LINQ基础篇（中）

聚合函数
Max 最大值
Min 最小值
Sum 求和
Average 求平均值
Aggregate 自定义累计
Count 统计元素个数
LongCount 统计元素个数返回long型

代码例子

            List<int> tempList = new List<int> { 56, 34, 23, 21, 78, 99 };

            var maxValue = tempList.Max();
            var minValue = tempList.Min();
            var sumValue = tempList.Sum();
            var avgValue = tempList.Average();
            var aggValue = tempList.Aggregate((initial, next) => initial+next * 2);
            var countValue = tempList.Count();
            var lcountValue = tempList.LongCount();
            Console.WriteLine("最大值：{0}
最小值：{1}
求和：{2}
平均值：{3}
自定义累计：{4}
元素个数：{5}
元素个数（long）：{6}",maxValue,minValue,sumValue,avgValue,aggValue,countValue,lcountValue);
            Console.ReadKey();            

结果

Aggregate

该函数有三种定义，其中Func是Lambada表达式，seed是初始增量

var aggValue = tempList.Aggregate(6,(initial, next) => initial+next * 2);

初始是6，表示2倍的和再加上6，即628，该函数灵活多变。

join连接

内连接 inner join

内连接和SQL是类似的

查询语法：var p=from a in A join b in B on a.No equals b.No select new {};

方法语法：var p=A.Join(B,a=>a.No,b=>b.No,(a,b)=>new {});

 

左连接 left outer join

查询语法：var p=from a in A join b in B on a.No equals b.No into abLink from c in abLink DefaultIfEmpty()

　　　　　　　 select new {aa=a.No,bb=c==null?null:c.cc}

 

代码例子

定义Student类

public class Student
    {
        public string StudentNo { get; set; }
        public string StudentName { get; set; }
        public string ClassNo { get; set; }
    }

定义ClassInfo类

 public class ClassInfo
    {
        public string ClassNo { get; set; }
        public string ClassName { get; set; }
    }

实现调用

            List<Student> listStudent = new List<Student>
            {
               new Student{StudentNo="1",StudentName="张三",ClassNo="1"},
               new Student{StudentNo="2",StudentName="李四",ClassNo="2"},
               new Student{StudentNo="3",StudentName="王五",ClassNo="3"},
               new Student{StudentNo="4",StudentName="刘三",ClassNo="1"},
               new Student{StudentNo="5",StudentName="关翊",ClassNo="2"},
               new Student{StudentNo="6",StudentName="张珊",ClassNo="3"},
               new Student{StudentNo="7",StudentName="赵四",ClassNo=""},
            };
            List<ClassInfo> listClassInfo = new List<ClassInfo>
            {
                new ClassInfo{ClassNo="1",ClassName="英才一班"},
                new ClassInfo{ClassNo="2",ClassName="英才二班"},
                new ClassInfo{ClassNo="3",ClassName="英才三班"},
            };
         
            var p = from s in listStudent
                    join c in listClassInfo
                    on s.ClassNo equals c.ClassNo
                    select new
                    {
                        studentname = s.StudentName,
                        classname=c.ClassName,
                    };
            var lp = listStudent.Join(listClassInfo, s =>s.ClassNo, c=>c.ClassNo,(s,c)=>new { studentname = s.StudentName,classname = c.ClassName });
            var linkp = from s in listStudent
                    join c in listClassInfo
                    on s.ClassNo equals c.ClassNo into scLink
                    from c in scLink.DefaultIfEmpty()
                    select new
                    {
                        studentname = s.StudentName,
                        classname = c?.ClassName
                        //classname = c==null?null:c.ClassName
                    };
            foreach (var item in linkp)
            {
                Console.WriteLine("姓名：{0}，班级：{1}",item.studentname,item.classname);
            }
            Console.ReadKey();        

输出结果

let

let用于设置临时变量储存结果，对数据进行操作

语法：let 变量a=操作。。。

   var linkp = from s in listStudent
                    join c in listClassInfo
                    on s.ClassNo equals c.ClassNo into scLink
                    from c in scLink.DefaultIfEmpty()
                    let sc="中华一番"+c?.ClassName
                    select new
                    {
                        studentname = s.StudentName,
                        classname = sc
                        //classname = c==null?null:c.ClassName
                    };
            foreach (var item in linkp)
            {
                Console.WriteLine("姓名：{0}，班级：{1}",item.studentname,item.classname);
            }
            Console.ReadKey()

上述代码在班级前加“中华一番”