C# 3.0 查詢運算式完整指南：LINQ 語法、翻譯機制、標準查詢運算子與模式實作

編輯：關於C語言

查詢運算式：語言整合查詢的宣告式語法

查詢運算式提供類 SQL 的宣告式語法，編譯器將其純語法轉譯為方法呼叫鏈，不綁定特定資料來源（記憶體集合、SQL、XML、自訂提供者）。

基本結構

from c in customers
where c.City == "London"
orderby c.Name
select new { c.Name, c.Phone }

from：定義範圍變數與資料來源
where：篩選條件
orderby：排序
select/group：投影或分組
into：查詢延續（子查詢）

翻譯機制：語法對應到標準查詢運算子

查詢子句	對應方法	說明
`where`	`Where`	Lambda 述詞篩選
`select`	`Select`	投影轉換
`group ... by`	`GroupBy`	分組，產生 `IGrouping<K,T>`
`orderby`	`OrderBy`/`ThenBy`	主要/次要排序
`orderby ... descending`	`OrderByDescending`/`ThenByDescending`	遞減排序
多重 `from`	`SelectMany`	扁平化/交叉聯結
`into`	巢狀查詢	前一查詢結果作為下一查詢來源

翻譯範例

Where + Select

from c in customers where c.City == "London" select c.Name
// 翻譯
customers.Where(c => c.City == "London").Select(c => c.Name)

GroupBy

from c in customers group c.Name by c.Country
// 翻譯
customers.GroupBy(c => c.Country, c => c.Name)

OrderBy + ThenBy

from c in customers orderby c.Country, c.Balance descending select c
// 翻譯
customers.OrderBy(c => c.Country).ThenByDescending(c => c.Balance)

SelectMany（多重 from）

from c in customers
from o in c.Orders
where o.Date.Year == 2005
select new { c.Name, o.Id }
// 翻譯
customers.SelectMany(c => c.Orders.Where(o => o.Date.Year == 2005)
    .Select(o => new { c.Name, o.Id }))

Into 查詢延續

from c in customers group c by c.Country into g
select new { Country = g.Key, Count = g.Count() }
// 翻譯
customers.GroupBy(c => c.Country)
    .Select(g => new { Country = g.Key, Count = g.Count() })

查詢運算式模式：自訂 LINQ 提供者的契約

型別實作下列方法即支援查詢運算式（可為實例方法或擴充方法）：

delegate R Func<A, R>(A arg);

class C<T> {
    public C<T> Where(Func<T, bool> predicate);
    public C<S> Select<S>(Func<T, S> selector);
    public C<S> SelectMany<S>(Func<T, C<S>> selector);
    public O<T> OrderBy<K>(Func<T, K> keySelector);
    public O<T> OrderByDescending<K>(Func<T, K> keySelector);
    public C<G<K, T>> GroupBy<K>(Func<T, K> keySelector);
    public C<G<K, E>> GroupBy<K, E>(Func<T, K> keySelector, Func<T, E> elementSelector);
}

class O<T> : C<T> {
    public O<T> ThenBy<K>(Func<T, K> keySelector);
    public O<T> ThenByDescending<K>(Func<T, K> keySelector);
}

class G<K, T> {
    public K Key { get; }
    public C<T> Group { get; }
}