关于C#3.0和.net一些问题的理解

1.string 和 System.String

string和System.String 是完全相同的

写下面的代码，编译之后，使用ildasm.exe查看生成的IL代码，可以发现是完全一样的

class Program
{
static void Main(string[] args)
{
var pString = new PersonString("Xg", "Luxv");
System.Console.WriteLine(pString);

var pSystemString = new PersonSystemString("Xg", "Luxv");
System.Console.WriteLine(pSystemString);

}
}

internal class PersonString
{
public string FirstName { get; private set; }
public string LastName { get; private set; }

public PersonString(string FirstName, string LastName)
{
this.FirstName = FirstName;
this.LastName = LastName;
}
public void Replace(string TextToFind, string ReplacingText)
{
FirstName = FirstName.Replace(TextToFind, ReplacingText);
LastName = LastName.Replace(TextToFind, ReplacingText);
}
public override string ToString()
{
return FirstName + " " + LastName;
}
}
internal class PersonSystemString
{
public System.String FirstName { get; private set; }
public System.String LastName { get; private set; }

public PersonSystemString(System.String FirstName,
System.String LastName)
{
this.FirstName = FirstName;
this.LastName = LastName;
}
public void Replace(System.String TextToFind,
System.String ReplacingText)
{
FirstName = FirstName.Replace(TextToFind, ReplacingText);
LastName = LastName.Replace(TextToFind, ReplacingText);
}
public override System.String ToString()
{
return FirstName + " " + LastName;
}
}

2.匿名类型 Var

匿名类型在第一次赋值时已经确定了类型，所以下面的代码是通不过编译的，不是可变类型，呵呵

class Program
{
static void Main()
{
var v = "Hello World!";
v = 10;
System.Console.WriteLine(v);
}
}

3.引用类型 和 值类型

System.String虽然是引用类型（Reference Types），但他显示出来的特性很像值类型，而System.Array就不一样了，是标准的引用类型的表现

执行下面的代码：

static void Main()
{
var stringValue = "Hello World!";
var stringValue2 = stringValue;
stringValue = "Hello Austria!";
System.Console.WriteLine(stringValue2);

var array = new[] { 1, 2, 3, 4 };
var array2 = array;
array[0] = 99;
System.Console.WriteLine(array2[0]);
}

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果应该是

Hello Wrold!

99

4.Finalizing Objects

像c++一样，c#的类在解构函数中释放资源是很好的方法，但这还不是足够的。看下面的代码：

internal class FileGenerator : IDisposable
{
public FileGenerator()
{
}
~FileGenerator()
{
// Just a debug output
Console.WriteLine("Closing file!");
}
public void Generate(int Length)
{
// Here some work is done...
}
public void Dispose()
{
// Just a debug output
Console.WriteLine("Disposing object!");
}
}

class Program
{
static void Generate()
{
using ( var fGen = new FileGenerator() )
fGen.Generate(512);
}
static void Main(string[] args)
{
Generate();
// Here we do some work; simulated by ReadLine statement
Console.Write("Please Press Enter...");
Console.ReadLine();
}
}

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

我们可以发现，"Closing file!",每次都要等到程序退出时才执行，解构函数是有.net框架的垃圾收集器调用的，什么时候调用，我们是不能控制的，所以如果需要手工控制资源释放的时候，我们需要从接口IDisposable继承一个类，然后重写Dispose（），在代码中使用using语句，这样程序将立即自动调用Dispose()的执行。

5.Nullable 类型

看下面的代码

static void Main()
{
int a = 10;
int? b = 20;
int? c = null;
System.Console.WriteLine(a + c ?? b);
System.Console.Read();
}

上面的结果是20

在c#中可以给所以的值定义Nullable类型，就是在类型前加上一个问号。两个问号??操作符 是和?配套使用的，当Nullable的值为null时，就会默认取??后面的值，

代码中c为null，所以a+c也为null,这样就会取b值，就是20

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }