LINQ Cheat Sheet

September 11th, 2016

LINQ 語言整合查詢（Language Integrated Query），提供標準且容易學習的資料查詢與更新方法，並且從 .NET Framework 3.5 開始就可以在 C# 以及 VB.NET 中使用此查詢語法。寫法分兩種方式：

宣告式(Declarative) -> LINQ
編程式(Imerative) -> Lambda 表示式

LINQ 架構圖

LINQ 如何改善我們對資料存取的開發效率，以及在各種使用情境下如何有效撰寫 LINQ 語法，請參考下列情境。

Filtering 過濾

將取回來的資料在程式中做進一步的資料篩選，在同一批資料中做不同條件的篩選資料時，不用再做一次資料庫連線及重新抓資料，降低資料庫負擔。

// Query Syntax
var col1 = from o in Orders
           where o.CustomerID == 23
           select o;

// Lambda Syntax
var col2 = Orders.Where(o => o.CustomerID == 23);

Return Anonymous Type 回傳匿名型別

匿名型別，讓開發人員建立物件執行個體，不需要先撰寫型別的類別定義，簡單來說就是沒有名稱的 class，作為暫時的物件，方便後續使用。

如果要使用具名型別，在 new 後面加上你要的資料型別即可。

// Query Syntax
var col1 = from o in Orders
           select new
           {
               OrderID = o.OrderID,
               Cost = o.Cost
           }

// Lambda Syntax
var col2 = Orders.Select(o => new
{
    OrderID = o.OrderID,
    Cost = o.Cost
});

Ording 排序

// Query Syntax
var col1 = from o in Orders
           orderby o.Cost ascending
           select o;

// Lambda Syntax
var col2 = Orders.OrderBy(o => o.Cost);

// Query Syntax
var col1 = from o in Orders
           orderby o.Cost descending
           select o;

// Lambda Syntax
var col2 = Orders.OrderByDescending(o => o.Cost);

// Query Syntax
var col1 = from o in Orders
           orderby o.CustomerID, o.Cost descending
           select o;
var col2 = from o in Orders
           orderby o.Cost descending
           orderby o.CustomerID
           select o;

// Lambda Syntax
var col3 = Orders.OrderBy(o => o.CustomerID)
     .ThenByDescending(o => o.Cost);

Joining 合併

LINQ 的 Join 很容易讓人看不懂，建議 Join 的動作在 SQL server 中完成，程式會有比較好的維護性。

// Query Syntax
var col1 = from c in Customers
           join o in Orders on
           c.CustomerID equals o.CustomerID
           select new
           {
               c.CustomerID,
               c.Name,
               o.OrderID,
               o.Cost
           };

// Lambda Syntax
var col2 = Customers.Join(Orders,
           c => c.CustomerID, o => CustomerID,
           (c, o) => new
           {
               c.CustomerID,
               c.Name,
               o.OrderID,
               o.Cost
           });

Grouping 群組

使用 Grouping 時，其 Key 的型別會和 Value 一樣，例如 o.CustomerID 型別是 int 則其 Key 也是 int型別。

// Query Syntax
var OrderCounts1 = from o in Orders
                   group o by o.CustomerID into g
                   select new
                   {
                       CustomerID = g.Key,
                       TotalOrders = g.Count()
                   };

// Lambda Syntax
var OrderCounts2 = Orders.GroupBy(o => o.CustomerID)
                   .Select(g => new
                   {
                       CustomerID = g.Key,
                       TotalOrders = g.Count()
                   });

Paging 分頁

分頁基本上就是 Skip 和 Take 的應用。

// select top 3
// Query Syntax
var col1 = (from o in Orders
            where o.CustomerID == 23
            select o).Take(3);

// Lambda Syntax
var col2 = Orders.Where(o => o.CustomerID == 23)
                 .Take(3);

// skip first 2 and return the 2 after
// Query Syntax
var col1 = (from o in Orders
            where o.CustomerID == 23
            orderby o.Cost
            select o).Skip(2).Take(2);

// Lambda Syntax
var col2 = Orders.Where(o => o.CustomerID == 23)
                 .Skip(2).Take(2);

Element Operators

使用 Single、Last、First、ElementAt 時，如果遇到資料來源是空的，會拋例外。

使用 SingleOrDefault、LastOrDefault、FirstOrDefault、ElementAtOrDefault 時，如果遇到資料來源是空的，會回傳 default(T)。會有以下兩種情境：

若 T 是參考型別或是 nullable 資料型別，會回傳 NULL。
若 T 是非 nullable 資料型別（如：int、bool）則會回傳 default(T)。

// throws exception if no elements
// Query Syntax
var cust1 = (from c in Customers
            where c.CustomerID == 23
            select c).Single();

// Lambda Syntax
var cust2 = Customer.Single(c => c.CustomerID == 23);

// returns null if no elements
// Query Syntax
var cust1 = (from c in Customers
            where c.CustomerID == 23
            select c).SingleOrDefault();

// Lambda Syntax
var cust2 = Customer.SingleOrDefault(c => c.CustomerID == 23);

// returns a new customer instance if no elements
// Query Syntax
var cust1 = (from c in Customers
            where c.CustomerID == 21
            select c)
            .DefaultIfEmpty(new Customer()).Single();

// Lambda Syntax
var cust2 = Customer.Where(c => c.CustomerID == 21)
                    .DefaultIfEmpty(new Customer()).Single();

// First, Last and ElementAt used in same way
// Query Syntax
var cust1 = (from o in Orders
            where o.CustomerID == 23
            orderby o.Cost
            select o).Last();

// Lambda Syntax
var cust2 = Orders.Where(o => o.CustomerID == 23)
                  .OrderBy(o => o.Cost).Last();

// returns 0 if no elements
// Query Syntax
var i = (from c in Customers
        where c.CustomerID == 23
        select c.CustomerID).SingleOrDefault();

// Lambda Syntax
var cust2 = Customer.Where(c => c.CustomerID == 23)
                    .Select(c => c.CustomerID).SingleOrDefault();

Conversions 轉換

LINQ 提供將查詢結果轉成各種 IEnumerable 資料型別，方便後續做資料處理

Array
Dictionary
List
ILookup

// To Array
string[] names = (from c in Customers
                  select c.Name).ToArray();

// To Dictionary
Dictionary<int, Customer> col = Customers.ToDictionary(c => c.CustomerID);
Dictionary<string, double> customerOrdersWithMaxCost = (from oc in 
    (from o in Orders
    join c in Customers on o.CustomerID equals c.CustomerID
    select new { c.Name, o.Cost})

    group oc by oc.Name into g
    select g).ToDictionary(g => g.Key, g => g.Max(oc => oc.Cost));

// ToList
List<Order> ordersOver10 = (from o in Orders
    where o.Cost > 10
    orderby o.Cost).ToList();

//To Lookup
ILookup<int, string> customerLookup =
    customers.ToLookup(c => c.CustomerID, c => c.Name);

參考資料：

Poy Chang

Trial and Error

Recent Posts