嘿，在下面的表单类中创建people对象

Hey, create people object outside but within the form class like below

Public FormPeople: Window
{
 People people = new People();
 IList<People> peoples = new List<People>();
 
Public FormPeople()  {  
people.dates = new  List<Date>();
 }
private void btnAddDate_Click_1(object sender, EventArgs e)
        {
            
            Date d = new Date();            

            
            d.date = dtpDate.Value; //date time picker              
            
            people.dates.Add(d); //trying to add the dates into the People list
          }
}
private void AddPeople()
{
  peoples.add(people);
}

我假设你使用Visual Studio IDE，在项目中你可以声明类。你已经定义了这些类并不重要。

I assumed you use Visual Studio IDEand within the project you can declare classes. It doesn't matter you have already defined those classes.