Getting NullReferenceException

I have such a funny question.

I have the following architecture:

For example, Manager class is implemented like this:

public sealed class Manager : Interface.Abstract.Employee
{
    private Interface.IEmployee chief = null;
    private readonly Decimal bonuslimit = Convert.ToDecimal(0.4F * Convert.ToSingle(BaseSalary));

    public Manager(Person person, DateTime hiredate) 
    : base(person, hiredate)
    {
    }

    public override List<Interface.IEmployee> Subordinates
    {
        get;
        set;
    }
    public override Interface.IEmployee Chief
    {
        get
        {
            return this.chief;
        }
        set
        {
            //if(value is Associate)
            //{                    
            //    throw new SystemException("Associate can't be a chief");
            //}
            this.chief = value;
        }
    }
    public override Decimal Salary
    {
        get
        {
            var actualbonus = Convert.ToDecimal(0.01F * Convert.ToSingle(this.YearsSinceHired * BaseSalary));
            var bonus = (actualbonus > bonuslimit) ? bonuslimit : actualbonus;
            var additional = 0M;

            if(this.HasSubordinates)
            {
                foreach(Interface.Abstract.Employee employee in this.Subordinates)
                {
                    if(employee is Sales)
                    {
                        additional += employee.Salary;
                    }
                } 
            }
            return Convert.ToDecimal(Convert.ToSingle(additional) * 0.005F) + BaseSalary + bonus;
        }
    }
}

And 'factory client' that looks like this:

public class EmployeeFactoryClient
{
    private IDictionary<String, IEmployee> employees = new Dictionary<String, IEmployee>();              

    public EmployeeFactoryClient()
    { 
        this.Factory = new EmployeeFactory();            
    }
    public EmployeeFactoryClient(IEmployeeFactory factory)
    {
        this.Factory = factory;            
    }
    public IEmployeeFactory Factory { get; set; }

    public void HireEmployee(Person person, String type, String code)
    {
        this.employees.Add(
            new KeyValuePair<String, IEmployee>(
                code,
               开发者_运维技巧 this.Factory.Create(person, type, DateTime.Now)
            )
        );
    }
    public void DismissEmployee(String code)
    {
        this.employees.Remove(code);
    }
    public IEmployee GetEmployee(String code)
    {
        return this.employees[code];
    }
    public IEmployee this[String index]
    {
        get { return this.employees[index]; }
        private set { this.employees[index] = value; }
    }

    public Decimal TotalSalary
    {
        get
        {
            var result = 0M;
            foreach(var item in this.employees)
            {
                result += item.Value.Salary;
            }
            return result;
        }
    }        
}

And finally I have some test code:

public void SalaryTest()
    {
        #region [Persons]            
        var SalesPerson01 = new Person
        {
            Birthday = new DateTime(1980, 11, 03),
            Forename = "Corey",
            Surname = "Black",
            Gender = SexType.Female
        };
        var SalesPerson02 = new Person
        {
            Birthday = new DateTime(1980, 11, 03),
            Forename = "John",
            Surname = "Travis",
            Gender = SexType.Male
        }; 
        #endregion

        this.company.HireEmployee(SalesPerson01, "Sales", SalesPerson01.GetHashCode().ToString());
        ((Employee)this.company[SalesPerson01.GetHashCode().ToString()]).YearsSinceHired = 10;

        this.company.HireEmployee(SalesPerson02, "Sales", SalesPerson02.GetHashCode().ToString());
        ((Employee)this.company[SalesPerson02.GetHashCode().ToString()]).YearsSinceHired = 3;            

        ///////////////////////////////////////////////////////////////////
        ((Employee)this.company[SalesPerson01.GetHashCode().ToString()]).Subordinates.Add(
            this.company[SalesPerson02.GetHashCode().ToString()]
        );

        Assert.AreEqual(1405M, this.company.TotalSalary);
    }

Line ((Employee)this.company[SalesPerson01.GetHashCode().ToString()]).Subordinates.Add(this.company[SalesPerson02.GetHashCode().ToString()]); throws NullReferenceExeption. In the this.company[SalesPerson02.GetHashCode().ToString()] indexer returns IEmployee interface but not a class instance. Am I right? And if it is so how do I fix that?

I don't see anywhere that you are initializing the Subordinates member, so I suspect that it still has the default value which is null (not the empty list). The fix is to initialize it to an empty list in the constructor:

public Manager(Person person, DateTime hiredate) : base(person, hiredate)
{
    Subordinates = new List<Interface.IEmployee>();
}

it seems you're putting in Person but then casting to Employee which are unrelated classes

indexer returns IEmployee interface but not a class instance. Am I right?

You don't right indexer must return instance, create instance from interface impossible.

I think this.company[SalesPerson02.GetHashCode().ToString()] returns null cause you don't add SalesPerson02 instance to your company object.