Question

OK THIS IS WHAT HAPPENED:

      decimal Price = 0m;
      string TotalPrice = "";

     if (boxes[i].Checked == true)
    {
  select += SecondMenuList[i].ToString() + "   :  " + "\t\t" +
   Check[i].ToString("c") + "\r\n";//THIS GOES TO THE MESSAGE BOX

    Price += Check[i];

   TotalPrice = Price.ToString("c");//THIS ONLY SHOWS CURRENT PRICE VALUES

   TotalSales.Text = TotalPrice;//THIS IS NOT NECESSARY, DON'T EVEN DO IT YET.

     txtTotalPrice.Text = (String.IsNullOrEmpty(txtTotalPrice.Text)? Price:  

  (Int32.Parse(txtTotalPrice.Text) + Price)).ToString("DON'T ADD CURRENCY
   FORMAT");//SO THAT THE STRING CAN BE CONVERTED BACK TO DECIMAL
    //TO ADD THE NEW Price TO IT, RECONVERT TO STRING AND DISPLAY IN THE
   //TotalSales TEXTBOX
    }

WHEN YOU DECLARE DECIMALS WHOSE VALUES WOULD BE ASSIGNED TO EACH OTHER, LEAVE 
ALL OF   THEM IN THE SAME FORMAT. EG: decimal TotalPrice = 0m; decimal Price = 0m;
NOT decimal TotalPrice = .00m; AND decimal Price = 0m;

---

*****PLEASE THE SECOND QUESTION IS STILL HANGIN**************

sealed class SerialPortDataRead//for serial port communication
{

        // Create the serial port with basic settings

        private readonly SerialPort port = new SerialPort("COM5", 9600, Parity.None, 8, StopBits.One);
        private readonly Form1 form1;

        public SerialPortDataRead(Form1 form1)
        {
            // TODO: Complete member initialization
            this.form1 = form1;
        }

        public SerialPortDataRead()
        {
            // TODO: Complete member initialization

        }

        private void Serial()
        {
            //beep the default system sound to indicate an incoming data
            SystemSounds.Beep.Play();

            // Attach a method to be called when there is data waiting in 
            //the port's buffer

            port.DataReceived += port_DataReceived;

            // Begin communications

            port.Open();

            // Enter an application loop to keep this thread alive

            Application.Run();

           }

        private void port_DataReceived(object sender, SerialDataReceivedEventArgs e)
        { // Arrange the incoming data in an array

            string[] PortVal = { port.ReadExisting() };

            double[] Check = new double[6]
        {120, 120, 100, 100, 150, 100};//this is the price array

            string[] SecondMenuList = new string[6]
        {"Screw Driver", "Tape", "Glue", "Marker", "Halogen"};//this the mainarray

            string FirstSelection = "";
            string value = "";
            using (Form1 frm = new Form1())
            {
                string[] c = { "1", "2", "3", "4" };
                for (int i = 0; i < 3; i++)
                    if (PortVal[i] == SecondMenuList[i])//this is where the 

                   //problem lies

// I want to only compare the first three letters of each elements in PortVal at

//any index with the first three letters of the elements in the

//SecondMenulist array. but i can only compare the elements

//what should i do here PLEASE?

                 {
                        FirstSelection += String.Format("{0}\t:{1:c}\r\n", 

                        SecondMenuList[i], Check[i]);

                        value = Check[i].ToString("c");

                         if (PortVal.Contains(c[i]))

                           {

//after the comparison, if there is any match, add the matched element

//in the SecondMenuList array to the DataGrid Table (named Table)

                          this.form1._names.Add("TABLE");
                            this.form1._dataArray.Add(new string[] { c[i] });
                        }
                        this.form1._names.Add("ITEM");
                        this.form1._dataArray.Add(new string[] { SecondMenuList[i] });

                        this.form1._names.Add("PRICE");
                        this.form1._dataArray.Add(new string[] { 
                        Check[i].ToString("c") });
                    }
                frm.txtTotalSales.Text = value;
            }
            DateTime Date = new DateTime();
            Date = DateTime.Now;
            PrinterHelper.SendStringToPrinter(String.Format("Order(s)\r\n\n\t{0}\t:{1}

                \r\n\n\t{2}",
                                 FirstSelection,
                                   value,
                                    Date));

        }
    }

    public Form1()
    {
        InitializeComponent();


        dataGridView1.DataSource = GetResultsTable();

    }

    public Form1(TextBox txtTotalSales)
    {
        // TODO: Complete member initialization
        this.txtTotalSales = txtTotalSales;
    }

    private static SerialPortDataRead port_DataReceived()
    {
        throw new NotImplementedException();
    }

    public DataTable GetResultsTable()
    {
        DataTable Table = new DataTable();
        for (int i = 0; i < this._dataArray.Count; i++)
        {
            string name = this._names[i];
            Table.Columns.Add(name);
            List<object> objectNumbers = new List<object>();
            foreach (string number in this._dataArray[i])
            {
                objectNumbers.Add((object)number);
            }
            while (Table.Rows.Count < objectNumbers.Count)
            {
                Table.Rows.Add();
            }
            for (int a = 0; a < objectNumbers.Count; a++)
            {
                Table.Rows[a][i] = objectNumbers[a];
            }
        }
        return Table;
    }

Answer 1

try this

if (boxes[i].Checked == true)
{
select += SecondMenuList[i].ToString() + "   :  " + "\t\t" +
    Check[i].ToString("c") + "\r\n";

Price += Check[i];

TotalPrice = Price.ToString("c");
txtTotalPrice.Text = (String.IsNullOrEmpty(txtTotalPrice.Text)?0:(Int32.Parse(txtTotalPrice.Text)+TotalPrice)).ToString();
}