C#: ? ?? => {}

1. ? – Nullable<T>

 
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[SerializableAttribute]

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public struct Nullable<T> where T : struct, new()

C#里像int, bool, double这样的struct和enum类型都不能为null。如果确实想在值域上加上null的话，Nullable就派上用场了。T?是Nullable&ly;T>的语法糖。要将T?转为T可以通过类型转换，或者通过T?的Value属性，当然后者要高雅些。

 
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// Nullable<int> arg = -1;

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int? arg = -1;

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if (arg.HasValue) {

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    // int value = (int)arg;

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    int value = arg.Value;

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}

2. ?? – operator ??

o ?? v可以看作是o == null ? v : o的语法糖。??运算符在左操作数非null时返回左操作数，否则返回右操作数。

 
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string result = gao();

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Console.WriteLine(result ?? "<NULL>");

3. => – lambda expression

看别人代码的过程中才发现原来C#也有lambda了，也才发现自己真的out了。当然，感觉C#里的lambda并没有带来什么革命性的变化，更像是一个语法糖。毕竟这不是Scale，MS也有F#了。

 
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Func<double, double, double> hypot = (x, y) => Math.Sqrt(x * x + y * y);

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Func<double, double, string> gao = (x, y) =>

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    {

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        double z = hypot(x, y);

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        return String.Format("{0} ^ 2 + {1} ^ 2 = {2} ^ 2", x, y, z);

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    };

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Console.WriteLine(gao(3, 4));

4. {} – initializer

collection initializer使得初始化一个List, Dictionary变得简单。

 
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List<string> list = new List<string>{"watashi", "rejudge"};

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Dictionary<string, string> dic = new Dictionary<string, string>

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{

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    {"watashi", "watashi wa watashi"},

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    {"rejudge", "-rejudge -pia2dea4"}

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};

而object initializer其实就是调用完成构造后执行属性操作的语法糖，它使得代码更加简洁，段落有致。试比较：

 
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Sequence activity = new Sequence()

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{

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    DisplayName = "Root",

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    Activities =

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    {

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        new If()

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        {

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            Condition = true,

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            Else = new DoWhile()

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            {

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                Condition = false

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            }

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        },

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        new WriteLine()

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        {

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            DisplayName = "Hello",

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            Text = "Hello, World!"

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        }

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    }

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};

 
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Sequence activity2 = new Sequence();

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activity2.DisplayName = "Root";

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If if2 = new If();

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if2.Condition = true;

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DoWhile doWhile2 = new DoWhile();

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doWhile2.Condition = false;

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if2.Else = doWhile2;

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activity2.Activities.Add(if2);

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WriteLine writeLine2 = new WriteLine();

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writeLine2.DisplayName = "Hello";

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writeLine2.Text = "Hello, World!";

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activity2.Activities.Add(writeLine2);

5. { get; set; } – auto-implemented properties

又是一个方便的语法糖，只要简单的一句

 
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public string Caption { get; set; }

就可以代替原来的一大段代码。

 
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private string caption;

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public string Caption

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{

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    get

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    {

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        return caption;

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    }

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    set

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    {

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        caption = value;

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    }

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}

6. var – implicit type

var并不是代表任意类型，毕竟C#是个强类型的语言。var只是个在声明变量时代替实际的类型名的语法糖，只能使用在编译器能根据上下文推出其实际类型的地方。这在类型名称藏在好几层namespace或class里的时候，还有在foreach语句中非常有用。

 
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foreach (var a in dict)

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{

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    Console.WriteLine("{0} => {1}", a.Key, a.Value);

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}