25LINQ拾遺及實例

? 投影

□ 遍歷數組索引，Select獲取

            int[] indexes = {0, 2};
            string[] strs = {"a", "b", "c", "d"};
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            var result = from i in indexes
                select strs[i];
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            foreach (string str in result)
            {
                Console.Write(str + " ");
            }
            Console.ReadKey();

結果：a c

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□ 使用集合索引投影匿名對象

        static void Main(string[] args)
        {
            int[] numbers = {0, 3, 4, 5, 7};
            var result = numbers.Select((n, index) => new {Num = n, IsValueEqualIndex = (n == index)});
            foreach (var item in result)
            {
                Console.WriteLine("{0}:{1}",item.Num,item.IsValueEqualIndex);
            }
            Console.ReadKey();
        }

結果：

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□ 2個集合的笛卡爾積

            int[] numberA = {0,2};
            int[] numberB = {1,3};
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            var result = from a in numberA
                from b in numberB
                select new {a, b};
            foreach (var item in result)
            {
                Console.WriteLine("{0},{1}",item.a,item.b);
            }
            Console.ReadKey();

結果：

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□ 笛卡爾積應用，列出所有客戶的所有訂單

   public class Customer
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public List<Order> Orders { get; set; }
    }
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    public class Order
    {
        public int Id { get; set; }
        public decimal Total { get; set; }
    }
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            Order order1 = new Order(){Id = 1,Total = 10.00m};
            Order order2 = new Order(){Id = 2,Total = 20.00m};
            Order order3 = new Order(){Id = 3,Total = 30.00m};
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            List<Customer> customers = new List<Customer>()
            {
                new Customer()
                {
                    Id = 1,
                    Name = "張三",
                    Orders = new List<Order>(){order1,order2}
                },
                new Customer()
                {
                    Id = 2,
                Name = "李四",
                Orders = new List<Order>() { order3 }
                }
            };
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            var result = from c in customers
                from o in c.Orders
                select new {c.Id, c.Name, o.Total};
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            foreach (var item in result)
            {
                Console.WriteLine("客戶編號：{0}，姓名：{1},訂單金額：{2}", item.Id,item.Name,item.Total);
            }
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            Console.ReadKey();
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結果：

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? 分組

□ group n by 條件 into g, g.Key就是分組標準和條件, g代表分組里所有元素的集合

            int[] numbers = { 5, 4, 1, 3, 9, 8, 6};
            var result = from n in numbers
                group n by n%5 into g
                select new {Remainder = g.Key, Numbers = g};
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            foreach (var item in result)
            {
                Console.WriteLine("元素除以5的余數為：{0}",item.Remainder);
                foreach (var n in item.Numbers)
                {
                    Console.WriteLine(n);
                }
            }
            Console.ReadKey();

結果：

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? 連接

□ 根據2個集合的相同部分連接

    public class Product
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string Category { get; set; }
    }
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            List<Product> products = new List<Product>()
            {
                new Product(){Id=1,Name = "體恤衫",Category = "服飾"},
                new Product(){Id = 2,Name = "香水",Category = "化妝品"}
            };
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            string[] categoreis = new string[]
            {
                "服飾",
                "化妝品",
                "電子"
            };
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            var result = from c in categoreis
                join p in products on c equals p.Category
                select new {Category = c, p.Name};
            foreach (var item in result)
            {
                Console.WriteLine(item.Name + "屬于：" + item.Category);
            }

            Console.ReadKey();

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結果：

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□ 把其中一個集合的所有元素放到一個別名集合中

            List<Product> products = new List<Product>()
            {
                new Product(){Id=1,Name = "體恤衫",Category = "服飾"},
                new Product(){Id = 2,Name = "香水",Category = "化妝品"}
            };
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            string[] categoreis = new string[]
            {
                "服飾",
                "化妝品",
                "電子"
            };
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            var result = from c in categoreis
                join p in products on c equals p.Category into ps
                select new {Category = c, Products = ps};
            foreach (var item in result)
            {
                Console.WriteLine("類別：{0}包括：",item.Category);
                foreach (var p in item.Products)
                {
                    Console.WriteLine("      " + p.Name);
                }
            }

            Console.ReadKey();

結果：