Here is my service method signature:

    [System.ServiceModel.Web.WebInvoke(UriTemplate = "/RegisterUser", Method = "POST")]
    void RegisterNewUser(User user);

Also Type User have DataContract attribute on class and DataMember attributes on its properties

and here is how I am calling the service method:

 String data = "{"user":{"__type" : "User:#PingMe","EmailID": "[email protected]","RegistrationID": "sdfhjklsdgkdfjgklgdjfklg"}}";  
        HttpWebRequest httpWebRequest = (HttpWebRequest)WebRequest.Create("http://localhost:2443/NotificationService.svc/RegisterUser");
        httpWebRequest.Method = "POST";
        byte[] bytes = Encoding.UTF8.GetBytes(data);
        httpWebRequest.ContentLength = bytes.Length;
        httpWebRequest.ContentType = "text/json; charset=utf-8";
            httpWebRequest.KeepAlive = false;
        Stream requestStream = httpWebRequest.GetRequestStream();

        HttpWebResponse httpWebResponse = (HttpWebResponse)httpWebRequest.GetResponse();

It call the service method succesfully, but in service method’s user parameter user.EmailID and User.RegistrationID always come ‘NULL’

Any Idea what I am missing here?

Do I need to set RequestFormat property as WebMessageFormat.JSON? in OperationContract attribute?