.net Lambda表达式与Linq (LINQ TO object)

lambda表达式，是用来写匿名方法的。

在委托用得比较多，因为委托是传递方法的。

 

定义几个委托：

public delegate void donothing();//无参无返回值

 

public delegate void donothingwithpara(sting name,int age);//有参无返回值

 

public delegate sting donothingwithreturn();//无参有返回值

 

public delegate int donothingwithparaandreturn(stiing name,int age);//有参有返回值

 

 

实例化委托

 

donothing dnt = ()=>{}; //无参无返回值方法

 

donothingwithpara dtwp = (x,y)=>{};//有参无返回值

 

donothingwithreturn dtwr = ()=>"hello"; //无参有返回值

 

donothingwithparaandreturn dntwpr = (x,y)=> 123; //有参有返回值

 

 

这就是lambda表达式的写法，本质就是方法。

 

---

linq to object

准备一个类：学生类student

 public class student
    {
        public int id { get; set; }

        public string name { get; set; }

        public int age { get; set; }

        public int classid { get; set; }
    }

准备学生数据：

list<student> stus= new list<student> {
            new student(){id=1,name="张三1",age=27,classid=1 },
            new student(){id=2,name="张三2",age=27,classid=1 },
            new student(){id=3,name="张三3",age=27,classid=1 },
            new student(){id=4,name="张三4",age=27,classid=1 },
            new student(){id=5,name="张三5",age=27,classid=1 },
            new student(){id=6,name="张三6",age=27,classid=2},
            new student(){id=7,name="张三7",age=19,classid=2},
            new student(){id=8,name="张三8",age=19,classid=2},
            new student(){id=9,name="张三9",age=19,classid=2},
            new student(){id=10,name="李四",age=32,classid=2 },
            new student(){id=11,name="李四1",age=32,classid=2 },
            new student(){id=12,name="李四2",age=32,classid=3 },
            new student(){id=13,name="李四3",age=32,classid=3 },
            new student(){id=14,name="李四4",age=32,classid=3 },
            new student(){id=15,name="李四5",age=32,classid=3 },
            new student(){id=16,name="李四6",age=32,classid=3 },
            new student(){id=17,name="李四7",age=37,classid=3 },
            new student(){id=18,name="李四8",age=37,classid=4 },
            new student(){id=19,name="王五",age=37,classid=4 },
            new student(){id=20,name="王五1",age=37,classid=4 },
            new student(){id=21,name="王五2",age=37,classid=4 },
            new student(){id=22,name="王五3",age=37,classid=4 },
            new student(){id=23,name="王五4",age=37,classid=4 },
            new student(){id=24,name="王五5",age=37,classid=4 },
            new student(){id=25,name="王五6",age=37,classid=4 },
            new student(){id=26,name="王五7",age=37,classid=4 }
            
        };

            查询班级id是1的
            var list = from s in stus
                       where s.classid == 1
                       select new
                       {
                           name = s.name,
                           classid = s.classid
                       };


            foreach (var item in list)
            {
                console.writeline(item.name + "---" + item.classid);
            }

//或者用框架的方法,查询年龄大于30的学生
            console.writeline("**********************");
            var list1 = stus.where(s => s.age > 30).select(s => new { id = s.id, name = s.name, age = s.age });
            foreach (var item in list1)
            {
                console.writeline(item.id + "---" + item.name + "--" + item.age);
            }

下面写一个分页的linq

var list = stus.where(s => s.age > 30)//条件筛选
                .select(s => new //投影
                {
                    name = s.name,
                    age = s.age,
                    classid = s.classid
                }).orderby(s => s.age)//排序
                .skip(2)//跳过几条
                .take(3);//获取几条 ，用于分页

            foreach (var item in list)
            {
                console.writeline(item.name + "--" + item.age);
            }

内连接，准备另外一个班级类

public class classinfo
    {
        public int id { get; set; }

        public string classname { get; set; }
    }


list<classinfo> classes = new list<classinfo>()
            {
                new classinfo(){id=1,classname="初级班" },
                new classinfo(){id=2,classname="中级班" },
                new classinfo(){id=3,classname="高级班" },
              //  new classinfo(){id=4,classname="超级班" },
            };

 

var list = from s in stus
                       join cla in classes
                       on s.classid equals cla.id
                      
                       select new
                       {
                           name = s.name,
                           classname = cla.classname,
                           age = s.age
                       };

            foreach (var item in list)
            {
                console.writeline(item.name+"========"+item.classname+"==="+item.age);
            }

左外连接

            var list1 = from s in stus
                        join c in classes
                        on s.classid equals c.id
                        into slist
                        from sc in slist.defaultifempty()
                        select new
                        {
                            name = s.name,
                            classname = sc== null ? "没有班级":sc.classname,
                            age = s.age
                        };
            foreach (var item in list1)
            {
                console.writeline(item.name + "========" + item.classname + "===" + item.age);
            }