C# 将类的复杂对象序列化为json

我想将以下类的新对象转换为 json 字符串。为此，我使用 JavaScriptSerializer 和 Newtonsoft 库。但它们的输出都是空括号（ {[],[]} ）！

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;

namespace My_Entity
{
    using My_Entity.Interfaces;
    using My_Entity.Abstracts;

    public class tbl_CategoryEntity : Entity<tbl_CategoryEntity>, Itbl_Category
    {
        private Int32? _CategoryID;
        private String _CategoryName;
        private Int32? _TypeID;
        private Boolean? _IsDel;
        private static readonly string _IdentityField = "CategoryID";
        private static readonly SqlDbType _IdentitySqlDbType = SqlDbType.Int;
        private readonly Dictionary<string, SqlDbType> _FieldsSqlDbType;

        public Int32? CategoryID
        {
            get { return _CategoryID; }
            set { _CategoryID = value; }
        }

        public String CategoryName
        {
            get { return _CategoryName; }
            set { _CategoryName = value; }
        }

        public Int32? TypeID
        {
            get { return _TypeID; }
            set { _TypeID = value; }
        }

        public Boolean? IsDel
        {
            get { return _IsDel; }
            set { _IsDel = value; }
        }

        public tbl_CategoryEntity()
        {
            _FieldsSqlDbType = new Dictionary<string, SqlDbType>()
            {
                { "CategoryID", SqlDbType.Int },
                { "CategoryName", SqlDbType.NVarChar },
                { "TypeID", SqlDbType.Int },
                { "IsDel", SqlDbType.Bit }
            }.Union(base._FilterFieldsSqlDbType).ToDictionary(k => k.Key, v => v.Value);
        }

        public static string GetIdentityField()
        {
            return _IdentityField;
        }

        public static SqlDbType GetIdentitySqlDbType()
        {
            return _IdentitySqlDbType;
        }

        public override SqlDbType GetSqlDbType(string PropertyName)
        {
            return _FieldsSqlDbType[PropertyName];
        }

        public override bool IsIdentity(string PropertyName)
        {
            return PropertyName.Equals(_IdentityField);
        }
    }
}

tbl_CategoryEntity a = new tbl_CategoryEntity()
{
    CategoryID = 12,
    CategoryName = "hi"
};
string json = new JavaScriptSerializer().Serialize(a);

我该如何修复它？

基类（实体）是：

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.Data.Sql;

namespace My_Entity.Abstracts
{
    using My_Entity.Interfaces;

    public abstract class Entity<T> : List<T>, IEntity where T : new()
    {
        private String _OrderColumn;
        private String _Order = "asc";
        private Int32? _PageIndex = 1;
        private Int32? _RowsPage = 10;
        protected readonly Dictionary<string, SqlDbType> _FilterFieldsSqlDbType;

        public String OrderColumn
        {
            get { return _OrderColumn; }
            set { _OrderColumn = value; }
        }
        public String Order
        {
            get { return _Order; }
            set { _Order = value; }
        }
        public Int32? PageIndex
        {
            get { return _PageIndex; }
            set { _PageIndex = value; }
        }
        public Int32? RowsPage
        {
            get { return _RowsPage; }
            set { _RowsPage = value; }
        }

        public Entity()
        {
            _FilterFieldsSqlDbType = new Dictionary<string, SqlDbType>()
            { 
                { "OrderColumn", SqlDbType.VarChar },
                { "Order", SqlDbType.VarChar },
                { "PageIndex", SqlDbType.Int },
                { "RowsPage", SqlDbType.Int },
            };
        }

        public abstract SqlDbType GetSqlDbType(string PropertyName);

        public abstract bool IsIdentity(string PropertyName);
    }
}