Authenticate user in OpenLDAP with username and password

[dorrit.Riemenschneider]

I need to validate a user with username and password against our
OpenLDAP active directory. This is my code:

Private bool ValidateUser (string username, string password)
{
DirectoryEntry userEntry = new DirectoryEntry(
ldapPath, username, password,
AuthenticationT ypes.Anonymous) ;
//Bind to the native AdsObject to force authentication.

Object obj = userEntry.Nativ eObject;
DirectorySearch er search = new
DirectorySearch er(userEntry);
search.Filter = "(cn=" + username + ")";
search.Properti esToLoad.Add("c n");
SearchResult result = search.FindOne( );
if (result != null)
return true;
else
return false;
}

The problem is, it returns also true if the username is correct, but
the password is false.
It looks like the user is located but not authenthicated.

I have already tried with several Authenthication Types:
I get an exeption "invalid dn-syntax" for Authenthication Types.None,
Authenthication Types.Delegatio n, Authenthication Types.FastBind,
Authenthication Types.ReadOnlyS erver, Authenthication Types.Sealing.

I get an exception "Die angeforderte Authentifizieru ngsmethode wird
durch den Server nicht unterstützt" (authenthicatio n method not
supported by server) for Authenthication Types.Secure or if I don't
specify an Authenthication Type.

Any help is appreciated!
Dorrit