想当年学C#的时候.Net还是2.0，学了就直接丢掉了，要不是到MS来实习，估计也未必会再捡起来了，结果.Net都4.0了。最近coding才发现C# 3.0多了好多新特性，而自己还把一些C# 2.0的特性彻底忘了，真不敢说自己会C#了。于是总结一下最近遇到并且常用到的特性吧。

1. ? – Nullable<T>

[SerializableAttribute]
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。

// Nullable<int> arg = -1;
int? arg = -1;
if (arg.HasValue) {
    // int value = (int)arg;
    int value = arg.Value;
}

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。

string result = gao();
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。

Func<double, double, double> hypot = (x, y) => Math.Sqrt(x * x + y * y);
Func<double, double, string> gao = (x, y) =>
    {
        double z = hypot(x, y);
        return String.Format("{0} ^ 2 + {1} ^ 2 = {2} ^ 2", x, y, z);
    };
Console.WriteLine(gao(3, 4));

4. {} – initializer

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

List<string> list = new List<string>{"watashi", "rejudge"};
Dictionary<string, string> dic = new Dictionary<string, string>
{
    {"watashi", "watashi wa watashi"},
    {"rejudge", "-rejudge -pia2dea4"}
};

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：

Sequence activity = new Sequence()
{
    DisplayName = "Root",
    Activities =
    {
        new If()
        {
            Condition = true,
            Else = new DoWhile()
            {
                Condition = false
            }
        },
        new WriteLine()
        {
            DisplayName = "Hello",
            Text = "Hello, World!"
        }
    }
};

Sequence activity2 = new Sequence();
activity2.DisplayName = "Root";

If if2 = new If();
if2.Condition = true;
DoWhile doWhile2 = new DoWhile();
doWhile2.Condition = false;
if2.Else = doWhile2;
activity2.Activities.Add(if2);

WriteLine writeLine2 = new WriteLine();
writeLine2.DisplayName = "Hello";
writeLine2.Text = "Hello, World!";
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

public string Caption { get; set; }

就可以代替原来的一大段代码。

private string caption;
public string Caption
{
    get
    {
        return caption;
    }
    set
    {
        caption = value;
    }
}

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。

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

This entry was posted on Sunday, January 24th, 2010 at 9:12 pm and is filed under summary. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.