Foreach statement cannot operate on variables of type because the do not contain public definition for GetEnumerator

Question

I have a simple class, Employee, and another class, Company, which contains an array of employees. The company class has 2 nested classes with iterators. One orders the items from first to last, and the second one reverses the order. The NormalEnumeration is the default. When I attempt to use ReverseOrder" in a foreach loop, I get this error:

foreach statement cannot operate on variables of type 'Zadanie_1.Company.ReverseEnumeration' because 'Zadanie_1.Company.ReverseEnumeration' does not contain a public definition for 'GetEnumerator'

My questions is: How can I implement the custom iterators?

public class Employee
{
    private string surname, position;
    public string Stanowisko { get { return position; } }
    public Employee (string nazwisko, string stanowisko)
    {
        this.surname = nazwisko;
        this.position = stanowisko;
    }
    public override string ToString()
    {
        return string.Format("nazwisko: {0}, stanowisko: {1}", surname, position);
    }
}

public class Company:IEnumerable
{
    string name;
    private Employee[] employeeArray;
    public Company(string nazwa, Employee[] pracownicy)
    {
        this.name = nazwa;
        this.employeeArray = pracownicy;
    }
    public IEnumerator GetEnumerator()
    {
        //foreach (Pracownik item in pracownicy)
        //{
        //    yield return item;
        //}

      return new NormalEnumeration(this);
    }
    public IEnumerator Ordered()
    {
        return new NormalEnumeration(this);
    }

    public override string ToString()
    {
        return string.Format("Nazwa firmy: {0}, liczba pracowników: {1}", name, employeeArray.Length);
    }
    public class ReverseEnumeration : IEnumerator
    {
        Company f;
        int counter;
        public ReverseEnumeration(Company f)
        {
            this.f = f;
            counter = f.employeeArray.Length;
        }

        public object Current
        {
            get
            {
                Console.WriteLine("Current");
                return f.employeeArray[counter];
            }
        }

        public bool MoveNext()
        {
            if (counter > 0)
            {
                Console.WriteLine("Move next:");
                counter--;
                return true;
            }
            else
                return false;
        }

        public void Reset()
        {
            counter = f.employeeArray.Length;
        }
    }
    public class NormalEnumeration : IEnumerator
    {
        Company f;
        int counter = -1;
        public NormalEnumeration(Company f)
        {
            this.f = f;
        }
        public object Current
        {
            get
            {
                Console.WriteLine("Current");
                return f.employeeArray[counter];
            }
        }

        public bool MoveNext()
        {
            if (counter >= f.employeeArray.Length-1)
                return false;
            else
            {
                counter++;
                return true;
            }

        }

        public void Reset()
        {
            counter = -1;
        }
    }
}

Answer 1

Comment under the question:

I tried to call the reverse iterator this way:

foreach (Employee item in new Company.ReverseEnumeration(instancenamehere)) { Console.WriteLine(item); }

For this to work, ReverseEnumration would have to implement IEnumerable. If you want to be able to modify the enumerator, just add a new property to the Company class.

public class Company : IEnumerable
{
    public IEnumerator Enumerator { get; set; }

    public IEnumerator GetEnumerator()
    {
        return Enumerator;
    }

    // other members
}

You can set the default enumerator in the constructor.

public Company(string nazwa, Employee[] pracownicy)
{
    Enumerator = new NormalEnumeration(this);

    // other stuff
}

You should extract your enumerators out of the Company class, because they don't use it's internals anyway. This will enable you to easily change enumerators from the outside.

var c = new Company(null, null);
c.Enumerator = new ReverseEnumeration(c);