C#利用webrequest计算待下载的文件大小

C#利用webrequest计算待下载的文件大小
string URL = textBox1.Text;
string filetype = URL.Substring(URL.LastIndexOf(".") + 1, (URL.Length - URL.LastIndexOf(".") - 1));
filetypevalue.Text = filetype.ToUpper();
string filename = URL.Substring(URL.LastIndexOf("/") + 1, (URL.Length - URL.LastIndexOf("/") - 1));
namelabel.Text = filename;
System.Net.WebRequest req = System.Net.HttpWebRequest.Create(textBox1.Text);
req.Method = "HEAD";
System.Net.WebResponse resp = req.GetResponse();
long ContentLength = 0;
long result;
if (long.TryParse(resp.Headers.Get("Content-Length"), out ContentLength))
{
string File_Size;


if (ContentLength >= 1073741824)
{
result = ContentLength / 1073741824;
kbmbgb.Text = "GB";
}
else if (ContentLength >= 1048576)
{
result = ContentLength / 1048576;
kbmbgb.Text = "MB";
}
else
{
result = ContentLength / 1024;
kbmbgb.Text = "KB";
}
File_Size = result.ToString("0.00");
sizevaluelabel.Text = File_Size;
}