using System;

using System.Web;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(ModifyUrlParameters("http://example.org?wID=xxx", "wID", "[[WID]]"));

    }

    public static string ModifyUrlParameters(string rawUrl, string key, string val)

    {

        if (string.IsNullOrWhiteSpace(rawUrl))

        {

            return string.Empty;

        }

        var uriBuilder = new UriBuilder(rawUrl);        

        var queryItems = HttpUtility.ParseQueryString(uriBuilder.Uri.Query);

        queryItems.Set(key, val);

        uriBuilder = new UriBuilder(rawUrl);

        uriBuilder.Query = queryItems.ToString();

        return uriBuilder.Uri.ToString();

    }

}