C# IEnumerable Interface

Question

I am trying to understand this example from a MSDN article, to my understanding with the IEnumberable interface, we will be able to use Foreach to loop through the class collection, I am confused at the Main method, why don't we just use:

   foreach (Person p in peopleArray)
            Console.WriteLine(p.firstName + " " + p.lastName);

instead of

   People peopleList = new People(peopleArray);
    foreach (Person p in peopleList)
        Console.WriteLine(p.firstName + " " + p.lastName);

Example:

using System;
using System.Collections;

public class Person
{
    public Person(string fName, string lName)
    {
        this.firstName = fName;
        this.lastName = lName;
    }

    public string firstName;
    public string lastName;
}

public class People : IEnumerable
{
    private Person[] _people;
    public People(Person[] pArray)
    {
        _people = new Person[pArray.Length];

        for (int i = 0; i < pArray.Length; i++)
        {
            _people[i] = pArray[i];
        }
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
       return (IEnumerator) GetEnumerator();
    }

    public PeopleEnum GetEnumerator()
    {
        return new PeopleEnum(_people);
    }
}

public class PeopleEnum : IEnumerator
{
    public Person[] _people;

    // Enumerators are positioned before the first element 
    // until the first MoveNext() call. 
    int position = -1;

    public PeopleEnum(Person[] list)
    {
        _people = list;
    }

    public bool MoveNext()
    {
        position++;
        return (position < _people.Length);
    }

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

    object IEnumerator.Current
    {
        get
        {
            return Current;
        }
    }

    public Person Current
    {
        get
        {
            try
            {
                return _people[position];
            }
            catch (IndexOutOfRangeException)
            {
                throw new InvalidOperationException();
            }
        }
    }
}

class App
{
    static void Main()
    {
        Person[] peopleArray = new Person[3]
        {
            new Person("John", "Smith"),
            new Person("Jim", "Johnson"),
            new Person("Sue", "Rabon"),
        };

        People peopleList = new People(peopleArray);
        foreach (Person p in peopleList)
            Console.WriteLine(p.firstName + " " + p.lastName);

    }
}

I don't see any reason to have the extra enumerator. You are right that you can directly traverse through the peopleArray instead of creating the instance of IEnumerable. — Venki, Feb 04 '14 at 12:23

score 0 · Answer 1 · answered Feb 05 '14 at 18:40

You're right, you could simply use the first version because arrays implement IEnumerable.

The reason they chose to iterate over People is simply for academic purposes; to demonstrate how iterators work (and how to implement IEnumerable). If they simply iterated over peoplearray, they wouldn't be using the People class, which is the main focus of that example.

C# IEnumerable Interface

1 Answers1