C#对象初始化器语法小深入探讨
C#3.0提供了一个新的语法，对象初始化器(Object Initializers)。 该语法是为了简化一个常见的编程模式：构造一个新对象，逐个设置对象的公共属性。
初探
实例如下：
class Program
{
static void Main(string[] args)
{ //对象初始化器
Person p = new Person() {
FirstName = "Hello",
LastName = "World"
};
Console.WriteLine(p);
Console.ReadKey();
}
}
class Person
{
public string FirstName { get;set; }
public string LastName { get;set; }
public override string ToString()
{
return string.Format("{0} {1}", FirstName, LastName);
}
}
从一开始我就知道这个语法等价于下面的语句：
Person p = new Person();
p.FirstName = "Hello";
p.LastName = "World";
...
我自然也就认为编译器也会用这样的代码进行替换。偶然一次起了好奇心， 想看看编译器是不是真的这么做的，于是就查看生成的IL，结果真发现了点东西。IL代码我只能看个大概， 下面是在Reflector后的结果(选项设置为C#, .net2.0)：
Person <>g__initLocal0 = new Person();
<>g__initLocal0.FirstName = "Hello";
<>g__initLocal0.LastName = "World";
Person p = <>g__initLocal0;
Console.WriteLine(p);
Console.ReadKey();
多了个局部变量了<>g__initLocal0， 构建的对象完成所有属性设置后再赋值给p。虽然我不理解为什么要加一个"临时变量"， 但是这不会影响代码的最终结果。
再探
原本到此就该结束了，但是在好奇心的驱使下又做了一个新的测试。 我把Person的定义改成了struct，这样他就是个值类型了。对象初始化器的语法对值类型依然有效，Reflector的结果没有变化， 但是这个赋值语句的意义就不一样了:
- 如果Person是引用类型，那么此处仅仅是引用的复制。
- 如果Person是值类型，那么这里就是完整的复制了<>g__initLocal0。
- 引用的复制应该比值拷贝要快，如果这个值类型有n多个字段的话，这个差别就很大。
- 值类型的局部变量整个对象在线程栈上，占用更多线程栈的内存，包含的字段越多占用就越多。
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