using System;

using System.Linq;

​

public class Program

{

    public static void Main(string[] args)

    {

        var urlencoded = urlEncode("http://stackoverflow.com/text?a es b");

        Console.WriteLine(urlencoded);

    }

​

    public static string urlEncode(string source)

    {

        var fixture = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".ToList();

        return string.Join("", source.Select(c => fixture.Contains(c) ? c.ToString() : "%" + Convert.ToByte(c).ToString("x2"))).Replace("%20", "+");

    }

}