


return ergebnis;
}



So, I'm finally there where I don't have any more ideas. I'm still trying,
but I'm feeling like any idea is a very long shot...

I'd be glad about any help you can provide. Many thanks in advance!
.