關於迭代器中IEnumerable與IEnumerator的區別

首先是IEnumerable與IEnumerator的定義：

1.IEnumerable介面允許使用foreach迴圈，包含GetEnumerator()方法，可以迭代集合中的項。

2.IEnumerator介面是一個真正的集合訪問器，它包含MoveNext()方法和Current屬性，在foreach迴圈中，如果MoveNext()返回True,則就是用IEnumerator介面的Current屬性來獲取對象的一個引用，用於foreach迴圈。

3.如果要迭代一個類，可以使用GetEnumerator(),其返回類型是IEnumerator.

　如果要迭代一個類成員，則用IEnumerable.

下麵的例子是迭代Person類中的類成員Ages,使用了IEnumerable。第二個例子則是迭代一個類，所以使用了IEnumerator作為返回值。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Collections;

namespace _10_5_5
{
    public class person
    {
        private string name;
        private int age;
        public string Name
        {
            get
            {
                return name;
            }
            set
            {
                name = value;
            }
        }
        public int Age
        {
            get
            {
                return age;
            }
            set
            {
                age = value;
            }
        }
        public person(string PName, int PAge)
        {
            Name = PName;
            Age = PAge;
        }
        public static bool operator >(person a, person b)
        {
            if (a.Age > b.Age)
                return true;
            else
                return false;
        }
        public static bool operator <(person a, person b)
        {
            if (a.Age > b.Age)
                return false;
            else
                return true;
        }
        public static bool operator >=(person a, person b)
        {
            if (a.Age >= b.Age)
            {
                return true;
            }
            else
                return false;
        }
        public static bool operator <=(person a, person b)
        {
            if (a.Age <= b.Age)
                return true;
            else
                return false;
        }
    }
    public class People : DictionaryBase
    {
        public IEnumerable Ages//註意是IEnumerable
        {
            get
            {
                foreach (object person in Dictionary.Values)
                {
                    yield return (person as person).Age;
                }
            }
        }
        public person[] GetOldest()
        {
            People oldPeople = new People();
            person oldPerson = null;
            person currentPerson;
            foreach (DictionaryEntry myPeople in Dictionary)
            {
                currentPerson = myPeople.Value as person;
                if (oldPerson == null)
                {
                    oldPerson = currentPerson;
                    oldPeople.Add(oldPerson);
                }
                else
                {
                    if (currentPerson > oldPerson)
                    {
                        oldPeople.Clear();
                        oldPeople.Add(currentPerson);
                        oldPerson = currentPerson;
                    }
                    else
                    {
                        if (currentPerson >= oldPerson)
                        {
                            oldPeople.Add(currentPerson);
                        }
                    }
                }
            }
            person[] oldestPeopleArray = new person[oldPeople.Count];
            int copyIndex = 0;
            foreach (DictionaryEntry p in oldPeople)
            {
                oldestPeopleArray[copyIndex] = p.Value as person;
                copyIndex++;
            }
            return oldestPeopleArray;
        }
        public void Add(person p)
        {
            Dictionary.Add(p.Name, p);
        }
        public person this[string SName]
        {
            get
            {
                return (person)Dictionary[SName];
            }
            set
            {
                Dictionary[SName] = value;
            }
        }

    }
    class Program
    {
        static void Main(string[] args)
        {
            person a = new person("Jack", 11);
            person b = new person("Json", 10);
            People s = new People();
            s.Add(a);
            s.Add(b);
            foreach(int age in s.Ages)
            {
                Console.WriteLine("{0}\t", age);
            }
            Console.ReadKey();
        }
    }
}

下麵是自定義的一個迭代器的例子：

Primer.CS

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Ch11Ex03_Exam
{
    public class Primes
    {
        private long min;
        private long max;
        public Primes():this(2,100)
        {
            
        }
        public Primes(long minNum,long maxNum)
        {
            if(minNum<2)
            {
                min=2;
            }else{
                min = minNum;
            }
            max = maxNum;
        }
        public IEnumerator GetEnumerator()//返回的是IEnumerator
        {
            for(long i=min;i<max;i++)
            {
                int flag = 1;
                for(long j=2;j<Math.Sqrt(min);j++)
                {
                    if(i%j==0)
                    {
                        flag = 0;
                        break;
                    }
                }
                if(flag==1)
                {
                    yield return i;
                }
            }
        }
    }
}

Program.CS:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Ch11Ex03_Exam
{
    class Program
    {
        static void Main(string[] args)
        {
            Primes s = new Primes(2, 100);
            foreach(long i in s)
            {
                Console.WriteLine("{0}\t", i);
            }
            Console.ReadKey();
        }
    }
}