将元素添加到字典中的列表会产生意外的结果 c#

我正在将后端转换为 c#，我注意到一个奇怪的行为

当我向字典中的列表添加元素时

（实际上在另一本字典中）：

很快，它会将值添加到字典中每个元素的每个列表中。

这是代码：

       public class Validator_Counter_Model
        {
            public readonly  Dictionary<string,Info_Model> Info;
            private Dictionary<string,Dictionary<DateTime,List<int>>> _map = new Dictionary<string, Dictionary<DateTime,List<int>>>();
            public Validator_Counter_Model(Dictionary<string, Info_Model> info)
            {
                Info = info;
            }
            public void Add(Validation_Element element)
            {

/// #### PROBLEM IS HERE!
                if (!_map.ContainsKey(element.Id))
                {
                    Dictionary<DateTime, List<int>> newmap = new Dictionary<DateTime, List<int>>();
                    _map.Add(element.Id, newmap);
                }

                DateTime fulldate = new Custom_Time_Model(element.Time).RegularTime;
                if (!_map[element.Id].ContainsKey(fulldate.Date))
                {
                    List<int> newList = new List<int>();
                    _map[element.Id].Add(fulldate.Date, newList);
                }
                _map[element.Id][fulldate.Date].Add(element.Time);
/// #### PROBLEM IS HERE!

            }

            public void Del(Validation_Element element)
            {
                DateTime fulldate = new Custom_Time_Model(element.Time).RegularTime;
                DateTime date = new DateTime(fulldate.Year, fulldate.Month, fulldate.Day);
                _map[element.Id][date].Remove(element.Time);
            }

            public void Update(Dictionary<string, Dictionary<DateTime, List<int>>> newMap) => _map = newMap;

            public Dictionary<string, Dictionary<DateTime, List<int>>> map => _map;

        }
    }