Entity Framework（一）

相关知识点复习：

1、var 类型推断： var p=new Person();

2、匿名类型： var a=new {Name="wang",Age=12 };

3、给新创建的对象属性赋值的简化方法： var P=new Person{Name="wang" ,Age=12 } 等价于 Person p=new Person() ; p.Name="wang"; p.Age=12;

4、.NET高级扩展方法：

            var s0 = new Person { Name = "wang", Age = 12, Salary = 1200 };
            var s1 = new Person { Name = "chen", Age = 13, Salary = 2400 };
            var s2 = new Person { Name = "li", Age = 16, Salary = 2300 };
            var s3 = new Person { Name = "qian", Age = 18, Salary = 1500 };
            var s4 = new Person { Name = "liu", Age = 21, Salary = 2100 };
            var s5 = new Person { Name = "zhao", Age = 23, Salary = 3211 };
            var s6 = new Person { Name = "sun", Age = 27, Salary = 2390 };

            List<Person> ps = new List<Person>();
            ps.Add(s0);
            ps.Add(s2);
            ps.Add(s3);
            ps.Add(s6);

            var teacher1 = new Teacher() { Name = "wang",Students = {s0,s1,s2 } };
            var teacher2 = new Teacher() { Name = "li" , Students = { s3, s4, s5 } };

            Teacher[] ts = { teacher1, teacher2 };

（1）Any() ,判断集合是否包含元素，返回值是bool，一般比Count()>0效率高，Any还可以指定条件表达式。bool b=list.Any(p=>p.Age>50) 等价于bool b=list.Where(p=>=.Age>50).Any();

Console.WriteLine(ps.Any(p=>p.Salary>2000));

(2) Distinct(),剔除完全重复的数据。(*)注意自定义对象的Equal问题，需要重写Equal和GetHashCode方法来进行内容比较

（3）排序：升序 list.OrderBy(p=>p.Age); 降序 list.OrderByDiscending(p=>p.Age) . 指定多个排序规则，而不是多个OrderBy，而是：list.OrderByDiscending(p=>p.Age).ThenBy(p=>p.Salary),也支持ThenByDiscending()

            var p1 = ps.OrderBy(p => p.Salary);
            foreach (var p in p1)
            {
                Console.WriteLine("姓名："+p.Name+"\n"+"薪水"+p.Salary);
            }

(4) skip(n)跳过前n条数据；Take(n)获取最多n条数据，如果不足也不会报错，常用来分页获取数据。 List.Skip(3).Take(2) 跳过前3条数据获取2条数据

            var p1 = ps.Skip(2).Take(3);
            foreach (var p in p1)
            {
                Console.WriteLine("姓名："+p.Name+"\n"+"薪水"+p.Salary);
            }

（5）Except(item1) 排除当前集合中在item1中存在的元素；

            int[] num1 = { 3, 5, 8, 11, 15 };
            int[] num2 = { 5, 8, 9,  15 };
            var num3 = num1.Except(num2);

            foreach (var p in num3)
            {
                Console.WriteLine(p);
            }

（6）Union(item1) 把当前集合和item1中组合；

            int[] num1 = { 3, 5, 8, 11, 15 };
            int[] num2 = { 5, 8, 9,  15 };
            var num3 = num1.Union(num2);

            foreach (var p in num3)
            {
                Console.WriteLine(p);
            }

（7）Intersect(item1)把当前集合和item1中取交集

            int[] num1 = { 3, 5, 8, 11, 15 };
            int[] num2 = { 5, 8, 9,  15 };
            var num3 = num1.Intersect(num2);

            foreach (var p in num3)
            {
                Console.WriteLine(p);
            }

(8) 分组：

            foreach (var p in ps.GroupBy(g1=>g1.Gender))
            {
                Console.WriteLine(p.Key);
                foreach (var p2 in p)
                {
                    Console.WriteLine(p2);
                }
            }

            Console.ReadKey();

（9）SelectMany: 把集合中的每个对象的另外集合属性的值重新拼接为一个新的集合；

            foreach (var s in ts.SelectMany(t=>t.Students))
            {
                Console.WriteLine(s);
                
            }

            Console.ReadKey();

Entity Framework（一）

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