将列表中的两个不同实例序列化为单个json字符串

I have two types of classes:

我有两种类:

public class HolidayClass
{
    public int ID { get; set; }
    public string Name { get; set; }
    public DateTime StartDate { get; set; }
    public DateTime EndDate { get; set; }
    public bool Active { get; set; }

    public HolidayClass(int ID, string Name, DateTime StartDate, DateTime EndDate, bool Active)
    {
        this.ID = ID;
        this.Name = Name;
        this.StartDate = StartDate;
        this.EndDate = EndDate;
        this.Active = Active;
    }

    public HolidayClass()
    {
    }
}

public class ProjectClass
{
    public int ID { get; set; }
    public string NetsisID { get; set; }
    public string Name { get; set; }
    public string Address { get; set; }
    public bool Active { get; set; }

    public ProjectClass(int ID, string NetsisID, string Name, string Address, bool Active)
    {
        this.ID = ID;
        this.NetsisID = NetsisID;
        this.Name = Name;
        this.Address = Address;
        this.Active = Active;
    }
    public ProjectClass()
    {
    }
}
public