Linq select 语法

文档：https://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b

1.可以对查询出来的结果做一些转换，下面的例子在数组中查找以"B"开头的名字，然后全部转成小写输出

?

1 2 3 4 5 6 7 8

string[] names = { "Jack", "Bob", "Bill", "Catty", "Willam" };             var rs = from n in names                      where n.StartsWith("B")                      select n.ToLower();             foreach (var item in rs)             {                 Response.Write(item+"<br />");             }

2.返回匿名类型，只返回需要的信息

?

1 2 3 4 5 6 7

var users = from c in UserList                         where c.Mobile.StartsWith("135")                         select new                                {                                    Name = c.Name,                                    Contact = c.Address + " " + c.Mobile                                };

3.使用原数组，产生两组新数组

?

1 2 3 4 5 6 7 8 9 10 11 12 13

public void Linq9() {     string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };         var upperLowerWords =         from w in words         select new { Upper = w.ToUpper(), Lower = w.ToLower() };         foreach (var ul in upperLowerWords)     {         Console.WriteLine("Uppercase: {0}, Lowercase: {1}", ul.Upper, ul.Lower);     } }

4.判断数字是否与他的下标一致

?

1 2 3 4 5 6 7 8 9

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };               var numsInPlace = numbers.Select((num, index) => new { Num = num, InPlace = (num == index) });               Console.WriteLine("Number: In-place?");             foreach (var n in numsInPlace)             {                 Response.Write(string.Format("{0}: {1}<br />", n.Num, n.InPlace));             }

5.获得A小于B的所有可能的集合，返回 拿A中的每个元素和B中的每个元素对比的结果

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14

int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };     int[] numbersB = { 1, 3, 5, 7, 8 };         var pairs =         from a in numbersA         from b in numbersB         where a < b         select new { a, b };         Console.WriteLine("Pairs where a < b:");     foreach (var pair in pairs)     {         Console.WriteLine("{0} is less than {1}", pair.a, pair.b);     }

6.从一个集合的子集中过滤，金额小于500的订单

?

1 2 3 4 5 6 7

List<Customer> customers = GetCustomerList();               var orders =                 from c in customers                 from o in c.Orders                 where o.Total < 500.00                 select new { c.CustomerID, o.OrderID, o.Total };

7.查询所有大于指定日期的用户订单

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

List<Customer> customers = GetCustomerList();     var orders =         from c in customers         from o in c.Orders         where o.OrderDate >= new DateTime(1998, 1, 1)         select new { c.CustomerID, o.OrderID, o.OrderDate };延伸：List<Customer> customers = GetCustomerList();          DateTime cutoffDate = new DateTime(1997, 1, 1);          var orders =          from c in customers          where c.Region == "WA"          from o in c.Orders          where o.OrderDate >= cutoffDate          select new { c.CustomerID, o.OrderID };

8.代替二重for循环

参考：http://www.cnblogs.com/manupstairs/archive/2012/11/27/2790114.html

http://www.cnblogs.com/lyout/archive/2012/11/28/2792622.html

?

1 2 3 4 5 6

List<Customer> customers = GetCustomerList();         var customerOrders =         customers.SelectMany(             (cust, custIndex) =>             cust.Orders.Select(o => "Customer #" + (custIndex + 1) + " has an order with OrderID " + o.OrderID));

9.Cross join 交叉连接查询

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

public void Linq102() {         string[] categories = new string[]{          "Beverages",           "Condiments",           "Vegetables",           "Dairy Products",           "Seafood" };          List<Product> products = GetProductList();         var q =         from c in categories         join p in products on c equals p.Category         select new { Category = c, p.ProductName };        foreach (var v in q)     {         Console.WriteLine(v.ProductName + ": " + v.Category);      } }

?

1 2 3 4 5 6 7 8 9

Result： Chai: Beverages Chang: Beverages Guaraná Fantástica: Beverages Sasquatch Ale: Beverages Steeleye Stout: Beverages Côte de Blaye: Beverages Chartreuse verte: Beverages Ipoh Coffee: Beverages

10. Group join 分组查询；将数据按照不同的类型划分，归入到不同的组中。

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

public void Linq103() {     string[] categories = new string[]{          "Beverages",          "Condiments",          "Vegetables",          "Dairy Products",          "Seafood" };         List<Product> products = GetProductList();         var q =         from c in categories         join p in products on c equals p.Category into ps         select new { Category = c, Products = ps };         foreach (var v in q)     {         Console.WriteLine("组"+v.Category + ":");         foreach (var p in v.Products)         {             Console.WriteLine("   " + p.ProductName);         }     } }

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Result：   组Beverages: Chai Chang   组Condiments: Aniseed Syrup Chef Anton's Cajun Seasoning Chef Anton's Gumbo Mix   组Vegetables: Dairy Products: Queso Cabrales

11.Left Outer Join 左外连接查询，如果连接表没有数据，这使用空值/默认值填充

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

public void Linq105()  {     string[] categories = new string[]{           "Beverages",          "Condiments",           "Vegetables",           "Dairy Products",          "Seafood" };         List<Product> products = GetProductList();                 var q =         from c in categories         join p in products on c equals p.Category into ps         from p in ps.DefaultIfEmpty()         select new { Category = c, ProductName = p == null ? "(No products)" : p.ProductName };         foreach (var v in q)     {         Console.WriteLine(v.ProductName + ": " + v.Category);     } }

?

1 2 3 4 5 6

Result： Chai: Beverages Aniseed Syrup: Condiments (No products): Vegetables Queso Cabrales: Dairy Products Konbu: Seafood