REST api with C#.NET complex json data in body posting

You have two problems here:

1. You need to set RequestFormat = DataFormat.Json before the call to AddBody:

       req.RequestFormat = DataFormat.Json;
       req.AddBody(json);
   

   Without setting the parameter, RestSharp tries to serialize the JObject to XML and falls into an infinite recursion somewhere -- most likely trying to serialize JToken.Parent.

2. The more recent versions of RestSharp no longer use Json.NET as their JSON serializer:

   There is one breaking change: the default Json*Serializer* is no longer 
   compatible with Json.NET. To use Json.NET for serialization, copy the code 
   from https://github.com/restsharp/RestSharp/blob/86b31f9adf049d7fb821de8279154f41a17b36f7/RestSharp/Serializers/JsonSerializer.cs 
   and register it with your client:
   
   var client = new RestClient();
   client.JsonSerializer = new YourCustomSerializer();
   

   RestSharp's new built-in JSON serializer doesn't understand JObject so you need to follow the instructions above if you are using one of these more recent versions, Create:

   public class JsonDotNetSerializer : ISerializer
   {
       private readonly Newtonsoft.Json.JsonSerializer _serializer;
   
       /// <summary>
       /// Default serializer
       /// </summary>
       public JsonDotNetSerializer() {
           ContentType = "application/json";
           _serializer = new Newtonsoft.Json.JsonSerializer {
               MissingMemberHandling = MissingMemberHandling.Ignore,
               NullValueHandling = NullValueHandling.Include,
               DefaultValueHandling = DefaultValueHandling.Include
           };
       }
   
       /// <summary>
       /// Default serializer with overload for allowing custom Json.NET settings
       /// </summary>
       public JsonDotNetSerializer(Newtonsoft.Json.JsonSerializer serializer){
           ContentType = "application/json";
           _serializer = serializer;
       }
   
       /// <summary>
       /// Serialize the object as JSON
       /// </summary>
       /// <param name="obj">Object to serialize</param>
       /// <returns>JSON as String</returns>
       public string Serialize(object obj) {
           using (var stringWriter = new StringWriter()) {
               using (var jsonTextWriter = new JsonTextWriter(stringWriter)) {
                   jsonTextWriter.Formatting = Formatting.Indented;
                   jsonTextWriter.QuoteChar = '"';
   
                   _serializer.Serialize(jsonTextWriter, obj);
   
                   var result = stringWriter.ToString();
                   return result;
               }
           }
       }
   
       /// <summary>
       /// Unused for JSON Serialization
       /// </summary>
       public string DateFormat { get; set; }
       /// <summary>
       /// Unused for JSON Serialization
       /// </summary>
       public string RootElement { get; set; }
       /// <summary>
       /// Unused for JSON Serialization
       /// </summary>
       public string Namespace { get; set; }
       /// <summary>
       /// Content type for serialized content
       /// </summary>
       public string ContentType { get; set; }
   }
   

   And then do:

       RestRequest req = new RestRequest(@"apicall/smsmessaging/v2/outbound/3313/requests", Method.POST);
       req.JsonSerializer = new JsonDotNetSerializer();