Hi Axiomatik, thank you so much again. :)

I changed everything in my code like you said:

1) edited  try catch;

2) added multiply inheritance in every my interface (my inheritance in every my class is like yours);

3) edited my class Mission and added interface IMission;

In the end I added blank space in my ToString() when I print listPrivates and listRepairs, because I missed them.

And the result is also 20/100 with three correct tests, one incorrect and three RunTime Errors. I'm very disparate.

Maybe I missed something important or might have been something else.

RunTime errors are most likely triggered from your StartUp class (ie Engineer has its for-loop run until inputArray.Length - 1 instead of inputArray.Length), unless some errors in your class builds. You can compare with different StartUp classes from other solutions, to try to fix some errors but also do watch how the SoftUni instructors solve this exercise and then come back to your solution and eliminate all of the problems.

StartUp:

using System;
using System.Linq;
using System.Collections.Generic;

using MilitaryElite.Classes;
using MilitaryElite.Interfaces;

namespace MilitaryElite.Core
{
    public class Engine
    {
        public Engine()
        {
        }

        public void Run()
        {
            List<Soldier> soldierList = new List<Soldier>();

            string[] soldierInput = Console.ReadLine()
                .Split(" ", StringSplitOptions.RemoveEmptyEntries);


            while (soldierInput[0] != "End")
            {

                string typeSoldier = soldierInput[0];
                try
                {
                    if (typeSoldier == "Private")
                    {
                        AddPrivate(soldierList, soldierInput);
                    }
                    else if (typeSoldier == "LieutenantGeneral")
                    {
                        AddGeneral(soldierList, soldierInput);
                    }
                    else if (typeSoldier == "Engineer")
                    {
                        AddEngineer(soldierList, soldierInput);
                    }
                    else if (typeSoldier == "Commando")
                    {
                        AddCommando(soldierList, soldierInput);
                    }
                    else if (typeSoldier == "Spy")
                    {
                        AddSpy(soldierList, soldierInput);
                    }
                }
                catch (Exception ex)
                {

                }

                soldierInput = Console.ReadLine()
                    .Split(" ", StringSplitOptions.RemoveEmptyEntries);
            }


            foreach (var soldier in soldierList)
            {
                Console.WriteLine(soldier.ToString());
            }
        }

        private static void AddSpy(List<Soldier> soldierList, string[] soldierInput)
        {
            int id = int.Parse(soldierInput[1]);
            string firstName = soldierInput[2];
            string lastName = soldierInput[3];
            int codeNumber = int.Parse(soldierInput[4]);

            Spy spy = new Spy(id, firstName, lastName, codeNumber);
            soldierList.Add(spy);
        }

        private static void AddCommando(List<Soldier> soldierList, string[] soldierInput)
        {
            int id = int.Parse(soldierInput[1]);
            string firstName = soldierInput[2];
            string lastName = soldierInput[3];
            decimal salary = decimal.Parse(soldierInput[4]);
            string corps = soldierInput[5];

            Commando commando = new Commando(id, firstName, lastName, salary, corps);
            if (soldierInput.Length > 6)
            {
                for (int i = 6; i < soldierInput.Length; i+=2)
                {
                    string missionName = soldierInput[i];
                    string missionState = soldierInput[i + 1];
                    try
                    {
                        Mission mission = new Mission(missionName, missionState);
                        commando.AddMission(mission);
                    }
                    catch (Exception ex)
                    {

                    }
                }
            }

            soldierList.Add(commando);
        }

        private static void AddEngineer(List<Soldier> soldierList, string[] soldierInput)
        {
            int id = int.Parse(soldierInput[1]);
            string firstName = soldierInput[2];
            string lastName = soldierInput[3];
            decimal salary = decimal.Parse(soldierInput[4]);
            string corps = soldierInput[5];

            Engineer engineer = new Engineer(id, firstName, lastName, salary, corps);
            if (soldierInput.Length > 6)
            {
                for (int i = 6; i < soldierInput.Length; i+=2)
                {
                    string partName = soldierInput[i];
                    int repairTime = int.Parse(soldierInput[i + 1]);

                    try
                    {
                        Repair repair = new Repair(partName, repairTime);
                        engineer.AddRepair(repair);
                    }
                    catch (Exception ex)
                    {

                    }
                }
            }

            soldierList.Add(engineer);
        }

        private static void AddGeneral(List<Soldier> soldierList, string[] soldierInput)
        {
            int id = int.Parse(soldierInput[1]);
            string firstName = soldierInput[2];
            string lastName = soldierInput[3];
            decimal salary = decimal.Parse(soldierInput[4]);

            LieutenantGeneral general = new LieutenantGeneral(id, firstName, lastName, salary);
            if (soldierInput.Length > 5)
            {
                for (int i = 5; i < soldierInput.Length; i++)
                {
                    int targetId = int.Parse(soldierInput[i]);
                    var currentPrivate = (Private)soldierList.First(sol => sol.Id == targetId);
                    general.AddPrivate(currentPrivate);
                }
            }

            soldierList.Add(general);
        }

        private static void AddPrivate(List<Soldier> soldierList, string[] soldierInput)
        {
            int id = int.Parse(soldierInput[1]);
            string firstName = soldierInput[2];
            string lastName = soldierInput[3];
            decimal salary = decimal.Parse(soldierInput[4]);

            Private soldier = new Private(id, firstName, lastName, salary);
            soldierList.Add(soldier);
        }
    }
}

Repair-class (variant 1):

using System;
using MilitaryEliteDemo.Contracts;

namespace MilitaryEliteDemo.Models
{
    public class Repair : IRepair
    {
        public Repair(string partName, int hoursWorked)
        {
            this.PartName = partName;
            this.HoursWorked = hoursWorked;
        }

        public string PartName { get; private set; }

        public int HoursWorked { get; private set; }

        public override string ToString()
        {
            return $"Part Name: {this.PartName} Hours Worked: {this.HoursWorked}";
        }
    }
}

Repair class (variant 2):

using System;
using System.Text;
using MilitaryElite.Interfaces;

namespace MilitaryElite.Classes
{
    public class Repair : IRepair
    {
        private int hours;

        public Repair(string name, int hours)
        {
            Name = name;
            Hours = hours;
        }

        public string Name { get; }

        public int Hours
        {
            get
            {
                return this.hours;
            }
            private set
            {
                if (value < 0)
                {
                    throw new ArgumentException();
                }
                this.hours = value;
            }
        }

        public override string ToString()
        {
            return $"Part Name: {this.Name} Hours Worked: {this.Hours}";
        }
    }
}

@ Axiomatik  ,

I got 100/100 after three days working and nearly thirty attemps - https://github.com/elipopovadev/CSharp-OOP/tree/main/InterfacesAndAbstraction-Exercise/MilitaryElite. You really help me to understand how important is this exercise and to found out my mistakes in my last solution. This exercise is closest to the real web project, isn't it?!  Can you  tell me more exercises like this one, because I'm not very confident to work with Engine, Writer and Reader?

I made some folders:

- Core with Engine and IEngine;

-  Enums with enumeration for State and Corps;

-  Exceptions with three exceptions - "InvalidCorpsException", "InvalidMissionCompletion Exception", "InvalidStateException";

-  Interfaces;

-  IO with ConsoleReader, ConsoleWriter, IReader and IWriter;

-  Models;

And StartUp:

using MilitaryElite.Core;
using MilitaryElite.IO;
using MilitaryElite.IO.Contracts;

namespace MilitaryElite
{
    public class StartUp
    {
        static void Main(string[] args)
        {
            IReader reader = new ConsoleReader();
            IWriter writer = new ConsoleWriter();
            IEngine engine = new Engine(reader, writer);
            engine.Run();
        }
    }
}

Best Regards!