QueryBuilder.cs

    internal static class QueryBuilder
    {
        internal static string Build(string uri, IDictionary<string, string> queryString)
        {
            var sb = new StringBuilder(uri);

            var hasQuery = uri.IndexOf('?') != -1;

            foreach (var item in queryString)
            {
                sb.Append(hasQuery ? '&' : '?');

                sb.Append(item.Key);
                sb.Append('=');
                sb.Append(item.Value);
                hasQuery = true;
            }

            return sb.ToString();
        }
    }
    
    //usage
    var url = QueryBuilder.Build(url, new Dictionary<string, string>
    {
      ["query1"] = "Some value",
      ["query2"] = "Some value",
    }),