C# LinQ的基本子句、协变与异变

            //1.from 子句：
            int[] arr = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            var query = from n in arr
                        where n > 3
                        select n;
            foreach (var i in query)
            {
                Console.WriteLine("" + i);
            }


            //2.select 子句：
            int[] arr = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            var query = from n in arr
                        select n * 2;
            foreach (var i in query)
            {
                Console.WriteLine("" + i);
            }


            //3.where 子句：
            int[] arr = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            var query = from n in arr
                        where test(n) > 5
                        select n;
            foreach (var i in query)
            {
                Console.WriteLine(i);
            }

            //4.let 子句：
            int[] arr = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            var query = from n in arr
                        let num = n * 2
                        where num > 5
                        select n;
            foreach (var i in query)
            {
                Console.WriteLine(i);
            }

            //5.order by 子句：
            int[] arr = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            var query = from n in arr
                        where n > 3
                        orderby n descending
                        select n;
            foreach (var i in query)
            {
                Console.WriteLine(i);
            }


            //6.group by 子句：
            int[] arr = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            var query = from n in arr
                        where n > 1
                        group n by n % 2;
            foreach (var i in query)
            {
                foreach (var j in i)
                {
                    Console.WriteLine(j);
                }
            }

            //7.into  子句：
            int[] arr = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            var query = from n in arr
                        where n > 1
                        group n by n % 2 into temp
                        from sn in temp
                        select sn;
            foreach (var i in query)
            {
                Console.WriteLine(i);
            }


            //8.join  子句：
            int[] arrA = { 0, 1, 2, 4, 5, 6, 7, 8, 9 };
            int[] arrB = { 0, 3, 5, 6, 9 };
            var query = from n in arrA
                        join ni in arrB on n equals ni
                        select ni;
            foreach (var i in query)
            {
                Console.WriteLine(i);
            }

            /*
             * 1.协变与异变：
             * 协变：指能够使用与原始指定的派生类型相比，派生程度更大的类型
             * string-->object
             * 逆变：指能够使用派生程度更新的类型
             * object-->string
             * */
            string[] arr = { "C#", "java", "javascript", "VC", "VB", "VB.net", "C++" };
            var query = from n in arr
                        group n by n.Length into lentGroup
                        orderby lentGroup.Key  //协变与逆变
                        select lentGroup;
            foreach (var i in query)
            {
                Console.WriteLine("" + i.Key);
                foreach (var str in i)
                {
                    Console.WriteLine("" + str);
                }
            }