今天看了一个多文件上传的例子，有个别不太懂的地方，请教一下

protected void btnUpload_Click(object sender, EventArgs e)
        {            {
                HttpFileCollection files = HttpContext.Current.Request.Files;  
                  System.Text.StringBuilder strmsg = new System.Text.StringBuilder("");    //这句是什么用啊
                string[] rd = Request.Form[1].Split(',');//这句是什么用啊
                int ifile;
                for (ifile = 0; ifile < files.Count; ifile++)
                {
                    if (files[ifile].FileName.Length > 0)
                    {
                        System.Web.HttpPostedFile postedfile = files[ifile];
                        if (postedfile.ContentLength / 1024 > 1024)
                        {
                            strmsg.Append(Path.GetFileName(postedfile.FileName) + "---不能大于1024k<br>");
                            break;
                        }
                        string fex = Path.GetExtension(postedfile.FileName);
                        if (fex != ".jpg" && fex != ".JPG" && fex != ".gif" && fex != ".GIF")
                        {
                            strmsg.Append(Path.GetFileName(postedfile.FileName) + "---图片格式不对，只能是jpg或gif<br>");
                            break;
                        }
                    }
                }
                if (strmsg.Length <= 0)//这句就不懂为什么？
                {
                    
                    string dirpath = Server.MapPath("51aspx");                    if (Directory.Exists(dirpath) == false)
                    {
                        Directory.CreateDirectory(dirpath);
                    }
                    Random ro = new Random();
                    int name = 1;
                    for (int i = 0; i < files.Count; i++)
                    {
                        System.Web.HttpPostedFile myFile = files[i];
                        string FileName = "";
                        string FileExtention = "";
                        FileName = System.IO.Path.GetFileName(myFile.FileName);
                        string stro = ro.Next(100, 100000000).ToString() + name.ToString();
                        string NewName = DateTime.Now.Minute.ToString() + DateTime.Now.Second.ToString() + DateTime.Now.Millisecond.ToString() + stro;
                        if (FileName.Length > 0)                        {
                            FileExtention = System.IO.Path.GetExtension(myFile.FileName);                            string ppath = dirpath + @"\" + NewName + FileExtention;
                            myFile.SaveAs(ppath);                        }                        name = name + 1;                    }                    Response.Write("<script>alert('恭喜，图片上传成功！')</script>");
                }
                else
                {
                    lblMessage.Text = strmsg.ToString();
                    lblMessage.Visible = true;
                }
            }        }