.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);
            }