yield是C#為了簡化遍歷操作實(shí)現(xiàn)的語法糖，我們知道如果要要某個(gè)類型支持遍歷就必須要實(shí)現(xiàn)系統(tǒng)接口IEnumerable，這個(gè)接口后續(xù)實(shí)現(xiàn)比較繁瑣要寫一大堆代碼才能支持真正的遍歷功能。舉例說明

using?System;
using?System.Collections.Generic;
using?System.Collections;
using?System.Linq;
using?System.Text;
namespace {
????class?Program
????{
????????static?void?Main(string[]?args)
????????{
????????????HelloCollection?helloCollection?=?new?HelloCollection();
????????????foreach?(string?s?in?helloCollection)
????????????{
????????????????Console.WriteLine(s);
????????????}
????????????Console.ReadKey();
????????}
????}
????//public?class?HelloCollection?:?IEnumerable
????//{
????//????public?IEnumerator?GetEnumerator()
????//????{
????//????????yield?return?"Hello";
????//????????yield?return?"World";
????//????}
????//}????public?class?HelloCollection?:?IEnumerable
????{
????????public?IEnumerator?GetEnumerator()
????????{
????????????Enumerator?enumerator?=?new?Enumerator(0);
????????????return?enumerator;
????????}
????????public?class?Enumerator?:?IEnumerator,?IDisposable
????????{
????????????private?int?state;
????????????private?object?current;
????????????public?Enumerator(int?state)
????????????{
????????????????this.state?=?state;
????????????}
????????????public?bool?MoveNext()
????????????{
????????????????switch?(state)
????????????????{
????????????????????case?0:
????????????????????????current?=?"Hello";
????????????????????????state?=?1;
????????????????????????return?true;
????????????????????case?1:
????????????????????????current?=?"World";
????????????????????????state?=?2;
????????????????????????return?true;
????????????????????case?2:
????????????????????????break;
????????????????}
????????????????return?false;
????????????}
????????????public?void?Reset()
????????????{
????????????????throw?new?NotSupportedException();
????????????}
????????????public?object?Current
????????????{
????????????????get?{?return?current;?}
????????????} ????????????public?void?Dispose()
????????????{
????????????}
????????}
????}
}

??? 上面注釋的部分引用了"yield return”，其功能相當(dāng)于下面所有代碼！可以看到如果不適用yield需要些很多代碼來支持遍歷操作。

??? yield return 表示在迭代中下一個(gè)迭代時(shí)返回的數(shù)據(jù)，除此之外還有yield break, 其表示跳出迭代，為了理解二者的區(qū)別我們看下面的例子

class?A?:?IEnumerable
{
? ??private int[] array = new int[10] {1, 1, 2, 3, 4, 5, 6, 7, 87, 10};
????public?IEnumerator?GetEnumerator()
????{
????????for?(int?i?=?0;?i?<?10;?i++)
????????{
????????????yield?return?array[i];
????????}
????}
}

??? 如果你只想讓用戶訪問ARRAY的前8個(gè)數(shù)據(jù),則可做如下修改.這時(shí)將會用到y(tǒng)ield break,修改函數(shù)如下

public?IEnumerator?GetEnumerator()
{
????for?(int?i?=?0;?i?<?10;?i++)
????{
????????if?(i?<?8)
????????{
????????????yield?return?array[i];
????????}
????????else
????????{
????????????yield?break;
????????}
????}
}

??? 這樣,則只會返回前8個(gè)數(shù)據(jù).

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