using System;

using System.Web;

public class Program

{

    public static void Main()

    {

        string url = "http://example.com";

        Uri uri1 = new Uri(url);

        uri1 = AddQuery(uri1, "name", "value");

        uri1 = AddQuery(uri1, "name", "value2");

        Console.WriteLine(uri1);

    }

    public static Uri AddQuery(Uri uri, string name, string value)

    {

        var ub = new UriBuilder(uri);

​

        // decodes urlencoded pairs from uri.Query to HttpValueCollection

        var httpValueCollection = HttpUtility.ParseQueryString(uri.Query);

        httpValueCollection.Remove(name);

​

        httpValueCollection.Add(name, value);

​

        // urlencodes the whole HttpValueCollection

        ub.Query = httpValueCollection.ToString();

​

        return ub.Uri;

    }

}

​

​