CS1061: 'page' does not contain a definition for 'BtnExportGrid_Click' and no extension method 'BtnExportGrid_Click'

Question

Here is the Code for my Code Behind

public partial class PIX_Report_by_PIX : System.Web.UI.Page
{
    /// <summary>
    /// 
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    /// <summary>
    /// 
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="args"></param>
    protected void BtnExportGrid_Click(object sender, EventArgs args)
    {
        //  pass the grid that for exporting ...
        GridViewExportUtil.Export("Customers.xls", this.gvCustomers);
    }
}

And here is the Code for GridViewExportUtil.cs

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();

            //  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();
        }
    }
  }

  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);
        }
    }
  }
}

How I add this definition? How can I fix this error. All inputs are really appreciated.