Preventing resubmission when page is refreshed

Question

I have created an email website form where user inputs their name, email, and message then submit a button to send. Once the email is sent, the lblResults on the page will state "Thank you." What I want to achieve is that if the page is changed, closed, or refreshed, the lblResults would return "". I was able to make this work but the problem is that when I refresh, it would resend the same email.

In the following is the code to my code behind:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using System.Net.Mail;

public partial class Contact : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Session["ClickedLink"] = "Contact";

        if (!IsPostBack)
            Session["Check_Page_Refresh"] = DateTime.Now.ToString();
    }

    protected void Page_PreRender(object sender, EventArgs e)
    {
        ViewState["Check_Page_Refresh"] = Session["Check_Page_Refresh"];
    }

    protected void btnSend_Click(object sender, EventArgs e)
    {
        if (Page.IsValid)
        {
            string fileName = Server.MapPath("~/App_Data/Contact.txt");
            string mailBody = File.ReadAllText(fileName);
            mailBody = mailBody.Replace("##Name##", txtName.Text);
            mailBody = mailBody.Replace("##Email##", txtEmail.Text);
            mailBody = mailBody.Replace("##Message##", txtMessage.Text);
            MailMessage myMessage = new MailMessage();
            myMessage.Subject = "Response from web site";
            myMessage.Body = mailBody;
            myMessage.From = new MailAddress("intern2@gmail.com", "Sender Name");
            myMessage.To.Add(new MailAddress("intern2@gmail.com", "Receiver Name"));
            myMessage.ReplyToList.Add(new MailAddress(txtEmail.Text));
            SmtpClient mySmtpClient = new SmtpClient();
            mySmtpClient.Send(myMessage);

            txtName.Text = "";
            txtEmail.Text = "";
            txtMessage.Text = "";
        }
        if (ViewState["Check_Page_Refresh"].ToString() == Session["Check_Page_Refresh"].ToString())
        {
            Session["Check_Page_Refresh"] = DateTime.Now.ToString();
            lblResults.Text = "Thank you";
        }
        else
        {
            lblResults.Text = "";
        }
    }
}

Please note, I haven't learn JS yet so if possible the focus would be on HTML5, CSS3, ASP.NET, or C#. Thanks in advance. Any suggestions would help!

Answer 1

The pattern to deal with this is known as Post-Redirect-Get . Essentially you don't respond using the same Url so that the duplicate post back cannot happen. Instead the user posts the form and the POST responds with a redirect to a GET .

Wrong Way:

Better Way:

More detailed blog post here