How to call a method in .cs file in App_Code folder in aspx.cs file?

My .cs file in App_Code folder

namespace OnlineShoppingSite.App_Code
{
    public class Product
    {
        public Product(string prodID, string prodName, string prodDesc,
                       decimal unitPrice, string prodImage, string prodImage2, string prodDetails, string prodMeasurement, string prodCare)
        public Product getProduct(string prodID)
        {
        }
}

My .aspx.cs file

using OnlineShoppingSite.App_Code;
namespace OnlineShoppingSite
{
    public partial class ProductDetails : BasePage
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }
    }
}

How do I retrieve the getProduct method in the .aspx.cs file? I added in using "OnlineShoppingSite.App_Code;" at the top of the file but it shows me this error 'App_Code' does not exist in your namespace 'OnlineShoppingSite'(are you missing an online assembly reference?) "