Grid View Export Excel

protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
        {           
            BindGridView();
            this.GridView1.AllowPaging = false;
            this.GridView1.DataBind();
            Export("Customers.xls", this.GridView1);
        }

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If you are Using Master Page or User Control in the page Please Add This

<Triggers>
          <asp:PostBackTrigger ControlID="ImageButton1" />
</Triggers>

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public void BindGridView()
        {
            con.Open();
            SqlCommand cmd = new SqlCommand("", con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);
            GridView1.DataSource = ds;
            GridView1.DataBind();
        }

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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.GridLines = gv.GridLines;
                    //  add the header row to the table
                    if (gv.HeaderRow != null)
                    {
                        PrepareControlForExport(gv.HeaderRow);
                        table.Rows.Add(gv.HeaderRow);
                    }
                    //  add each of the data rows to the table
                    foreach (GridViewRow row in gv.Rows)
                    {
                        PrepareControlForExport(row);
                        table.Rows.Add(row);
                    }
                    //  add the footer row to the table
                    if (gv.FooterRow != null)
                    {
                        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();
                }
            }
        }

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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())
                {
                    PrepareControlForExport(current);
                }
            }
        }