how to export only visible columns to excel in asp.net

Try
protected void Button1_Click(object sender, EventArgs e)
{
try
{

GridView1.AllowPaging = false;
GridView1.DataBind();

PrepareGridViewForExport(GridView1);

Context.Response.ClearContent();
Context.Response.ContentType = "application/ms-excel";
Context.Response.AddHeader("content-disposition", string.Format("attachment;filename={0}.xls", "enquiry_request_details"));
Context.Response.Charset = "";
System.IO.StringWriter stringwriter = new System.IO.StringWriter();
HtmlTextWriter htmlwriter = new HtmlTextWriter(stringwriter);
GridView1.RenderControl(htmlwriter);
Context.Response.Write(stringwriter.ToString());
Context.Response.End();

}
catch (Exception ex)
{

}
}
public override void VerifyRenderingInServerForm(Control control)
{

}
private void PrepareGridViewForExport(Control gridView)
{
for (int i = 0; i < gridView.Controls.Count; i++)
{
//Get the control
Control currentControl = gridView.Controls[i];
if (currentControl is LinkButton)
{
gridView.Controls.Remove(currentControl);
gridView.Controls.AddAt(i, new LiteralControl((currentControl as LinkButton).Text));
}
else if (currentControl is ImageButton)
{
gridView.Controls.Remove(currentControl);
gridView.Controls.AddAt(i, new LiteralControl((currentControl as ImageButton).AlternateText));
}
else if (currentControl is HyperLink)
{
gridView.Controls.Remove(currentControl);
gridView.Controls.AddAt(i, new LiteralControl((currentControl as HyperLink).Text));
}
else if (currentControl is DropDownList)
{
gridView.Controls.Remove(currentControl);
gridView.Controls.AddAt(i, new LiteralControl((currentControl as DropDownList).SelectedItem.Text));
}
else if (currentControl is CheckBox)
{
gridView.Controls.Remove(currentControl);
gridView.Controls.AddAt(i, new LiteralControl((currentControl as CheckBox).Checked ? "True" : "False"));
}
if (currentControl.HasControls())
{
// if there is any child controls, call this method to prepare for export
PrepareGridViewForExport(currentControl);
}
}
}