How to Attach My upload files

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C#

ASP.NET

Visual-Studio

Hi everyone
I am trying to upload some files attached to a list box into my computer but I get an error that says you cannot open a closed file(s)
Below is my code:

protected void btnUpload_Click(object sender, EventArgs e)
{
  int sessionCount = Session.Count;

  for (int i = sessionCount - 1; i >= 0; i--)
  {
    if (sessionCount <= 3)
    {
      HtmlInputFile hif = (HtmlInputFile)Session["myupload" + i];

      if (hif.PostedFile.ContentLength <= 500000)
      {
        string storePath = Server.MapPath("~") + "/MultipleUpload";

        if (!Directory.Exists(storePath))

          Directory.CreateDirectory(storePath);
        hif.PostedFile.SaveAs(storePath + "/" + Path.GetFileName
          (hif.PostedFile.FileName));
        lblError.Text = "Your Files are uploaded successfully";
        ListBox1.Items.Clear();
      }
      else
          lblError.Text = "An error occured";
    }
    else
        lblError.Text =
        "You have exceeded the maximum number of files to be uploaded (3)";

  }
  Session.RemoveAll();
}

I dont know what I am doing wrong please I need help

Thanks in advance