LinQ的第一天（老规矩：还是案例分析，从简单到难）

LINQ-TO-SQL:
案例1：基础模型

static void Main(string[] args)
        {
            int[] int_array = { 1, 8, 5, 9, 15, 20 };

            var filtered = from value in int_array//注意是var，可以帮助我们减少很多麻烦
                        where value > 4
                        select value;

            Console.WriteLine(filtered.GetType());//System.Linq.Enumerable+<WhereIterator>d__0`1[System.Int32]
            foreach (int i in filtered)
            {
                Console.Write(i + "");//8  5  9  15  20
            }
            Console.WriteLine();


            Console.WriteLine("The Next:");
            double[] double_array={1.6,2.5,1.5,7.5};
            IEnumerable<double> double_arrayQuery =//标准格式
                from value in double_array
                where value > 2
                select value;
            Console.WriteLine(double_arrayQuery.GetType());//System.Linq.Enumerable+<WhereIterator>d__0`1[System.Double]
            foreach (double testQuery in double_arrayQuery)
            {
                Console.Write(testQuery+"");//2.5 7.5
            }
        }

案例2：

 static void Main(string[] args)
        {
            int[] int_array = { 1, 8, 5, 9, 15, 20 };

            var filtered = from value in int_array//注意是var
                        where value > 4
                        orderby value//按默认的规则升序排列
                        select value;

            Console.WriteLine(filtered.GetType());//System.Linq.Enumerable+<WhereIterator>d__0`1[System.Int32]
            foreach (int i in filtered)
            {
                Console.Write(i + "");//5  8  9  15  20
            }
            Console.WriteLine();


            Console.WriteLine("The Next:");
            double[] double_array={1.6,2.5,1.5,7.5};
            IEnumerable<double> double_arrayQuery =//标准格式
                from value in double_array
                where value > 2
                orderby value descending//降序排列
                select value;
            Console.WriteLine(double_arrayQuery.GetType());//System.Linq.Enumerable+<WhereIterator>d__0`1[System.Double]
            foreach (double testQuery in double_arrayQuery)//注意这里查询的是哪个变量，是SQL的变量
            {
                Console.Write(testQuery+"");//7.5 2.5
            }



            string[] names = { "Mr.w","Jack","Angrw"};
            Console.WriteLine("The names is :");
            foreach (string nametest in names)
            {
                Console.Write(nametest + "");
            }
            Console.WriteLine();

            IEnumerable<string> names_query =
                from name in names
                orderby name
                select name;
            Console.WriteLine("The Orderby of name is :");
            foreach (string nametest in names_query)
            {
                Console.Write(nametest + "");
            }
            Console.WriteLine();

           names_query =
                from name in names
                orderby name descending
                select name;
           Console.WriteLine("The Orderby...Descending of name is :");
            foreach (string nametest in names_query)
            {
                Console.Write(nametest + "");
            }
            Console.WriteLine();
        }
The Result:
/*
System.Linq.OrderedEnumerable`2[System.Int32,System.Int32]
5  8  9  15  20
The Next:
System.Linq.OrderedEnumerable`2[System.Double,System.Double]
7.5  2.5  The names is :
Mr.w  Jack  Angrw
The Orderby of name is :
Angrw  Jack  Mr.w
The Orderby...Descending of name is :
Mr.w  Jack  Angrw
*/

案例3：简化上面的代码

static void Main(string[] args)
        {
            int[] int_array = { 1, 8, 5, 9, 15, 20 };
            var filtered = from value in int_array//注意是var
                        where value > 4
                        orderby value//按默认的规则升序排列
                        select value;
            Console.WriteLine(filtered.GetType());//System.Linq.Enumerable+<WhereIterator>d__0`1[System.Int32]
            Display(filtered, "Int");

            double[] double_array={1.6,2.5,1.5,7.5};
            IEnumerable<double> double_arrayQuery =//标准格式
                from value in double_array
                where value > 2
                orderby value descending//降序排列
                select value;
            Console.WriteLine(double_arrayQuery.GetType());//System.Linq.Enumerable+<WhereIterator>d__0`1[System.Double]
            Display(double_arrayQuery, "Double");


