C# .net 获取外网ip

public string GetIP()
{
string strUrl = "http://www.ip138.com/ip2city.asp"; //获得IP的网址了
Uri uri = new Uri(strUrl);
WebRequest wr = WebRequest.Create(uri);
Stream s = wr.GetResponse().GetResponseStream();
StreamReader sr = new StreamReader(s, Encoding.Default);
string all = sr.ReadToEnd(); //读取网站的数据
int i = all.IndexOf("[") + 1;

int j = all.IndexOf("]");

return ip;
}