string url="http://forum.csdn.net/PointForum/Forum/PostTopic.aspx?forumID=e2798a59-79d5-4833-9c57-87d46a8b907a";
string[] urlArrays=url.Split('forumID');
string aStr=urlArrays[0];

------解决方案--------------------
 System.Text.RegularExpressions.Regex reg=new System.Text.RegularExpressions.Regex("a=([^&]+)");
       Uri uri = new Uri("http://www.contoso.com/catalog/shownew.htm?date=today&a=1&b=2");
       System.Text.RegularExpressions.Match match= reg.Match(uri.Query);
       string a= match.Groups[1].Value;