Issue with Session Variables

Question

So I'm trying to make a simple banking website using C# and ASP.net and I'm just learning about Session Variables for the first time. I have a starting account balance of 1000 dollars and I want to transfer that to another page and have that balance updated via withdrawals or deposits and then update the balance. Heres my code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace Project4.Forms
{
    public partial class Services : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }
        protected void btnSubmit_Click1(object sender, EventArgs e)
        {
            double userInput = double.Parse(txtAmount.Text);
            Session["userInput"] = userInput;

            Session["balance"] = 1000.00;
            double balance = Convert.ToDouble(Session["balance"]);

            Session["userAction"] = ddlSelectService.SelectedItem.Text;

            if (ddlSelectService.SelectedItem.Text == "Withdrawl")
            {
                balance -= userInput;
            }
            else if (ddlSelectService.SelectedItem.Text == "Deposit")
            {
                balance += userInput;
            }
            else
            {

            }

            Response.Redirect("Status.aspx");
        }
    }
}

I understand the root of my problem is probably the original Session["balance"] = 1000.00; but I'm not sure how to declare a starting amount and then it not affect the balance when I return to this page from the results page. Any help is appreciated, also as I said in the beginning this is a barebones simple atm website please don't suggest anything too crazy because I am a beginner.

Answer 1

I don't do asp.net, so this is just a guess, but maybe try only setting the default value if it's null, something like: if (Session["balance"] == null) Session["balance"] = 1000;. It also seems like this should be in the Page_Load event, not the btnSubmit_Click event, since it only has to happen once.

Finally, don't forget to update the session variable after setting the new balance:

protected void Page_Load(object sender, EventArgs e)
{
    if (Session["balance"] == null) Session["balance"] = 1000;
}

protected void btnSubmit_Click1(object sender, EventArgs e)
{
    double userInput;
    if (!double.TryParse(txtAmount.Text, out userInput))
    {
        ClientScript.RegisterStartupScript(this.GetType(), "myalert", 
            "alert('Input must be a valid double');", true);

        return;
    }

    Session["userInput"] = userInput;

    double balance = Convert.ToDouble(Session["balance"]);

    Session["userAction"] = ddlSelectService.SelectedItem.Text;

    if (Session["userAction"] == "Withdrawl")
    {
        balance -= userInput;
    }
    else if (Session["userAction"] == "Deposit")
    {
        balance += userInput;
    }

    Session["balance"] = balance;

    Response.Redirect("Status.aspx");
}