Linq 大觀園：統一查詢體驗的三大支柱與實戰對比

Linq 家族包含三大支柱，提供「殊途同歸」的查詢體驗：

• Linq to Objects：記憶體集合查詢 (System.Linq.Enumerable)
• Linq to XML：XML 文件查詢 (System.Xml.Linq.dll)
• Linq to SQL：關聯式資料庫查詢 (System.Data.Linq.dll，原生支援 SQL Server；DbLinq 擴充支援 MySQL/Oracle/PostgreSQL/SQLite)

查詢語法 vs 方法語法：編譯器視角無差別

// 查詢語法 (語法糖)
var result = from book in books
             where book.Title.StartsWith("I")
             orderby book.Price
             select new { Key = book.Title, Value = book.Price };

// 編譯後等同方法語法
var result = books.Where(b => b.Title.StartsWith("I"))
                  .OrderBy(b => b.Price)
                  .Select(b => new { Key = b.Title, Value = b.Price });

Reflector 反編譯確認兩者生成完全相同的 IL 代碼。

實戰對比：同一查詢邏輯，三種資料源零改動

Linq to Objects (記憶體集合)

IList<Book> books = new List<Book> {
    new Book { Title = "Inside COM", Price = 20 },
    new Book { Title = "Inside C#", Price = 100 },
    new Book { Title = "Linq", Price = 120 }
};

var result = from book in books
             where book.Price > 50
             orderby book.Price
             select new { Key = book.Title, Value = book.Price };

Linq to SQL (SQL Server)

[Table]
public class Book
{
    [Column] public string Title { get; set; }
    [Column(DbType = "numeric(5,2)")] public float Price { get; set; }
    [Column] public string Author { get; set; }
    [Column] public string ISBN { get; set; }
}

// 查詢碼完全相同，僅資料源不同
DataContext db = new DataContext(connString);
var result = from book in db.GetTable<Book>()
             where book.Price > 50
             orderby book.Price
             select new { Key = book.Title, Value = book.Price };

輸出結果完全一致：Inside C# 100、Linq 120。資料儲存細節對查詢邏輯完全透明，正是 Linq「語言整合查詢」的核心價值。