Hello,  
This is my main class
            string Baseurl = " xxxx.limesurvey.net/index.php/admin/remotecontrol ";
            JsonRPCclient client = new JsonRPCclient(Baseurl);
            client.Method = "get_session_key";
            client.Parameters.Add("username", "username");
            client.Parameters.Add("password", "PW");
            client.Post();
            string SessionKey = client.Response.result.ToString();

I'm using RPCclient.cs

using System;
using System.IO;
using System.Net;
using System.Text;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

namespace limesurvey
{

    public class JsonRPCclient
    {

        private int id = 0;
        /// <summary>
        /// Set JSON-RPC webservice URL
        /// </summary>
        public string URL { set; get; }
        /// <summary>
        /// Set JSON-RPC method
        /// </summary>
        public string Method { set; get; }
        /// <summary>
        /// Add JSON-RPC params
        /// </summary>
        public JObject Parameters { set; get; }

        /// <summary>
        /// Results of the request
        /// </summary>
        public JsonRPCresponse Response { set; get; }


        /// <summary>
        /// Create a new object of RPCclient 
        /// </summary>
        public JsonRPCclient()
        {
            Parameters = new JObject();
            Response = null;
        }

        /// <summary>
        /// Create a new object of RPCclient
        /// </summary>
        /// <param name="URL"></param>
        public JsonRPCclient(string URL)
        {
            this.URL = URL;
            Parameters = new JObject();
            Response = null;
        }

        /// <summary>
        /// POST the request and returns server response
        /// </summary>
        /// <returns></returns>
        public string Post()
        {
            try
            {
                JObject jobject = new JObject();
                jobject.Add(new JProperty("jsonrpc", "2.0"));
                jobject.Add(new JProperty("id", ++id));
                jobject.Add(new JProperty("method", Method));
                jobject.Add(new JProperty("params", Parameters));

                string PostData = JsonConvert.SerializeObject(jobject);
                UTF8Encoding encoding = new UTF8Encoding();
                byte[] bytes = encoding.GetBytes(PostData);

                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(URL);
                request.Method = "POST";
                request.ContentType = "application/json";
                request.KeepAlive = true;
                request.ContentLength = bytes.Length;

                Stream writeStream = request.GetRequestStream();
                writeStream.Write(bytes, 0, bytes.Length);
                writeStream.Close();

                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                Stream responseStream = response.GetResponseStream();
                StreamReader readStream = new StreamReader(responseStream, Encoding.UTF8);

                JsonRPCresponse Response = new JsonRPCresponse();
                Response = JsonConvert.DeserializeObject<JsonRPCresponse>(readStream.ReadToEnd());
                Response.StatusCode = response.StatusCode;

                return Response.ToString();
            }
            catch (Exception e)
            {
                return e.ToString();
            }
        }

        public void ClearParameters()
        {
            this.Parameters = new JObject();
        }


    }

    public class JsonRPCresponse
    {
        public int id { set; get; }
        public object result { set; get; }
        public string error { set; get; }
        public HttpStatusCode StatusCode { set; get; }

        public JsonRPCresponse() { }

        public override string ToString()
        {
            return "{\"id\":" + id.ToString() + ",\"result\""" + result.ToString() + "\",\"error\":" + error + ((String.IsNullOrEmpty(error)) ? "null" : "\"" + error + "\"") + "}";
        }
    }

}

Please tell me why i get always null error in 
RPCclient.cs at line
           Response = JsonConvert.DeserializeObject<JsonRPCresponse>(readStream.ReadToEnd()); ????!!!

Thanks vey much!
 

Hello again!
I did my solution: i would like to share

First: using POSTMAN to get this

Creation a console projet like below: everything works!  (have to refactor my code  but i get my session key and survey list)
my Program.cs

using System;
using System.Net.Http;
using System.Text;
using Newtonsoft.Json;
using RestSharp;

namespace limesurvey
{
    class Program
    {
        private static readonly HttpClient client = new HttpClient();
        static void Main(string args)
        {
            var client = new RestClient(" xxxx.limequery.net/admin/remotecontrol ");
            var request = new RestRequest("", Method.Post);
            request.AddHeader("Content-Type", "application/json");
            request.AddHeader("Cookie", "PHPSESSID=e07219dc77786bfb3775003dd7e44e9c");
            var body = @"{
                " + "\n" +
                            @"    ""method"": ""get_session_key"",
                " + "\n" +
                            @"    ""params"": [
                " + "\n" +
                            @"        ""username"",
                " + "\n" +
                            @"        ""PW""
                " + "\n" +
                            @"    ],
                " + "\n" +
                            @"    ""id"": 1
                " + "\n" +
                            @"}";
            request.AddParameter("application/json", body, ParameterType.RequestBody);
            var response = client.Execute(request);
            Console.WriteLine(response.Content);
            if (response!=null && response.Content!=null && response.Content.Length> 0)
            {
                var sessionKey = SessionResult.FromJson(response.Content);
                // i get this: { "id":1,"result":"XiAwYJH6NyC5nFfts9l89hrZH4QouDkf","error":null}
                request = new RestRequest("", Method.Post);
                body = @"{
                        " + "\n" +
                         @"    ""method"": ""list_surveys"",
                        " + "\n" +
                         @"    ""params"": [
                        " + "\n" +
                         @"        ""sessionKey"",
                        " + "\n" +
                        sessionKey.Result
                         + "\n" +
                         @"    ],
                        " + "\n" +
                         @"    ""id"": 1
                        " + "\n" +
                         @"}";

                var getSurveyJson = JsonConvert.SerializeObject(new { method = "list_surveys", @params = new { sessionKey.Result }, id = 1 });
                request.AddParameter("application/json", getSurveyJson, ParameterType.RequestBody);
                response = client.Execute(request);
                
            }

            var survey_list = response.ToString();
            // I get survey list here
        }
    }
}

OutputSession.cs
using Newtonsoft.Json;
using Newtonsoft.Json.Converters;
using System.Globalization;

namespace limesurvey
{
    public partial class SessionResult
        {
            [JsonProperty("id")]
            public long Id { get; set; }

            [JsonProperty("result")]
            public string Result { get; set; }

            [JsonProperty("error")]
            public object Error { get; set; }
        }

        public partial class SessionResult
        {
            public static SessionResult FromJson(string json) => JsonConvert.DeserializeObject<SessionResult>;(json, limesurvey.Converter.Settings);
        }

        public static class Serialize
        {
            public static string ToJson(this SessionResult self) => JsonConvert.SerializeObject(self, limesurvey.Converter.Settings);
        }

        internal static class Converter
        {
            public static readonly JsonSerializerSettings Settings = new JsonSerializerSettings
            {
                MetadataPropertyHandling = MetadataPropertyHandling.Ignore,
                DateParseHandling = DateParseHandling.None,
                Converters =
            {
                new IsoDateTimeConverter { DateTimeStyles = DateTimeStyles.AssumeUniversal }
            },
            };
        }
    }

Cheer!
Nicolas P.




 

Sorry about POSTMAN image.
I attached it!
Cheer!