《网瘾战争》

Resharper:”Access to modified closure”

using System;

namespace ConsoleApplication1
{
    class Program
    {
        delegate void MyDelegate();
        static void Main()
        {
            MyDelegate d = null;
            string s;
            for (var i = 0; i < 10; i++)
            {
               s = i.ToString();
                d += () => Console.WriteLine(s);
            }
            d();
        }
    }
}

输出是什么？如果把s的定义放到循环内部呢？

using System;

namespace ConsoleApplication1
{
    class Program
    {
        delegate void MyDelegate();
        static void Main()
        {
            MyDelegate d = null;
            //string s;
            for (var i = 0; i <= 10; i++)
            {
                string s;
                s = i.ToString();
                d += () => Console.WriteLine(s);
            }
            d();
        }
    }
}

 

第一种情况下，由于s是在循环外部定义的，所有的delegate都是共享的，所以，每循环一次，s增1，到最后，s变为10，最后执行d()的时候，用的都是同一个s，因此输出都是10。resharper 会提定义局部变量保存s的拷贝，如下，这样，每一次执行都会使用自己的copy，也就会输出0 1 2 3 ….10

using System;

namespace ConsoleApplication1
{
    class Program
    {
        delegate void MyDelegate();
        static void Main()
        {
            MyDelegate d = null;
            string s;
            for (var i = 0; i <= 10; i++)
            {
               s = i.ToString();
                string s1 = s;
                d += () => Console.WriteLine(s1);
            }
            d();
        }
    }
}

 

C#: Anonymous methods are not closures

The implementation of anonymous methods in C# and its consequences (part 1)

Resharper 提示”lambda expression to statement”

最近用上了resharper，实时对代码进行改进，碰到这么一个提示“lambda expression to statement”,那什么是lambda expression，什么是lambda statement？二者有什么区别？

Expression：
var exprBooks = books.Find(book => book.Author.Contains("Fowler"));

Statement:

var stmtBooks = books.Find(book => { return book.Author.Contains("Fowler"); });

A lambda statement contains braces and a function body, and can potentially have multiple lines like a standard delegate. A lambda expression is the single line with an implicit return

http://www.lostechies.com/blogs/jimmy_bogard/archive/2008/07/18/expressions-and-lambdas.aspx

.net links 2010/1/26

Use .NET Built-in Methods to Save Time and Headaches 

  TryParse() and File.WriteAllText(file, str) are really useful to me!

If you are using a loop, you're doing it wrong

if you can use Linq, do not use for and foreach

.Net Tip: Convert a String to Title Case

string helloWorld = "hello world haha ds s w";

Console.WriteLine(
    System.Threading.Thread.CurrentThread.CurrentCulture.TextInfo.ToTitleCase(helloWorld));

可以写个扩展方法

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

Assembly.LoadFile() Assembly.LoadFrom() Assembly.Load()

1、Assembly.LoadFile（）只载入相应的dll文件，比如Assembly.LoadFile(”a.dll”)，则载入a.dll，假如a.dll中引用了b.dll的话，b.dll并不会被载入。Assembly.LoadFrom则不一样，它会载入dll文件及其引用的其他dll，比如上面的例子，b.dll也会被载入。
2、用Assembly.LoadFrom（）载入一个Assembly时，会先检查前面是否已经载入过相同名字的Assembly，比如a.dll有两个版本(版本1在目录1下，版本2放在目录2下)，程序一开始时载入了版本1，当使用Assembly.LoadFrom(”2″”a.dll”)载入版本2时，不能载入，而是返回版本1。Assembly.LoadFile的话则不会做这样的检查，比如上面的例子换成Assembly.LoadFile的话，则能正确载入版本2。

3、Assembly.Load（）方法参数为程序集的名称不包含扩展名

.net 4.0 中只保留了LoadFrom() 方法

http://msdn.microsoft.com/en-us/library/ee191568(VS.100).aspx

http://blog.csdn.net/nanqingfei/archive/2009/10/12/4659004.aspx

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