List<string> list = (from IDataRecord r in dataReader
                     select (string)r["FieldName"]
                    ).ToList();

List<int> list = new List<int>();

using (SqlDataReader reader = cmd.ExecuteReader())
{
    while (reader.Read())
    {
         list.Add(reader.GetInt32(0));
    }
}
return list.ToArray();

int size;
object[] data = new object[]{};
size = reader.GetValues(data);

DataTable dt as DataTable;
dt.fill(data);

ids_array = (from IDataRecord r in idReader
select (string)r["ID"]).ToArray<string>();

HashSet<string> hs = new HashSet<string>(
(from IDataRecord r in idReader select (string)r["ID"]).AsEnumerable<string>() );

public IList<UserAccount> List()
    {
        var list = new FluentCommand<UserAccount>("SELECT ID, UserName, Password FROM UserAccount")
            .SetMap(reader => new UserAccount
            {
                ID = reader.GetInt("ID"),
                Password = reader.GetString("Password"),
                UserName = reader.GetString("UserName"),
            })
            .AsList();

        return list;
    }