using System;

using System.Net;

using System.Web;

var url = new Uri("http://infinity.world.uk/Members/namelist.aspx?ID=-1&fid=X");

var query = HttpUtility.ParseQueryString(url.Query);

​

query["fid"] = 10.ToString();

​

var newUrl = url.GetLeftPart(UriPartial.Path) + "?" + query;

​

Console.WriteLine(newUrl);

​

query["fid"] = "%".ToString();

​

var newUrl2 = url.GetLeftPart(UriPartial.Path) + "?" + query;

Console.WriteLine(newUrl2);

​