private void Operation_txtBox_TextChanged(object sender, EventArgs e)
 {
     cs.GetCalcState(
         val,
         Operation_txtBox,
         CalcResult_txtBox
         );
     cs.PreviousValue.Add(val.result);
 }
        public void GetCalcState(Operation val, TextBox op, TextBox res)
        {
            CalcState cst = new CalcState(val, val.result);

            cst.GetCalcState(val, op, res);
        }