MessageBox is not working

protected void Button3_Click(object sender, EventArgs e)
    {
        string fileName = Path.GetFileName(FileUpload1.PostedFile.FileName);
        int fileLengthInKB = FileUpload1.PostedFile.ContentLength;
        if (FileUpload1.HasFile)
        {
            if (fileLengthInKB < 1048576)
            {
                FileUpload1.PostedFile.SaveAs(Server.MapPath("~/upload/") + fileName);
                Response.Redirect(Request.Url.AbsoluteUri);
            }
            else
            {
                MessageBox.Show("Enter a file less than 1 MB");
            }
        }

    }