LINQ标准查询操作符-阿里云开发者社区

1.投影操作符：将序列中的元素转换为一个由开发人员定义的形式的操作。

Select ：对单个序列或集合中的值进行投影。

string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };

var SelectDevices1 = from device in Devices

select device;

// var SelectDevices = Devices.Select(device=>device);

foreach (string dev in SelectDevices1)

{

Console.WriteLine("带电的设备：{0}", dev);

}

SelectMany：将序列的每个元素投影到 IEnumerable<(Of <(T>)>) 并将结果序列合并为一个序列。
SelectMany：将序列的每个元素投影到 IEnumerable<(Of <(T>)>) 并将结果序列合并为一个序列。

class Devices

{

public string Name

{ get; set; }

public List<double> Price

{ get; set; }

}

static void Main()

{

Devices[] DeviceGroup = { new Devices { Name = "电视", Price =new List<double>{ 3000,2000} }, new Devices { Name = "电冰箱", Price =new List<double> { 4000,5000 }}, new Devices { Name = "洗衣机", Price =new List<double> { 1500,1000 }} };

var devices = DeviceGroup.SelectMany(device => device.Price );

foreach (var v in devices)

{

Console.WriteLine(v);

}

}

2.限制操作符：按照一定的限制，对序列进行过滤。

Where：对单个序列或集合中的值进行投影。

string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };

var SelectDevices = from device in Devices

where device.StartsWith("电")

select device;

//var SelectDevices = Devices.Where(device=>device.StartsWith ("电"));

foreach (string dev in SelectDevices)

{

Console.WriteLine("带电的设备：{0}", dev);

}

3.排序操作符：按照升序和降序的方式对结果进行排序的功能。

OrderBy：将序列中的返回值按照升序的顺序进行排列。

string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };

var SelectDevices = from device in Devices

orderby device

select device;

// var SelectDevices = Devices.OrderBy (device => device);

foreach (string dev in SelectDevices)

{

Console.WriteLine("带电的设备：{0}", dev);

}

OrderByDescending：将序列中的返回值按照降序的顺序进行排列。

string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };

var SelectDevices = from device in Devices

orderby device descending

select device;

// var SelectDevices = Devices.OrderByDescending(device => device);

foreach (string dev in SelectDevices)

{

Console.WriteLine("带电的设备：{0}", dev);

}

ThenBy：排序依据的次关键字。

Devices[] DeviceGroup = { new Devices { Name = "电视", Price = 10000 }, new Devices { Name = "电话", Price = 240 }, new Devices { Name = "电视", Price = 3000 } };

var devices = from device in DeviceGroup

orderby device.Name,device .Price

select device;

// var devices = DeviceGroup.AsQueryable().OrderBy(device => device.Name).ThenBy(device => device.Price);

foreach (var dev in devices)

{

Console.WriteLine("品名：{0}，价格：{1}",dev.Name,dev.Price );

}

ThenByDescending：倒排序依据的次关键字。

Devices[] DeviceGroup = { new Devices { Name = "电视", Price = 10000 }, new Devices { Name = "电话", Price = 240 }, new Devices { Name = "电视", Price = 3000 } };

var devices = from device in DeviceGroup

orderby device.Name,device .Price descending

select device;

// var devices = DeviceGroup.AsQueryable().OrderBy(device => device.Name).ThenByDescending(device => device.Price);

foreach (var dev in devices)

{

Console.WriteLine("品名：{0}，价格：{1}",dev.Name,dev.Price );

}

ThenByDescending：倒排序依据的次关键字。

Devices[] DeviceGroup = { new Devices { Name = "电视", Price = 10000 }, new Devices { Name = "电话", Price = 240 }, new Devices { Name = "电视", Price = 3000 } };

var devices = from device in DeviceGroup

orderby device.Name,device .Price descending

select device;

// var devices = DeviceGroup.AsQueryable().OrderBy(device => device.Name).ThenByDescending(device => device.Price);

foreach (var dev in devices)

{

Console.WriteLine("品名：{0}，价格：{1}",dev.Name,dev.Price );

}

Reverse：反转序列。

string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };

var SelectDevices = from device in Devices.Reverse()

select device;

//var SelectDevices = Devices.Select(device=>device).Reverse ();

foreach (string dev in SelectDevices)

{

Console.WriteLine("带电的设备：{0}", dev);

}

4.联接操作符：将两个或多个数据源对象进行关联或联合。

Join：按照一定的条件，将两个数据源关联起来。

static void Main()

{

List<StudentMessage> A_Class = new List<StudentMessage>(); //声名一个A班，来存放学员

A_Class.Add(new StudentMessage { ID = 1001, Name = "张三", Sex = true, Age = 20 }); //添加学员

