AssemblaClientSettings.cs

public class AssemblaClientSettings
    {
        public string BaseUrl { get; set; } = "https://api.assembla.com/";
        public string ApiKey { get; set; }
        public string ApiSecret { get; set; }
    }

AssemblaRestService.cs

public class AssemblaRestService : IAssemblaRestService
    {
        private readonly AssemblaClientSettings _assemblaClientSettings;

        public AssemblaRestService(AssemblaClientSettings assemblaClientSettings)
        {
            _assemblaClientSettings = assemblaClientSettings;
        }

        public TResponse Get<TResponse>(string resource) where TResponse : class
        {
            using (var client = CreateClient(resource))
            {
                var response = client.GetAsync(client.BaseAddress).Result;
                return DeserializeResponse<TResponse>(response);
            }
        }

        public TResponse Post<TResponse>(string resource) where TResponse : class
        {
            using (var client = CreateClient(resource))
            {
                var response = client.PostAsync(client.BaseAddress, new StringContent("")).Result;
                return DeserializeResponse<TResponse>(response);
            }
        }

        protected virtual TResponse DeserializeResponse<TResponse>(HttpResponseMessage response) where TResponse : class
        {
            if (!response.IsSuccessStatusCode)
                return null;
            var responseJson = response.Content.ReadAsStringAsync().Result;
            return JsonConvert.DeserializeObject<TResponse>(responseJson);
        }

        protected virtual HttpClient CreateClient(string resource)
        {
            var client = new HttpClient {BaseAddress = CreateUrl(resource)};
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            client.DefaultRequestHeaders.Add("X-Api-Key", _assemblaClientSettings.ApiKey);
            client.DefaultRequestHeaders.Add("X-Api-Secret", _assemblaClientSettings.ApiSecret);
            return client;
        }

        private Uri CreateUrl(string resource)
        {
            return new Uri(new Uri(_assemblaClientSettings.BaseUrl), resource);
        }
    }

AssemblaTicketClient.cs

public class AssemblaTicketClient : AssemblaClientBase, IAssemblaTicketClient
    {
        public AssemblaTicketClient(IAssemblaRestService assemblaRestService) : base(assemblaRestService)
        {
        }

        public virtual Ticket GetTicket(string space, long ticketId)
            => AssemblaRestService.Get<Ticket>($"/v1/spaces/{space}/tickets/{ticketId}.json");

        public ICollection<Ticket> GetTicketsByMilestone(string space, long milestoneId)
        {
            var allTickets = new List<Ticket>();
            var result = GetTicketsByMilestone(space, milestoneId, 1);
            var page = 1;
            while (result != null)
            {
                allTickets.AddRange(result);
                page++;
                result = GetTicketsByMilestone(space, milestoneId, page);
            }
            return allTickets;
        }

        private IEnumerable<Ticket> GetTicketsByMilestone(string space, long milestoneId, int page)
            => AssemblaRestService.Get<IEnumerable<Ticket>>(
                $"/v1/spaces/{space}/tickets/milestone/{milestoneId}.json?per_page=100&page={page}&ticket_status=all");
    }