System.Runtime.Serializationを利用してJsonデータクラスを定義

自分で作成した一例

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.Serialization;

namespace TestApp.Json
{
    [DataContract]
    public class CreateModel_request
    {
        [DataMember]
        public string SessionID { get; set; }

        [DataMember]
        public int UserID { get; set; }

        [DataMember]
        public VoiceInfo_Object_CreateModel VoiceInfo { get; set; }
    }

    [DataContract]
    public class VoiceInfo_Object_CreateModel {

        [DataMember]
        public string Phrase { get; set; }

        [DataMember]
        public bool CleanRegist { get; set; }

        [DataMember]
        public List<VoiceList_Item> VoiceList { get; set; }
    }

    [DataContract]
    public class VoiceList_Item {

        [DataMember]
        public int VoiceID { get; set; }

        [DataMember]
        public string VoiceData { get; set; }
    }
}