A_Class.Add(new StudentMessage { ID = 1002, Name = "李四", Sex = false, Age = 21 });//添加学员

A_Class.Add(new StudentMessage { ID = 1003, Name = "王五", Sex = true, Age = 19 }); //添加学员

List<StudentScort> A_ScortSheet = new List<StudentScort>(); //声名一个成绩表

A_ScortSheet.Add(new StudentScort { ID = 1001, ChineseScort = 90f, MathsScort = 88f }); //添加学员成绩

A_ScortSheet.Add(new StudentScort { ID = 1002, ChineseScort = 80f, MathsScort = 68f }); //添加学员成绩

A_ScortSheet.Add(new StudentScort { ID = 1003, ChineseScort = 60f, MathsScort = 98f }); //添加学员成绩

var messages = from mes in A_Class

join sco in A_ScortSheet on mes.ID equals sco.ID

select new { mes.ID, mes.Name, sco.ChineseScort, sco.MathsScort };

//var messages=A_Class.Join (A_ScortSheet ,mes=>mes .ID ,sco=>sco.ID ,(mes,sco)=>new {mes.ID, mes.Name, sco.ChineseScort, sco.MathsScort });

Console.WriteLine("{0,4} {1,10} {2,3} {3,3}", "学号","姓名","语文","数学");

foreach (var scort in messages)

{

Console.WriteLine("{0,6} {1,10} {2,4} {3,4}",scort .ID ,scort .Name ,scort.ChineseScort ,scort .MathsScort );

}

}

GroupJoin：将主数据源中的每一个值或元素与次数据源中相应的值联接起来。

static void Main()

{

List<StudentMessage> A_Class = new List<StudentMessage>(); //声名一个A班，来存放学员

A_Class.Add(new StudentMessage { ID = 1001, Name = "张三", Sex = true, Age = 20 }); //添加学员

A_Class.Add(new StudentMessage { ID = 1002, Name = “李四”, Sex = false, Age = 21 });//添加学员

List<Scort> A_ScortSheet = new List<Scort>(); //声名一个成绩表

A_ScortSheet.Add(new ChineaseScort { ID = 1001, Scort = 100f, Subject = "语文" }); //添加语文成绩

A_ScortSheet.Add(new ChineaseScort { ID = 1002, Scort = 80f, Subject = “语文” }); //添加语文成绩

A_ScortSheet.Add(new MathsScort { ID = 1001, Scort = 90f, Subject = “数学” }); //添加数学成绩

A_ScortSheet.Add(new MathsScort { ID = 1002, Scort = 60f, Subject = “数学” }); //添加数学成绩

var messages = A_Class.GroupJoin(A_ScortSheet, mes => mes.ID, sco => sco.ID, (mess, scor) => new { Mess = mess.ID, Mename = mess.Name, Scor = scor.Select(Scor => Scor) });

foreach (var scort in messages)

{

Console.WriteLine("学号：{0,4} 姓名：{1}", scort.Mess, scort.Mename);

foreach (var sc in scort.Scor)

{

Console.WriteLine(" {0}：{1}分", sc.Subject, sc.Scort);

}

}

数据源1.GroupJoin(数据源2，变量1=>数据源1.关联字段，变量2=>数据源码2.关联字段，（数据源1数据变量，数据源2数据列表变量）=>new{变量a=数据源1.字段1，变量b=数据源1.字段2，变量c=数据源2列表变量.Select(变量c=变量c)})

5.分组操作符：按照一定的值将序列中的值或元素进行分组。

GroupBy：将序列中的返回值按照升序的顺序进行排列。

class Student

{

public string ClassName

{ get; set; }

public string Name

{ get; set; }

}

static void Main()

{

Student[] students = new Student[] {

new Student{ClassName="A", Name="张三"},

new Student{ClassName ="B", Name ="李四"},

new Student{ClassName ="A" ,Name ="王五"},

new Student {ClassName ="B", Name ="赵六"},

new Student{ClassName ="B" ,Name ="钱七"},

new Student {ClassName ="B", Name ="孙八"}

};

var stus = (from stu in students

select stu).GroupBy(st => st.ClassName);

//var stus = students.GroupBy(st => st.ClassName);

foreach (var v in stus)

{

Console.WriteLine("{0}班学生：", v.Key);

foreach (var va in v)

{

Console.WriteLine(" 姓名：{0}", va.Name);

}

}

}

6.合并操作符：将两个对象合并在一起。

Concat：

string[] Citys = new string[] { "北京","上海","东京"};

string[] Mobiles = new string[] {"诺基亚","摩托罗拉","三星" };

var Cont = Citys.Select(city => city).Concat(Mobiles.Select(mobil => mobil));

foreach (var v in Cont)

