From: 011netservice@gmail.com
Date: 2022-04-24
Subject: .txt

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IEnumerable vs IEnumerator

IEnumerable is an interface that defines one method GetEnumerator which returns an IEnumerator interface, 
this in turn allows readonly access to a collection. 
A collection that implements IEnumerable can be used with a foreach statement.


Definition
  IEnumerable                            // 實作 IEnumerable 的 Collection 就可以使用 foreach 語法!
    public IEnumerator GetEnumerator();  // IEnumerable 定義 GetEnumerator() 可取得唯讀的 collection.

  IEnumerator                  // 實作 IEnumerator 需提供 Current property and MoveNext and Reset methods. 
    public object Current;
    public void Reset();
    public bool MoveNext();


using System;
using System.Collections;

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

    public string firstName;
    public string lastName;
}

// Collection of Person objects. This class
// implements IEnumerable so that it can be used
// with ForEach syntax.
// IEnumerable 適用 non-generic 之類的物件.
public class People : IEnumerable  // 實作 IEnumerable 才能使用 ForEach 語法.
{
    private Person[] _people;
    public People(Person[] pArray)
    {
        _people = new Person[pArray.Length];

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

// Implementation for the GetEnumerator method.
    IEnumerator IEnumerable.GetEnumerator()  // IEnumerable 只需實作一個方法 GetEnumerator().
    {
       return (IEnumerator) GetEnumerator();
    }

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

// When you implement IEnumerable, you must also implement IEnumerator.
public class PeopleEnum : IEnumerator  // 實作 IEnumerator 需提供 Current property and MoveNext and Reset methods.
{
    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);
    }
}

/* This code produces output similar to the following:
 *
 * John Smith
 * Jim Johnson
 * Sue Rabon
 *
 */
 
 
 Remarks
IEnumerable is the base interface for all non-generic collections that can be enumerated. 
For the generic version of this interface see System.Collections.Generic.IEnumerable<T>. 
IEnumerable contains a single method, GetEnumerator, which returns an IEnumerator. 
IEnumerator provides the ability to iterate through the collection by exposing a Current property and MoveNext and Reset methods.

It is a best practice to implement IEnumerable and IEnumerator on your collection classes to enable the ( in Visual Basic) syntax, 
however implementing IEnumerable is not required. 
If your collection does not implement IEnumerable, you must still follow the iterator pattern to support this syntax by providing a method that returns an interface, class or struct. 
When using Visual Basic, you must provide an IEnumerator implementation, which is returned by . 
When developing with C# you must provide a class that contains a property, 
and and methods as described by IEnumerator, 
but the class does not have to implement IEnumerator.foreachFor 
EachGetEnumeratorGetEnumeratorCurrentMoveNextReset