C#2.0实现的Nullable数据类型，虽然说只是一个小小的cookie，但不得不说是C#矢志不渝的延续了它人性化的特点，我们终于不用再使用object来存放简单数据来通过==null测试。从表面上看这个功能或许并无太大的创新意义，但不知你是否也和我一样在记忆里埋有对类似int a=null;通不过编译时的抱怨？ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    关于Nullable的详细介绍可以参考C#2.0的新特新和很多的blog文章，这不是我主要想说的内容。只是2.0为了让Nullable类型和non-Nullable数据之间转换，提供了一个新的操作符"??"比较有意思。这个操作符的作用很简单，用法如下：ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
int? a =ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
1;ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
int? b =ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
null;ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
int c = a; // compile error ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
int c = a ??ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
100; // rightƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
int d = a + b; // compile error yetƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
int d = a + b ??ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
-1; // rightƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    看到这个"??"的使用，你第一时间能想到什么呢？我第一时间就想到了三元操作运算 ? :！ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    在代码中书写一定的三元运算表达式，很多时候能给我们的代码带来简洁性和紧凑感。不过任何东西都会美中不足，这个经典的三元操作必须有两个分支（嗯，如果一个分支就不是三元了），所以我有时不得不为了不使用if语句，而写下一些自感丑陋蹩脚代码：ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    1.ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
string param = Request.Params["param"];ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
if ( param ==ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
null )ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
{ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    param = defaultValue;ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
}ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    或ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
string param = Request.Params["param"] ==ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
nullƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
? defaultValue : Request.Params["param"];ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    我是比较反感把类似Request.Params["key"]、ViewState["key"]以及Hasttable["key"]这类的相同代码写超过一遍的，因为作为key的literal string不能被编译器检查，出现拼写错误后是非常让人抓狂的。ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    2.ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
publicƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
string GetValueƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
{ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    getƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    {ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
        if ( this.value ==ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
null )ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
        {ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
            returnƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
string.Empty;ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
        }ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
        elseƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
        {ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
            returnƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
this.value;ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
        }ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    }ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
}ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    或ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
publicƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
string GetValueƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
{ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    getƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    {ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
        returnƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
this.value ==ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
nullƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
?ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
string.Empty : this.value;ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    }ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
}ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    使用?:后貌似不错了，但似乎还不是我们希望的终极无间...ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
    在C#2.0中，借助"??"运算符，这类代码将变得非常sexy： 1.  stringƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
paramsƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
= Reqeust.Params["param"] ?? defaultValue;ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
2.  publicƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
string GetValue ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
{ getƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
{ returnƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
this.value ??ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’
string.Empty; } }ƒ['²7 ‘å<www.netcsharp.cn]i[¨HPwêC’