public class NullToEmptyStringResolver : DefaultContractResolver
    {
        /// <summary>
        /// 建立屬性
        /// </summary>
        /// <param name="type">型別</param>
        /// <param name="memberSerialization">序列化成員</param>
        /// <returns></returns>
        protected
 
 override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
        {
            return type.GetProperties()
                .Select(x =>
                {
                    var property = CreateProperty(x, memberSerialization);
                    property.ValueProvider 
 
= new NullToEmptyStringValueProvider(x);
                    return property;
                }).ToList();
        }

        /// <inheritdoc />
        /// <summary>
        /// 小寫
        /// </summary>
        /// <param name="propertyName"></param>
        /// <returns></returns>
 

        protected override string ResolvePropertyName(string propertyName)
        {
            return propertyName.ToLower();
        }
    }

public class NullToEmptyStringValueProvider : IValueProvider
    {
        private readonly PropertyInfo _memberInfo;

        /// <summary>
        /// 建構函式
        /// </summary>
        /// <param name="memberInfo"></param>
        public NullToEmptyStringValueProvider(PropertyInfo memberInfo)
        {
            _memberInfo = memberInfo;
        }

        /// <inheritdoc />
        /// <summary>
        /// 獲取Value
        /// </summary>
        /// <param name="target"></param>
        /// <returns></returns>
        public object GetValue(object target)
        {
            var result = _memberInfo.GetValue(target);
            if (_memberInfo.PropertyType == typeof(string) && result == Convert.DBNull)
            {
                result = string.Empty;
            }

            return result;

        }

        /// <inheritdoc />
        /// <summary>
        /// 設定Value
        /// </summary>
        /// <param name="target"></param>
        /// <param name="value"></param>
        public void SetValue(object target, object value)
        {
            _memberInfo.SetValue(target, value);
        }
    }