Using ASP.NET with C# and VS2010, how would you write code on a button click to export over 5,000 records to an Excel 2003 file using Web forms?

You can render your gridview on an HTMLWriter and then force a download to the client side.

A very detailed example is listed here:
http://www.c-sharpcorner.com/UploadFile/DipalChoksi/exportxl_asp2_dc11032006003657AM/exportxl_asp2_dc.aspx

and if you are looking for a downloadable sample:
http://mattberseth.com/blog/2007/04/export_gridview_to_excel_1.html

Chinmay's first link looks very good but the mime types are wrong. You should update this line:

Response.ContentType = "application/ms-excel";

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...to the correct mime type:

Response.ContentType = "application/vnd.ms-excel" // classic Excel .xls
-- OR -- 
Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" // modern Excel .xlsx

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@Angel:
application/ms-excel should work just fine give it a try.

Yes it may well work but it's wrong. If you use the correct mime type you can have more confidence it will continue to work in the future.

http://blogs.msdn.com/b/vsofficedeveloper/archive/2008/05/08/office-2007-open-xml-mime-types.aspx
http://en.wikipedia.org/wiki/Microsoft_Excel#File_formats

I had used the following code in the past as you suggested.

The problem is that when I export 5,000 rows of data, I get the error message:

Problems duing load
Problems came up in the following areas during load:
cell data too large
---------------------------------------------------------------------

 protected void Button1_Click1(object sender, EventArgs e)
        {
            string attachment = "attachment; filename=Contacts.xls";

            Response.ClearContent();
            Response.AddHeader("contenet-disposition", attachment);
            Response.ContentType = "application/vnd.ms-excel";

            StringWriter stringWriter = new StringWriter();
            HtmlTextWriter htmlTextWriter = new HtmlTextWriter(stringWriter);
            GridView1.RenderControl(htmlTextWriter);
            Response.Write(stringWriter.ToString());
            Response.End();
        }

        public override void VerifyRenderingInServerForm(Control control)
        {

        }

I'm only exporting 5 fields of data per row as follow:
-------------------------------------------------------
TransDate    file type  datetime

ProcessDate  file type  datetime

Bank             nvarchar(255)

Type             nvarchar(255)

Office          nvarchar(255)

Try this

Response.ContentType = "application/vnd.xls";

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Response.ContentType = "application/vnd.xls";
----------------------------------------------------------------

This revised code results in the following error:

The XML page cannot be displayed
Cannot view XML input using XSL style sheet. Please correct the error and then click the Refresh button, or try again later.


--------------------------------------------------------------------------------

The operation completed successfully. Error processing resource 'http://localhost:4380/default.aspx'. Line 8, Position 92

   <td>9/26/2013 12:00:00 AM</td><td>9/26/2013 12:00:00 AM</td><td>0184</td><td>A&...

Alright try this:

Response.ContentType = "application/vnd.xls"; 
Response.ContentEncoding = System.Text.Encoding.UTF8; 

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Source:
http://www.experts-exchange.com/Programming/Languages/.NET/ASP.NET/Q_24049444.html

string attachment = "attachment; filename=Contacts.xls";
                Response.ClearContent();
                Response.AddHeader("contenet-disposition", attachment);
                Response.ContentType = "application/vnd.xls";
                Response.ContentEncoding = System.Text.Encoding.UTF8;
                StringWriter stringWriter = new StringWriter();
                HtmlTextWriter htmlTextWriter = new HtmlTextWriter(stringWriter);
                GridView1.RenderControl(htmlTextWriter);
                Response.Write(stringWriter.ToString());
                Response.End();
--------------------------------------------
The XML page cannot be displayed
Cannot view XML input using XSL style sheet. Please correct the error and then click the Refresh button, or try again later.


--------------------------------------------------------------------------------

The operation completed successfully. Error processing resource 'http://localhost:4380/default.aspx'. Line 8, Position 92

   <td>9/26/2013 12:00:00 AM</td><td>9/26/2013 12:00:00 AM</td><td>0184</td><td>A&...

Can you send a screenshot of the error please. Thanks.

Hi zimmer9,

Was it a correct answer? If yes then it is ok I got bit confused as you are using C# code and that sample is in VB.

Regards,
Chinmay.

Thanks for asking. Yes the answer I chose works for my application.