string url = "http://www.***.com?id=123&username=test&userpwd=*****";

WebRequest wRequest = WebRequest.Create(url);

wRequest.Method = "GET";

wRequest.ContentType = "text/html;charset=UTF-8";

WebResponse wResponse = wRequest.GetResponse();

Stream stream = wResponse.GetResponseStream();

StreamReader reader = new StreamReader(stream, System.Text.Encoding.Default);

string str = reader.ReadToEnd();   //url返回的值

reader.Close();

wResponse.Close();