Form Controls not clearing after downloading csv in web forms

I have a web form where i download a csv. i want to clear the form. but the form controls are not clearing.

    protected void btndownload_Click(object sender, EventArgs e)
    {
        string inv = txtInvoiceNo.Text;
        clear();
        try
        {

        GetSelectedRecords(inv);

        }
        catch (Exception ex)
        {
            throw ex;
        }

    }

    private void clear()
    {
        txtInvoiceNo.Text = "";
        btndownload.Visible = false;
    }

When i run without the GetSelectedRecords, its clearing.

code for csv download

    private void GetCSV(DataTable dtsorted, string filename)
    {
        Response.Clear();
        Response.ClearContent();
        Response.ContentType = "application/text";
        Response.BufferOutput = true;
        Response.AddHeader("Content-Disposition", "attachment; filename=" + filename);
        Response.Charset = "";
        string sep = "";
        for (int i = 0; i <= dtsorted.Columns.Count - 1; i++)
        {
            Response.Write(sep + dtsorted.Columns[i].ColumnName + ',');
        }
        Response.Write("\r\n");
        for (int i = 0; i < dtsorted.Rows.Count; i++)
        {
            sep = "";
            for (int j = 0; j <= dtsorted.Columns.Count - 1; j++)
            {

                Response.Write(sep + dtsorted.Rows[i].ItemArray[j].ToString() + ',');
            }
            Response.Write("\r\n");
        }
        HttpContext.Current.Response.Flush();
        HttpContext.Current.Response.SuppressContent = true;
        HttpContext.Current.ApplicationInstance.CompleteRequest();
       // Response.Clear();
         //HttpContext.Current.Response.End();

    }