using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MessageLib
{
    public class Message
    {
        private string _content;

        public string Content
        {
            set
            {
                if (value.Length > 80 || value.Length <= 0)
                    throw new ArgumentException("Длина строки не может превышать 80 символов!");
            }
            get =>
                _content;
        }

        public DateTime SendDate
        {
            get;
            set;
        }

        public DateTime ReceiveDate
        {
            get
            {
                return SendDate.AddSeconds(1);
            }
            private set
            {

            }
        }

        public override string ToString() =>
            $"Mail: Content = {Content}, SendDate = {SendDate}, ReceiveDate = {ReceiveDate}";


    }
}


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MessageLib
{
    public class Dmessage : Message
    {

        public int Hours
        {
            get;
            set;
        }

        public new DateTime ReceiveDate
        {
            get
            {
                return SendDate.AddHours(-Hours);
            }
            private set
            {

            }
        }

        public override string ToString() =>
            $"D-Message: Content = {Content}, SendDate = {SendDate}, Hours = {Hours}, " +
            $"ReceiveData = {ReceiveDate}";
    }
}


using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MessageLib
{
    public class MessageBox : IEnumerable<Message>
    {
        private List<Message> _messages;

        public void ReceiveMail(Message msg) =>
            _messages.Add(msg);


        public IEnumerator<Message> GetEnumerator()
        {
            throw new NotImplementedException();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            throw new NotImplementedException();
        }
    }
}