随每个新请求更改webclient代理

如何在每次提出新请求时更改代理? 如果关闭程序并重新启动,代理将使用不同的IP地址。

string GetRequest() 
{
  WebClient client = new WebClient();
  WebProxy wp = new WebProxy(randomProxyAddress + ":" + portNum);
  wp.UseDefaultCredentials = false;
  wp.Credentials = new NetworkCredential(username, password);
  client.Proxy = wp;            
  string str = client.DownloadString("http://www.example.com");
  client.Proxy = null;
  client.Dispose();
  return str;
}