How do I calculate a value from a string in C# when taking data out of a list box?

Question

I have a form program that I need some help with. I need to calculate a total from a string. In this program I have a hardware store that lists items in a box. The user selects the item and then the quantity. The Receipt list box displays the items. I have this part just fine. However when I try to calculate what is in the box I am getting errors. Once calculated I need to enter in dollar amount to calculate change in a dialog box. Can anyone PLEASE help this newbie out???

namespace HardwareStore
{
    public partial class frmMain : Form
    {
        public frmMain()
        {
            InitializeComponent();

            ProductBox.Items.Add(new Hardware(){ItemNo = 1010, ProdName = "Hammer         ", Price = (decimal)14.99d });
            ProductBox.Items.Add(new Hardware(){ItemNo = 1056, ProdName = "Bag of Nails   ", Price = (decimal)19.99d });
            ProductBox.Items.Add(new Hardware(){ItemNo = 2001, ProdName = "Saw               ", Price = (decimal)29.99d });
            ProductBox.Items.Add(new Hardware(){ItemNo = 2005, ProdName = "Chainsaw       ", Price = (decimal)69.99d });
            ProductBox.Items.Add(new Hardware(){ItemNo = 3090, ProdName = "Ladder          ", Price = (decimal)109.99d });
        }

        private void frmMain_Load(object sender, EventArgs e)
        {

        }

        private void btnAddItem_Click(object sender, EventArgs e)
        {
           for (int i = 0; i < int.Parse(txtQuantity.Text); i++)
           ReceiptBox.Items.Add(ProductBox.Items[ProductBox.SelectedIndex]);
        }

       private void ReceiptBox_SelectedIndexChanged(object sender, EventArgs e)
        {
           int total = 0;
           //foreach (Hardware Current in ReceiptBox.Items) total += Current.

           //double subtotal = Math.Round((ReceiptBox.Items), 2);
           //double tax = Math.Round((subtotal * .075), 2);
           //double total = subtotal + tax;
        }

           private void btnCalculate_Click(object sender, EventArgs e)
        {
           //lblSub.Text = "$" + subtotal.ToString();
           //lblTax.Text = "$" + tax.ToString();
           //lblTotal.Text = "$" + total.ToString();

           lblSub.Visible = true;
           lblTax.Visible = true;
           lblTotal.Visible = true;
        }

       private void btnChange_Click(object sender, EventArgs e)
        {
           //MessageBox.Show("Change Due: $ ",);

        }
    }
}

Answer 1

ReceiptBox has no way of knowing what type of object is being stored in it. You will need to cast it as such

var subTotal = ReceiptBox.Items.Cast<Hardware>().Sum(item => item.Price);

Not too familiar with WinForms, but it looks like a better way of doing this would be utilizing ListBox.DataSource Property

Here is a stackoverflow thread on this matter

Answer 2

You need to convert your string to double. Try this:

double subtotal = Math.Round(double.Parse(ReceiptBox.Items), 2);