using System;

using System.Net;

using System.IO;

​

public class Program

{

    public static void Main()

    {

using (var wc = new WebClient())

{

    using (var imgStream = new MemoryStream(wc.DownloadData("https://ivd.gib.gov.tr/captcha/jcaptcha?imageID=33n42hjvdlrh8900")))

    {

            string result = Convert.ToBase64String(imgStream.ToArray());

        Console.WriteLine(result);

    }

}

    }

}