我试图将一个派生类对象强制转换为它的基类（通过JSON转换保存它）。每当我尝试从派生类对象获取基类时，它都会返回派生类的一个对象。我无法获得基类对象（尝试使用显式和隐式强制转换以及转换）。

public class PlanningListModel : INotifyPropertyChanged
{
    private DateTime _date;
    private List<PlanningEntryModel> _Plannings;
    private bool _isSaving = false;

    public List<PlanningEntryModel> Plannings {get=>_Plannings;} //field i want to serialize
    //Same declarations for public fields
}

public class PlanningListViewModel :PlanningListModel   INotifyPropertyChanged
{
    private DateTime _date;
    private List<PlanningEntryModel> _Plannings;
    private bool _isSaving = false;

    public List<PlanningEntryModel> Plannings{           
        get {
            if (App.Data == null || App.Data.User == null || App.Data.IsReplicating)
                return base.Plannings.FindAll(x => true);
            switch (App.Data.CurrentList) {
                case 0: return base.Plannings.FindAll(x => true);
                case 1: return base.Plannings.FindAll(x => x.Volonter.ID == App.Data.User.ID);
                case 2: return base.Plannings.FindAll(x =>  x.User.Referent==App.Data.User.ID);
                default: return base.Plannings.FindAll(x => true);
        }
    }
} 

我要做的是:

PlanningListViewModel A = new PlanningListViewModel ();
PlanningListModel B = (PlanningListModel)A;
typeof(B); // B stays PlanningListViewModel and not PlanningListModel

我需要访问基类的字段（因为派生类的字段被修改（被重写的对象，我无法强制转换为它的基类。

我做错了什么？我非常感谢任何帮助！