yield是C#為了簡化遍歷操作實現的語法糖，我們知道如果要要某個類型支持遍歷就必須要實現系統接口IEnumerable，這個接口后續實現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迭代時返回的數據，除此之外還有yield break, 其表示跳出迭代，為了理解二者的區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個數據,則可做如下修改.這時將會用到yield break,修改函數如下

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個數據.

總結

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