ASP.Net double-click problem

The general approach is twofold.

Serverside:

On load of the page, generate a token (using System.Random), save it in the session, and write it to a hidden form field
On submit, check that the hidden form field equals the session variable (before setting it again)
Do work

Clientside:

Similar to what you have, but probably just hide the button, and replace it with some text like ‘submitting’.

The important thing to note, client side, is that the user may cancel the post by hitting ‘escape’, so you should consider what to do here (depending on how far along they are the token won’t be used, so you’ll need to bring the button back from being disabled/hidden).

Complete example follows:

C# (includes code to see it in action):

<html>
<head runat="server">
    <title>double-click test</title>
    <script language="c#" runat="server">
    private Random
        random = new Random();

    private static int
        TEST = 0;

    public void Page_Load (object sender, EventArgs ea)
    {
        SetToken();
    }

    private void btnTest_Click (object sender, EventArgs ea)
    {
        if( IsTokenValid() ){
            DoWork();
        } else {
            // double click
            ltlResult.Text = "double click!";
        }
    }

    private bool IsTokenValid ()
    {
        bool result = double.Parse(hidToken.Value) == ((double) Session["NextToken"]);
        SetToken();
        return result;
    }

    private void SetToken ()
    {
        double next = random.Next();

        hidToken.Value       = next + "";
        Session["NextToken"] = next;
    }


    private void DoWork ()
    {
        TEST++;
        ltlResult.Text = "DoWork(): " + TEST + ".";
    }
    </script>
</head>
<body>
    <script language="javascript">
        var last = null;
        function f (obj)
        {
            obj.src = "http://www.gravatar.com/avatar/4659883ec420f39723c3df6ed99971b9?s=32&d=identicon&r=PG";
            // Note: Disabling it here produced strange results. More investigation required.
            last = obj;
            setTimeout("reset()", 1 * 1000);
            return true;
        }

        function reset ()
        {
            last.src = "http://www.gravatar.com/avatar/495ce8981a5127a9fd24bd72e7e3664a?s=32&d=identicon&r=PG";
            last.disabled = "false";
        }
    </script>
    <form id="form1" runat="server">
        <asp:HiddenField runat="server" ID="hidToken" />
        <asp:ImageButton runat="server" ID="btnTest"
            OnClientClick="return f(this);"
            ImageUrl="http://www.gravatar.com/avatar/495ce8981a5127a9fd24bd72e7e3664a?s=32&d=identicon&r=PG" OnClick="btnTest_Click" />
        <pre>Result: <asp:Literal runat="server" ID="ltlResult" /></pre>
    </form>
</body>
</html>

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