Per esportare dati da una gridview ad un file excel utilizzo il seguente codice che ho trovato in rete:

public class GridViewExportUtil
{

public static void Export(string fileName, GridView gv)
{
HttpContext.Current.Response.Clear();
HttpContext.Current.Response.AddHeader(
"content-disposition", string.Format("attachment; filename={0}", fileName));
HttpContext.Current.Response.ContentType = "application/ms-excel";

using (StringWriter sw = new StringWriter())
{
using (HtmlTextWriter htw = new HtmlTextWriter(sw))
{
// Create a form to contain the grid
Table table = new Table();
//table.Style.Add("border-style", "solid");


// add the header row to the table
if (gv.HeaderRow != null)
{
GridViewExportUtil.PrepareControlForExport(gv.HeaderRow);
table.Rows.Add(gv.HeaderRow);
}

// add each of the data rows to the table
foreach (GridViewRow row in gv.Rows)
{
GridViewExportUtil.PrepareControlForExport(row);
table.Rows.Add(row);
}

// add the footer row to the table
if (gv.FooterRow != null)
{
GridViewExportUtil.PrepareControlForExport(gv.FooterRow);
table.Rows.Add(gv.FooterRow);
}

// render the table into the htmlwriter
table.RenderControl(htw);

// render the htmlwriter into the response
HttpContext.Current.Response.Write(sw.ToString());
HttpContext.Current.Response.End();
}
}
}


/// Replace any of the contained controls with literals
private static void PrepareControlForExport(Control control)
{
for (int i = 0; i < control.Controls.Count; i++)
{
Control current = control.Controls[i];
if (current is LinkButton)
{
control.Controls.Remove(current);
control.Controls.AddAt(i, new LiteralControl((current as LinkButton).Text));
}
else if (current is ImageButton)
{
control.Controls.Remove(current);
control.Controls.AddAt(i, new LiteralControl((current as ImageButton).AlternateText));
}
else if (current is HyperLink)
{
control.Controls.Remove(current);
control.Controls.AddAt(i, new LiteralControl((current as HyperLink).Text));
}
else if (current is DropDownList)
{
control.Controls.Remove(current);
control.Controls.AddAt(i, new LiteralControl((current as DropDownList).SelectedItem.Text));
}
else if (current is CheckBox)
{
control.Controls.Remove(current);
control.Controls.AddAt(i, new LiteralControl((current as CheckBox).Checked ? "True" : "False"));
}

if (current.HasControls())
{
GridViewExportUtil.PrepareControlForExport(current);
}
}
}
}

Funziona perfettamente ma ho un problema, se il dato che esporto è una stringa lunga composta di numeri per esempio 150193019543, sul file excel mi ritrovo questo 1,50193E+11. Che posso fare?
Buon lavoro a tutti

Mauro