using System;

using System.Web;

public class Program

{

    public static void Main()

    {

        string input="/article/dmf290484743?utm_source=hallo&deviceid=jfgghfhg";

        string prefix="https://testbrooklynapi.nieuwsblad.be";

        var uri=new Uri(new Uri(prefix),input);

        Console.WriteLine(uri.RemoveQueryStringByKey(prefix).PathAndQuery);

    }

}

public static class UriExtensions

{

    public static Uri RemoveQueryStringByKey(this Uri uri, string key)

        {

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

            newQueryString.Remove(key);

            var uriBuilder = new UriBuilder(uri);

            uriBuilder.Query=newQueryString.ToString();

            return uriBuilder.Uri;

        }

}