.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。

 

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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=,Name="张三1",Age=,ClassId= },
            new Student(){Id=,Name="张三2",Age=,ClassId= },
            new Student(){Id=,Name="张三3",Age=,ClassId= },
            new Student(){Id=,Name="张三4",Age=,ClassId= },
            new Student(){Id=,Name="张三5",Age=,ClassId= },
            new Student(){Id=,Name="张三6",Age=,ClassId=},
            new Student(){Id=,Name="张三7",Age=,ClassId=},
            new Student(){Id=,Name="张三8",Age=,ClassId=},
            new Student(){Id=,Name="张三9",Age=,ClassId=},
            new Student(){Id=,Name="李四",Age=,ClassId= },
            new Student(){Id=,Name="李四1",Age=,ClassId= },
            new Student(){Id=,Name="李四2",Age=,ClassId= },
            new Student(){Id=,Name="李四3",Age=,ClassId= },
            new Student(){Id=,Name="李四4",Age=,ClassId= },
            new Student(){Id=,Name="李四5",Age=,ClassId= },
            new Student(){Id=,Name="李四6",Age=,ClassId= },
            new Student(){Id=,Name="李四7",Age=,ClassId= },
            new Student(){Id=,Name="李四8",Age=,ClassId= },
            new Student(){Id=,Name="王五",Age=,ClassId= },
            new Student(){Id=,Name="王五1",Age=,ClassId= },
            new Student(){Id=,Name="王五2",Age=,ClassId= },
            new Student(){Id=,Name="王五3",Age=,ClassId= },
            new Student(){Id=,Name="王五4",Age=,ClassId= },
            new Student(){Id=,Name="王五5",Age=,ClassId= },
            new Student(){Id=,Name="王五6",Age=,ClassId= },
            new Student(){Id=,Name="王五7",Age=,ClassId= }

        };

            查询班级Id是1的
            var list = from s in stus
                       where s.ClassId ==
                       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 > ).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 > )//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()//跳过几条
                .Take();//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=,ClassName="初级班" },
                new ClassInfo(){Id=,ClassName="中级班" },
                new ClassInfo(){Id=,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);
            }