{

Console.WriteLine(v);

}

7.聚合操作符：在一系列值上执行特定的运算，并返回单个值。

Aggregate：从序列或集合中收集值。

string[] citys = {"北京","上海","东京"};

string newcity = citys.Aggregate((cityname,next)=>next+"-"+cityname);

Console.WriteLine(newcity);

Average：求一个数值序列的平均值。

Demo1：

double[] nums = {1.2,34.1,45.21,43.1 };

Console.WriteLine(nums.Average ());

Demo2：

List<StudentSoc> list = new List<StudentSoc>();

list.Add(new StudentSoc { Name = "张三", Chinese = 90f });

list.Add(new StudentSoc { Name = "李四", Chinese = 94f });

list.Add(new StudentSoc { Name = "王五", Chinese = 92f });

list.Add(new StudentSoc { Name = "赵六", Chinese = 78f });

var quer = from stu in list select stu.Chinese;

Console.WriteLine(quer .Average ());

//var quer = list.Average(lis=>lis.Chinese );

//Console.WriteLine(quer);

Count：求一个数值序列的个数。（LongCount用法相同）

Demo1：

double[] nums = {1.2,34.1,45.21,43.1 };

Console.WriteLine(nums.Count(num=>num<10));

Demo2：

List<StudentSoc> list = new List<StudentSoc>();

list.Add(new StudentSoc { Name = "张三", Chinese = 90f });

list.Add(new StudentSoc { Name = "李四", Chinese = 94f });

list.Add(new StudentSoc { Name = "王五", Chinese = 92f });

list.Add(new StudentSoc { Name = "赵六", Chinese = 78f });

var quer = from stu in list

where stu.Chinese >80

select stu.Chinese;

Console.WriteLine(quer.Count());

//var quer = list.Count(lis=>lis.Chinese >80);

//Console.WriteLine(quer);

Sum：求一个数值序列的和。

Demo1：

double[] nums = {1.2,34.1,45.21,43.1 };

Console.WriteLine(nums.Where (numm=>numm>10).Sum(num=>num));

Demo2：

List<StudentSoc> list = new List<StudentSoc>();

list.Add(new StudentSoc { Name = "张三", Chinese = 90f });

list.Add(new StudentSoc { Name = "李四", Chinese = 94f });

list.Add(new StudentSoc { Name = "王五", Chinese = 92f });

list.Add(new StudentSoc { Name = "赵六", Chinese = 78f });

var quer = from stu in list

where stu.Chinese < 93

select stu.Chinese;

Console.WriteLine(quer.Sum());

//var quer = list.Where(liss => liss.Chinese < 93).Sum(lis => lis.Chinese);

// Console.WriteLine(quer);

8.集合操作符：对元素的集合或序列集合进行操作，并近观回一个集合。

Distinct：删除集合中重复的值，并返回该项集合中互不相同的元素。

List<StudentSoc> list = new List<StudentSoc>();

list.Add(new StudentSoc { Name = "李四", Chinese = 94f });

list.Add(new StudentSoc { Name = "李四", Chinese = 94f });

list.Add(new StudentSoc { Name = "王五", Chinese = 92f });

StudentSoc s = new StudentSoc { Name = "赵六", Chinese = 78f };

list.Add(s);

list.Add(s);

//var quer = from stu in list

// select stu;

//foreach (var que in quer.Distinct())

//{

// Console.WriteLine(que.Name +":"+que.Chinese );

//}

var quer = list.Distinct();

foreach (var que in quer)

{

Console.WriteLine(que.Name + ":" + que.Chinese);

}

Union：将两个集合或序列合并后返回不同的元素。

int[] nums1 = {1,2,3,3,4};

int[] nums2 = { 2, 3, 4, 5, 6 };

var newnums = nums1.Union(nums2);

foreach (var num in newnums)

{

Console.WriteLine(num);

}

Intersect：返回两个集合的交集。

int[] nums1 = {1,2,3,3,4};

int[] nums2 = { 2, 3, 4, 5, 6 };

var newnums = nums1.Intersect(nums2);

foreach (var num in newnums)

{

Console.WriteLine(num);

}

Except：返回序列一中序列二没有的值。

int[] nums1 = {1,2,3,3,4,7};

int[] nums2 = { 2, 3, 4, 5, 6 ,9,23};

var newnums = nums1.Except(nums2);

foreach (var num in newnums)

{

Console.WriteLine(num);

}

9.生成操作符：从现有的序列中创建新的序列。

Empty：指定类型的空集。

var s= Enumerable.Empty<string>();

Console.WriteLine(s.Count ());

Range：创建一个包含数字序列的集合。

foreach (var num in Enumerable.Range(3, 9))

{

Console.WriteLine(num);

