25LINQ拾遗及实例

投影

□ 遍历数组索引，Select获取

            int[] indexes = {0, 2};

            string[] strs = {"a", "b", "c", "d"};

            var result = from i in indexes

                select strs[i];

            foreach (string str in result)

                Console.Write(str + " ");

            Console.ReadKey();

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

结果：a c

□ 使用集合索引投影匿名对象

        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();

结果：

□ 2个集合的笛卡尔积

            int[] numberA = {0,2};

            int[] numberB = {1,3};

            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();

结果：

□ 笛卡尔积应用，列出所有客户的所有订单

   public class Customer

        public int Id { get; set; }

        public string Name { get; set; }

        public List<Order> Orders { get; set; }

    public class Order

        public int Id { get; set; }

        public decimal Total { get; set; }

            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};

            List<Customer> customers = new List<Customer>()

                new Customer()

                    Id = 1,

                    Name = "张三",

                    Orders = new List<Order>(){order1,order2}

},

                new Customer()

                    Id = 2,

                Name = "李四",

                Orders = new List<Order>() { order3 }

};

            var result = from c in customers

                from o in c.Orders

                select new {c.Id, c.Name, o.Total};

            foreach (var item in result)

                Console.WriteLine("客户编号：{0}，姓名：{1},订单金额：{2}", item.Id,item.Name,item.Total);

            Console.ReadKey();

结果：

分组

□ group n by 条件 into g, g.Key就是分组标准和条件, g代表分组里所有元素的集合

            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};

            foreach (var item in result)

                Console.WriteLine("元素除以5的余数为：{0}",item.Remainder);

                foreach (var n in item.Numbers)

                    Console.WriteLine(n);

            Console.ReadKey();

结果：

连接

□ 根据2个集合的相同部分连接

    public class Product

        public int Id { get; set; }

        public string Name { get; set; }

        public string Category { get; set; }

            List<Product> products = new List<Product>()

                new Product(){Id=1,Name = "体恤衫",Category = "服饰"},

                new Product(){Id = 2,Name = "香水",Category = "化妆品"}

};

            string[] categoreis = new string[]

                "服饰",

                "化妆品",

                "电子"

};

            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 + "属于：" + item.Category);

            Console.ReadKey();

结果：

□ 把其中一个集合的所有元素放到一个别名集合中

            List<Product> products = new List<Product>()

                new Product(){Id=1,Name = "体恤衫",Category = "服饰"},

                new Product(){Id = 2,Name = "香水",Category = "化妆品"}

};

            string[] categoreis = new string[]

                "服饰",

                "化妆品",

                "电子"

};

            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("类别：{0}包括：",item.Category);

                foreach (var p in item.Products)

                    Console.WriteLine("      " + p.Name);

            Console.ReadKey();

结果：

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