domainAndUsername = domain + @"" + username;
 entry = new DirectoryEntry(_path, domainAndUsername, pwd);
 entry.AuthenticationType = FindAuthTypeMicrosoft(authType);

DirectoryEntry entry = new DirectoryEntry("LDAP://server-name/DC=domainContext,DC=com");
entry.Username = @"DOMAINaccount";
entry.Password = "...";
DirectorySearcher searcher = new DirectorySearcher(entry);
searcher.Filter = "(&(objectClass=user)(sn=Jones))";
SearchResultCollection results = searcher.FindAll();