Generics challenge

gistfile1.cs

namespace HolidayAPI
{
    public class Schedule<T> where T : ISchedulable<IComparable>
    {
        private List<T> values = new List<T>();

        public void AddEvent(in T Event)
        {
            values.Add(Event);
            values = values.OrderBy(r => r).ToList();
        }

        public Schedule<T> ApplyHoliday(Holiday Holiday)
        {
            return this;
        }

        public Schedule()
        {

        }
    }

    public interface ISchedulable<out T> where T : IComparable
    {
        T Value { get; }
    }

    public class DueDate : ISchedulable<Version>
    {
        public Version Value => throw new NotImplementedException();

    }
} 

namespace HolidayAPITests
{
    [TestClass]
    public class ScheduleTests
    {
        [TestMethod]
        public void TestCreate1()
        {
            var schedule = new Schedule<DueDate>();
        }
    }
}

Reformatted into a SharpLab which compiles

And working: SharpLab. Variance only works for reference types (for obvious reasons, when you stop and think about it), so we need to introduce that DateTime as its own generic type parameter.

I'd question the overall design, though...

@canton7 - I see, thanks. I had found that solution (I should have mentioned) and wanted to avoid the caller of the constructor having to kmow about the type used to implement DueDate, I want that detail to be an implementation detail. Is there any way?

When we use a ref type it works fine but a value type seems to be fundamentally upsetting it.