c# - Simple shopping cart : calculating total price (logic) -

i have 2 dropdownlists category , product, , 2 label controls price , subtotal , textbox control quantity. when select category, other dropdownlist product add products depending on category , when select product, label (price , subtotal) , textbox (qunatity) values depending on product. , when change quantity, subtotal changed depending on value of quantity.

problem below method

protected void calculatetotal() {     string cnnstring = configurationmanager.connectionstrings["connectionstring1"].connectionstring;     sqlconnection conn = new sqlconnection(cnnstring);     conn.open();     sqlcommand cmd1 = new sqlcommand("select * cart", conn);     sqldatareader dr = cmd1.executereader();      lblsubtotal.text = convert.tostring(total);     conn.close() } 

if want 1 value query can use executescalar

protected void calculatetotal() {     string cnnstring = configurationmanager.connectionstrings["connectionstring1"].connectionstring;     sqlconnection conn = new sqlconnection(cnnstring);     conn.open();     sqlcommand cmd1 = new sqlcommand("select sum (quantity * price) cart", conn);      lblsubtotal.text = convert.tostring(cmd1.executescalar());     conn.close() }