IEnumerable, IEnumerator, GetEnumerator()

如果想要自定义类型支持foreach，那么此类型必须实现IEnumerable 接口，然后在GetEnumerator()方法中返回实现IEnumerator接口的对象，实现Current，MoveNext，Reset方法。好消息是2.0中加入了yield关键字，一句yield return，编译器就会给你做剩下的事情

using System;
using System.Collections;

namespace ConsoleApplication2
{
public class Person
{
public Person(string fName, string lName)
{
FirstName = fName;
LastName = lName;
}

public string FirstName;
public string LastName;
}

public class People : IEnumerable
{
private Person[] _people;
public People(Person[] pArray)
{
_people = new Person[pArray.Length];

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

IEnumerator IEnumerable.GetEnumerator()
{
//return new PeopleEnum(_people);
// and you can delete below PeopleEnum class
for (int i = 0; i < _people.Length; i++)
{
yield return _people[i];
}
}
}

public class PeopleEnum : IEnumerator
{
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;
}

public object Current
{
get
{
try
{
return _people[position];
}
catch (IndexOutOfRangeException)
{
throw new InvalidOperationException();
}
}
}
}

class App
{
static void Main()
{
Person[] peopleArray = new Person[3]
{
new Person("Tom", "Cat"),
new Person("Jon", "Walker"),
new Person("Jet", "Li"),
};

People peopleList = new People(peopleArray);
foreach (Person p in peopleList)
Console.WriteLine(p.FirstName + " " + p.LastName);

}
}

或者是返回IEnumerable的方法

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication2
{
class Program
{
static void Main(string[] args)
{
foreach (var p in Primes(100))
{
Console.WriteLine(p);
}
}

public static IEnumerable<int> Primes(int max)
{
yield return 2;
var found = new List<int> {3};
var candidate = 3;
while (candidate <= max)
{
var isPrime = found.TakeWhile(
prime => prime*prime <= candidate).All(prime => candidate%prime != 0);
if (isPrime)
{
found.Add(candidate);
yield return candidate;
}
candidate += 2;
}
}
}
}

Implementing iterators with yield statements

Behind the scenes of the C# yield keyword

typeof() GetType() is

Suppose there are three Classes(type) Aninaml, Dog and Tree

and 3 instances animal dog and tree

if(dog is Animal) // true

dog.GetType() == typeof(Dog) // true

call to GetType gets resolved at runtime, while typeof is resolved at compile time.

throw ex 和 throw 一样么？

不一样，

throw 是输出整个stacktrack，考虑多层设计中，UI层有try catch, 逻辑层有 try catch， 数据层有try catch， 三者均使用 throw， 假设是在数据层出错，输出的exception 信息会显示这三层的throw point，由此可以知道 exception的源头在数据层；如果是在逻辑层出错，会输出两层的 throw point， 由此可知 exception 的源头在逻辑层

但是用 throw ex的话，会清空stacktrace，如上例，你只能看到UI层的exception，无法获知到底是发生在UI层还是逻辑层还是数据层

因此，总是使用throw 抛出原始 exception

JIT and NGEN

JIT, Just in time,即时编译，每次运行都会进行，会对代码进行优化

NGEN Native image generator，在运行前一次性的把代码转换为本地代码并存储为本地文件，会改善程序的启动性能，但对总体的性能，有可能提升，也有可能降低

一个有用的C# Attribute: DebuggerDisplay

namespace ConsoleApplication2

{

[DebuggerDisplay("count ={Count}")]

public class Test

{

public int Count{ get; set;}

public string Name { get; set; }

public void Clear()

{

}

}

}

如上代码示例所示，该属性可以让对象在鼠标悬停时按格式显示Count字段的值，而不必点开对象查找，当对象有非常多的Property时，这个属性尤其有用