if (!IsPostBack)
{
    int reccount = dsResult.Tables[0].Rows.Count;
    for (int count = 0; count < reccount;count++)
    {
        HashSet<string> arrayOrdId = new HashSet<string>();
        arrayOrdId.Add(dsResult.Tables[0].Rows[count][1].ToString());
        // arrayOrdId[count] = dsResult.Tables[0].Rows[count][1].ToString();
    }
}

var missingFromX = y.Except(x);
var missingFromY = x.Except(y);
// Do whatever you want with those differences

var orders1 = new HashSet<string>();
var orders2 = new HashSet<string>();

// add the orders to the sets, not stringbuilders, here

var order1sInOrders2 = orders1.Intersect(orders2);
var order2sNotInOrders1 = orders2.Except(orders1);