Error in Exporting Gridview to Excel

Question

Yeah i have found many question referencing about this but i cant seem to find the exact scenario like i have. so please take a time to look my code below:

I have this Script inside a content Place holder

   <script language="javascript" type="text/javascript">
        function GetMac2() {

            var macAddress = "";
            var ipAddress = "";
            var computerName = "";
            var wmi = GetObject("winmgmts:{impersonationLevel=impersonate}");
            e = new Enumerator(wmi.ExecQuery("SELECT * FROM Win32_NetworkAdapterConfiguration WHERE IPEnabled = True"));
            for (; !e.atEnd(); e.moveNext()) {
                var s = e.item();
                macAddress = s.MACAddress;
            }

            document.getElementById("<% =txtMACAdd.ClientID %>").value = unescape(macAddress);
        }  </script>

and a code for Exporting Gridview to excel

HtmlForm form = new HtmlForm();
            Response.Clear();
            Response.Buffer = true;
            string fileName = "Prenda of " + YearDropDownList.SelectedValue + "/" + MonthDropDownList.SelectedValue;
            Response.AddHeader("content-disposition", "attachment;filename=" + fileName);
            Response.Charset = "";
            Response.ContentType = "application/vnd.ms-excel";
            StringWriter sw = new StringWriter();
            HtmlTextWriter hw = new HtmlTextWriter(sw);
            PrendaGridView.AllowPaging = false;
            PrendaGridView.DataSource = (DataSet)ViewState["view"];
            PrendaGridView.DataBind();
            form.Controls.Add(PrendaGridView);
            this.Controls.Add(form);
            form.RenderControl(hw);
            Response.Output.Write(sw.ToString());
            Response.Flush();
            Response.End();

The problem is i Recive the error

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).

when i include the javascript above, if i remove it. it works well. so is there any work around on how to include my javascript?

i call my java script on page_load like this

if(!isPostback)
{
   string jvscript = "<script language='javascript'>GetMac2();</script>";
                    Page.ClientScript.RegisterStartupScript(this.GetType(), "invoke", jvscript);
}  

Hope you can help me thanks!

Answer 1

There is one workaround i can think of:
HTML:

<input id="tempInp" runat="server" type="hidden"></input>

C#:

tempInp.value = txtMACAdd.ClientID;

Javascript:

document.getElementById(document.getElementById("tempInp").value).value = unescape(macAddress);

This will not disturb the javascript and there are no server tags in the javascript so you can do this.Controls.Add(form);

EDIT:
Since you are using server tag to get the Client ID of the control, I think the Client ID and Server ID of the controls are not same.
There is one more non-standard but simple workaround. Just copy the Client ID of txtMACAdd from the final source of the page from the browser and paste it in document.getElementById("<% =txtMACAdd.ClientID %>").value .
Because when we are doing document.getElementById("tempInp").value the tempInp control's ClientID might be different and it will not be able to find the control and again will be the same problem.