How to prevent Re-Post action caused by pressing browser's Refresh button

The Problem

When a user presses the Browser's Refresh button (Or F5/Ctrl+R etc.) the browser resends the last request data to the server. This action causes an Unexpected Re-Post action (if there was one). In some cases, it can be very problematic (For example, in shopping website when the user clicks on "Add to Cart" button and then presses the Refresh button - the item will be added twice !!!)

The solution

We will store a unique code (in my case Guid) on both client and server and compare them on every request action. If the two codes are not equal, the action will be canceled and the page will reload ('GET' method). Add Web User Control with this code:

Markup

Add a hidden field to hold client code

<asp:HiddenField runat="server" ID="_repostcheckcode" />

Code behind

    protected void Page_Load(object sender, EventArgs e)
    {
        CancelUnexpectedRePost();
    }

    private void CancelUnexpectedRePost()
    {
        string clientCode = _repostcheckcode.Value;

        //Get Server Code from session (Or Empty if null)
        string serverCode = Session["_repostcheckcode"] as string  ?? "";

        if (!IsPostBack || clientCode.Equals(serverCode))
        {
            //Codes are equals - The action was initiated by the user
            //Save new code (Can use simple counter instead Guid)
            string code = Guid.NewGuid().ToString();  
            _repostcheckcode.Value = code;
            Session["_repostcheckcode"] = code;
        }
        else
        {
            //Unexpected action - caused by F5 (Refresh) button
            Response.Redirect(Request.Url.AbsoluteUri);
        }
    }

Now you can drag the User Control to every page you want to handle that problem, or you can drag it to your Master Page to handle the problem in the entire site.