Can't get users present in Domain users in Active directory?
string sADPath = "LDAP://" + tbDomain.Text;
string username = tbUsername.Text;
string password = tbPassword.Text;
DirectorySearcher mySearcher = new DirectorySearcher(directoryEntry);


int MaxResults = Int32.MaxValue - 1;


ComboBoxItem selectItem = (ComboBoxItem)ddlGroups.SelectedItem;
String value = selectItem.Value;

mySearcher.Filter = ("(&(objectCategory=person)(objectClass=User)(memberOf=" + value + "))");

mySearcher.SearchScope = SearchScope.Subtree;


foreach (SearchResult temp in mySearcher.FindAll())
{
}