[原创]如何改善Managed Code的Performance和Scalability系列之一：由for V.S. for each想到的...

一直想写一系列如何提高Performance和Scalability的文章，把我的相关经验和所知道的相关的技巧同大家分享。前一阵在园子里有一篇讨论for each 和 for两种循环那个具有更好的performance的blog，议论得沸沸扬扬。我觉得这是一个很好的切入点，我就已此作为引子，开始我的这个系列的文章。这篇文章的重点不是在于比较这两种循环孰优孰劣，我将讨论的重点是如何更好地定义Collection，如何在判断在什么时候该用Array，什么时候用Collection。

一、for each的本质

我们知道，所有实现了System.Collections. IEnumerable接口的类，我们都可以对它运用for each loop。为了弄清楚它的执行过程，我先给出一个Sample：

using System;
using System.Collections.Generic;
using System.Text;
using System.Collections;

namespace Artech.CustomCollection
{
structEmployee
{
PrivateFields#regionPrivateFields
privatestring_employeeID;
privatestring_name;
privateint_age;
#endregion

Constructor#regionConstructor
publicEmployee(stringemployeeID,stringname,intage)
{
this._employeeID=employeeID;
this._name=name;
this._age=age;
}
#endregion

PublicProperties#regionPublicProperties

publicstringEmployeeID
{
get{return_employeeID;}
set{_employeeID=value;}
}

publicstringName
{
get{return_name;}
set{_name=value;}
}

publicintAge
{
get{return_age;}
set{_age=value;}
}

#endregion

Tostring#regionTostring
publicoverridestringToString()
{
returnstring.Format("\tID:\t{0}\n\tName:\t{1}\n\tAge:\t{2}\n\t",this._employeeID,this._name,this._age);
}
#endregion
}

classEmployeeList:IEnumerable
{
privateEmployee[]_employees;

publicEmployee[]Employees
{
get{return_employees;}
set{_employees=value;}
}

IEnumerableMembers#regionIEnumerableMembers

publicvirtualIEnumeratorGetEnumerator()
{
Console.WriteLine("EmployeeList.GetEnumerator()isinvoked");
returnnewGenericEmployeeEnumerator(this._employees);
}

#endregion
}

classEmployeeEnumerator:IEnumerator
{
privateEmployee[]_employees;
privateint_index=-1;

publicEmployeeEnumerator(Employee[]employees)
{
this._employees=employees;
}

IEnumeratorMembers#regionIEnumeratorMembers

publicobjectCurrent
{
get
{
Console.WriteLine("EmployeeEnumerator.Currentisinvoked");
returnthis._employees[this._index];
}
}

publicboolMoveNext()
{
Console.WriteLine("EmployeeEnumerator.MoveNextisinvoked");

if(this._index<this._employees.Length-1)
{
this._index++;
returntrue;
}
returnfalse;
}

publicvoidReset()
{
this._index=-1;
}
#endregion
}
classProgram
{
staticvoidMain(string[]args)
{
Employee[]employees=newEmployee[]{newEmployee("0001","ZhangSan",21),newEmployee("0002","LiSi",30)};
EmployeeListempoyeeList=newEmployeeList();
empoyeeList.Employees=employees;

Console.WriteLine("Beginforeachloop");
foreach(EmployeeemployeeinempoyeeList)
{
Console.WriteLine(employee.ToString());
}
Console.WriteLine("\n\nBeginwhileloop");
IEnumeratorenumerator=empoyeeList.GetEnumerator();
while(enumerator.MoveNext())
{
Employeeemployee=(Employee)enumerator.Current;
Console.WriteLine(employee.ToString());
}
}
}
}

我们先运行一下上面的程序再来讲述具体的执行的流程。

在上面的Sample中我们先定义了一个Employee的struct，之所以使用struct而不用一般的class，我将在后面的部分介绍。

struct Employee
{
PrivateFields#regionPrivateFields
privatestring_employeeID;
privatestring_name;
privateint_age;
#endregion

Constructor#regionConstructor
publicEmployee(stringemployeeID,stringname,intage)
{
this._employeeID=employeeID;
this._name=name;
this._age=age;
}
#endregion

PublicProperties#regionPublicProperties

publicstringEmployeeID
{
get{return_employeeID;}
set{_employeeID=value;}
}

publicstringName
{
get{return_name;}
set{_name=value;}
}

publicintAge
{
get{return_age;}
set{_age=value;}
}

#endregion

Tostring#regionTostring
publicoverridestringToString()
{
returnstring.Format("\tID:\t{0}\n\tName:\t{1}\n\tAge:\t{2}\n\t",this._employeeID,this._name,this._age);
}
#endregion
}

然后我们基于这个Emplyee struct，定义了与之对应的Collection：EmplyeeList。EmplyeeList实现了System.Collections. IEnumerable接口。并以一个virtual 方法的形式实现了该接口的GetEnumerator方法。（为什么用virtual方法的原因，我会再后续部分解释）。

Public virtual IEnumeratorGetEnumerator()