Reset hiddenfield value from code behind

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VB

ASP.NET

Hye,

Is it possible to change/reset hiddenfield value to empty string from code behind?

I've tried methods below but none works;

1) hdnKey.Value = ""
2) Dim hdn As HiddenField = DirectCast(FindControl("hdnKey"), HiddenField)
hdn.Value = ""

Thanks.

---- UPDATE 1----

This is aspx codes:

<asp:UpdatePanel ID="UpdatePanel2" runat="server">
            <ContentTemplate>
            <asp:HiddenField ID="hdnKey" runat="server" />
               <%-- Other controls here --%>
            </ContentTemplate>
</asp:UpdatePanel>

* Even if i move the hiddenfield outside the update panel, it still didnt work.


--- UPDATE 2 ----

I assign value to hiddenfield using javascript.
After that, I want to reset back the value to empty string.

<script>

        document.onkeydown = KeyDownHandler;
        document.onkeyup = KeyUpHandler;

        var CTRL = false;
        var SHIFT = false;
        var ALT = false;
        var CHAR_CODE = -1;


        var confirm_value = document.createElement("INPUT");
        var hdnKey = document.getElementById('&lt;%= hdnKey.ClientID %&gt;');
        confirm_value.type = "hidden";
        confirm_value.name = "confirm_value";
        var a = hdnKey.value;

        function KeyDownHandler(e) {
            var x = '';
            if (document.all) {
                var evnt = window.event;
                x = evnt.keyCode;
            }
            else {
                x = e.keyCode;
            }
            DetectKeys(x, true);
        }

        function KeyUpHandler(e) {
            var x = '';
            if (document.all) {
                var evnt = window.event;
                x = evnt.keyCode;
            }
            else {
                x = e.keyCode;
            }
            DetectKeys(x, false);
        }

        function DetectKeys(KeyCode, IsKeyDown) {
            if (KeyCode == '16') {
                SHIFT = IsKeyDown;

                confirm_value.value = "16";
            }
            else if (KeyCode == '17') {
                CTRL = IsKeyDown;
                confirm_value.value = "17";
            }
            else if (KeyCode == '18') {
                ALT = IsKeyDown;
                confirm_value.value = "";
            }
            else {

                confirm_value.value = "";

            }

            document.forms[0].appendChild(confirm_value);
            document.forms[0].appendChild(hdnKey);
            hdnKey.value = confirm_value.value;

        }

    </script>