using System;

public class Program

{

    public static void Main()

    {

        var x = new Uri("http://skaap.com");

​

        var queryToAppend = "myvalue=123";

        var y = new UriBuilder(x);

        y.Query += "vark";

        if (y.Query != null && y.Query.Length > 1)

            y.Query = y.Query.Substring(1) + "&" + queryToAppend; 

        else

           y.Query = queryToAppend;         

        Console.WriteLine(y.ToString());

    }

}