Active directory users - Mutliple domain

public static DataTable SearchADUsers(string userName)
{
    //Create new datatable
    using (DataTable dtUsers = new DataTable())
    {
        dtUsers.Locale = CultureInfo.InvariantCulture;
        dtUsers.Columns.Add("key");
        dtUsers.Columns.Add("value");

        foreach (string domainToBeSearched in ConfigurationDetails.DomainTobeChecked.Split(','))
        {
            ValidateADUser(domainToBeSearched, userName, dtUsers);
        }
        return dtUsers;
    }
}

private static void ValidateADUser(string domainToBeSearched, string userName, DataTable dtUsers)
        {
            try
            {
                using (DirectoryEntry domain = new DirectoryEntry("LDAP://" + domainToBeSearched))
                {
                    using (DirectorySearcher searcher = new DirectorySearcher())
                    {
                        //Set root for searching
                        searcher.SearchRoot = domain;
                        //Set filters for searching user
                        searcher.Filter = FormFilter("user", userName + "*");
                        searcher.SearchScope = SearchScope.Subtree;
                        searcher.PageSize = 10;
                        //Get all registered user on basis of passed filter
                        using (SearchResultCollection adResult = searcher.FindAll())
                        {
                            DataRow dtrUsers = null;
                            foreach (SearchResult result in adResult)
                            {
                                if (dtUsers.Select("key='" + result.Properties["samaccountname"][0].ToString() + "'").Length <= 0)
                                {
                                    //Add user to datatable
                                    dtrUsers = dtUsers.NewRow();
                                    dtrUsers[0] = result.Properties["samaccountname"][0].ToString();
                                    dtrUsers[1] = result.Properties["DisplayName"][0];
                                    dtUsers.Rows.Add(dtrUsers);
                                }

                            }
                        }
                    }
                }
            }

            catch (Exception ex)
            {
                LogUtil.WriteLog(LogLevel.ERROR, ex.ToString());
            }
        }