C# 3.0 特性探秘 (五)：物件與集合初始化器語法糖與編譯器生成代碼

物件初始化器

// C# 3.0 前
Book b = new Book(); b.Title = "Inside C#"; b.ISBN = "123"; b.Price = 100;

// C# 3.0
Book b = new Book { Title = "Inside C#", ISBN = "123", Price = 100 };

集合初始化器

// 舊寫法
var books = new List();
books.Add(new Book { Title = "A", ISBN = "1", Price = 10 });
books.Add(new Book { Title = "B", ISBN = "2", Price = 20 });

// 新寫法
var books = new List {
    new Book { Title = "A", ISBN = "1", Price = 10 },
    new Book { Title = "B", ISBN = "2", Price = 20 }
};

編譯器生成代碼 (Reflector 反編譯)

List <>g__initLocal0 = new List();
Book <>g__initLocal1 = new Book();
<>g__initLocal1.Title = "A"; <>g__initLocal1.ISBN = "1"; <>g__initLocal1.Price = 10;
<>g__initLocal0.Add(<>g__initLocal1);
Book <>g__initLocal2 = new Book();
<>g__initLocal2.Title = "B"; <>g__initLocal2.ISBN = "2"; <>g__initLocal2.Price = 20;
<>g__initLocal0.Add(<>g__initLocal2);
// ...

意義

• 純語法糖，編譯器自動展開為建構子呼叫、屬性設定、Add 方法。
• 減少樣板代碼，提升可讀性，特別適合匿名類型初始化：var x = new { Key="k", Value="v" };。
• 編譯器優化方向從「機器效率」轉向「開發者效率」。

無此語法，匿名類型將難以初始化，影響 LINQ 投影運算。