            string[] names = { "Mr.w","Jack","Angrw"};
            IEnumerable<string> names_query =
                from name in names
                orderby name
                select name;
            Display(names_query,"String-orderby");

            names_query =
                from name in names
                orderby name descending
                select name;
           Display(names_query, "String-orderby..descending");
        }
            //为了简化我们写成方法调用
        static void Display(IEnumerable<string> string_query, string welcome)  //下面3个方法起到了方法重载作用
        {
            Console.WriteLine("The {0} is :", welcome);
            foreach (string test in string_query)
            {
                Console.Write(test + "");
            }
            Console.WriteLine();
        }

        static void Display(IEnumerable<int> int_query, string welcome)
        {
            Console.WriteLine("The {0} is :", welcome);
            foreach (int test in int_query)
            {
                Console.Write(test + "");
            }

            Console.WriteLine();
        }

        static void Display(IEnumerable<double> double_query, string welcome)
        {
            Console.WriteLine("The {0} is :", welcome);
            foreach (double test in double_query)
            {
                Console.Write(test + "");
            }

            Console.WriteLine();
        }
The Result:
/*
System.Linq.OrderedEnumerable`2[System.Int32,System.Int32]
The Int is :
5  8  9  15  20
System.Linq.OrderedEnumerable`2[System.Double,System.Double]
The Double is :
7.5  2.5
The String-orderby is :
Angrw  Jack  Mr.w
The String-orderby...descending is :
Mr.w  Jack  Angrw
*/

案例4：
 

class Program
    {
        static void Main(string[] args)
        {

            Member[] persons ={
                                  new Member(-10m, 21, "jack"),
                                  new Member(25m,25,"An"),
                                  new Member(100m,18,"Mr.w"),
                                  new Member(3000m,27,"deve")
                               };

            Display(persons,"The Array");

            var betwtenn100M3000M =
                from e in persons
                where e.Pay >= 100m && e.Pay <= 3000m
                select e;
            Display(betwtenn100M3000M, "100m-300m");

            var PaySort =
                from e in persons
                orderby e.Pay  //默认是月薪从小到大
                select e;
            Display(PaySort,"The Pay Sort");

            var AgeSort =
                from e in persons
                orderby e.age descending  //年龄从大到小
                select e;
            Display(AgeSort, "The Age Sort");
        }

        static void Display<T>(IEnumerable<T> T_query, string header)//泛型接口
        {
            Console.WriteLine("{0}  :", header);

            foreach (T test in T_query)
            {
                Console.WriteLine(test + "");
            }
            Console.WriteLine();
        }
    }

    class Member//成员
    {
        private decimal pay;// 月薪
        public decimal Pay//月薪属性
        {
            get { return pay; }
            set
            {
                if (value >= 0m)
                {
                    pay = value;
                }

            }
        }
       public string name;//这里如果不是public 外部将无法访问
       public  int age;

        public Member(decimal monthPay, int member_age, string member_name)
        {
            name = member_name;
            age = member_age;
            Pay = monthPay;//注意这里的大写Pay
        }

        public override string ToString()//重写string类的ToString方法
        {
            return string.Format("Name is: {0}   Age is:{1}   MonthPay is:{2}",name,age,pay);
            
        }

    }
The Result:
/*
The Array  :
Name is: jack   Age is:21   MonthPay is:0
Name is: An   Age is:25   MonthPay is:25
Name is: Mr.w   Age is:18   MonthPay is:100
Name is: deve   Age is:27   MonthPay is:3000

100m-300m  :
Name is: Mr.w   Age is:18   MonthPay is:100
Name is: deve   Age is:27   MonthPay is:3000

The Pay Sort  :
Name is: jack   Age is:21   MonthPay is:0
Name is: An   Age is:25   MonthPay is:25
Name is: Mr.w   Age is:18   MonthPay is:100
Name is: deve   Age is:27   MonthPay is:3000

The Age Sort  :
Name is: deve   Age is:27   MonthPay is:3000
Name is: An   Age is:25   MonthPay is:25
Name is: jack   Age is:21   MonthPay is:0
Name is: Mr.w   Age is:18   MonthPay is:100
*/