DownLoad の処理について

protected void Button1_Click(object sender, EventArgs e)
{
      int num = ListBox1.Items.Count;
      for (int i = 0; i < num; ++i)
      {
          DownLoad(i);
      }
}
protected void DownLoad(int i)
{
      string[] FileMei = (string[])Session["FilesFullPath"];

      if (ListBox1.Items[i].Selected)
      {
          string str = string.Format("attachment;filename={0}", ListBox1.Items[i].Text);
          Response.AddHeader("Content-Disposition", str);
          Response.ContentType = "application/octet-stream";
          System.Text.Encoding encoding = System.Text.Encoding.GetEncoding("Shift-JIS");

          FileStream MyFileStream;
          long FileSize;

          MyFileStream = new FileStream(FileMei[i], FileMode.Open);
          FileSize = MyFileStream.Length;

          byte[] Buffer = new byte[(int)FileSize];
          MyFileStream.Read(Buffer, 0, (int)FileSize);
          MyFileStream.Close();

          Response.BinaryWrite(Buffer/*encoding.GetBytes()*/);
     }
     Response.End();
}