}

Repeat：将值重复一定的次数。

foreach (var num in Enumerable.Repeat(“*****”,9))

{

Console.WriteLine(num);

}

10.转换操作符：将输入对象类型转变为序列的动作。

AsEnumerable：

int[] ArrInt = {1,2,3,4,5 };

foreach (int i in ArrInt .Where (arr=>arr>1))

{

Console.WriteLine(i);

}

foreach (int i in ArrInt.AsEnumerable().Where(arr => arr > 1))

{

Console.WriteLine(i);

}

Cast：将序列转换成指定的类型。

直接对Array应用Select是不可以的。

ArrayList AL = new ArrayList();

AL.Add(1);

AL.Add(2);

AL.Add(3);

AL.Add(4);

IEnumerable <int> que=AL.Cast<int>().Select (arr=>arr);

foreach (int i in que)

{

Console.WriteLine(i);

}

OfType：过滤序列中的某种类型。

ArrayList AL = new ArrayList();

AL.Add(1);

AL.Add(2);

AL.Add("3");

AL.Add(4);

var que = AL.OfType<int>();

foreach (var i in que)

{

Console.WriteLine(i);

}

ToArray：从Ienumerable序列创建一个数组。

List<StudentSoc> list = new List<StudentSoc>();

list.Add(new StudentSoc { Name = "张三", Chinese = 90f });

list.Add(new StudentSoc { Name = "李四", Chinese = 94f });

list.Add(new StudentSoc { Name = "王五", Chinese = 92f });

list.Add(new StudentSoc { Name = "赵六", Chinese = 78f });

var stu1 = list.Select(stud => stud.Chinese);

var stu = list.Select(stud => stud.Chinese).ToArray();

ToDictionary：从Ienumerable序列创建一个Dictionary序列。

class StudentSoc

{

public string Name

{ get; set; }

public float Chinese

{ get; set; }

}

List<StudentSoc> list = new List<StudentSoc>();

list.Add(new StudentSoc { Name = "张三", Chinese = 90f });

list.Add(new StudentSoc { Name = "李四", Chinese = 94f });

list.Add(new StudentSoc { Name = "王五", Chinese = 92f });

list.Add(new StudentSoc { Name = "赵六", Chinese = 78f });

Dictionary<float ,StudentSoc> stu = list.ToDictionary(stud => stud.Chinese );

foreach (KeyValuePair<float,StudentSoc > item in stu)

{

Console.WriteLine(item.Key +" "+item.Value.Name +" "+item.Value .Chinese );

}

ToList：从Ienumerable序列创建一个List序列。

class StudentSoc

{

public string Name

{ get; set; }

public float Chinese

{ get; set; }

}

List<StudentSoc> list = new List<StudentSoc>();

list.Add(new StudentSoc { Name = "张三", Chinese = 90f });

list.Add(new StudentSoc { Name = "李四", Chinese = 94f });

list.Add(new StudentSoc { Name = "王五", Chinese = 92f });

list.Add(new StudentSoc { Name = "赵六", Chinese = 78f });

var stu = list.Select(lis=>lis.Chinese).ToList();

foreach (var item in stu)

{

Console.WriteLine(item );

}

ToLookup：从Ienumerable序列创建一个 Lookup序列。Lookup是主键的集合，其中主键对应着一个或多个值。

class OrderTable

{

public int OrderNum

{ get; set; }

public double Amount

{ get; set; }

}

List<OrderTable> list = new List<OrderTable>();

list.Add(new OrderTable { OrderNum = 1001, Amount = 1234.5 });

list.Add(new OrderTable { OrderNum = 1002, Amount = 2340 });

list.Add(new OrderTable { OrderNum = 1001, Amount = 3000 });

var lis = list.ToLookup(num => num.OrderNum, num => num.Amount);

foreach (var item in lis)

{

Console.WriteLine("订单号：{0}",item.Key );

foreach (var v in item)

{

Console.WriteLine(" {0}元",v);

}

}

11.元素操作符：从一个序列返回单个特定的元素。

DefaultIfEmpty：将空集合替换为包含默认的单个值的集合。

List<OrderTable> list = new List<OrderTable>();

list.Add(new OrderTable { OrderNum = 1001, Amount = 1234.5 });

list.Add(new OrderTable { OrderNum = 1002, Amount = 2340 });

list.Add(new OrderTable { OrderNum = 1001, Amount = 3000 });

var lis = list.Where(num => num.OrderNum == 1001);

foreach (var item in lis.DefaultIfEmpty(new OrderTable { OrderNum =0,Amount =0}))

{

Console.WriteLine("订单号：{0}", item.Amount);

}

ElementAt：返回集合中给定索引的元素。

List<OrderTable> list = new List<OrderTable>();

list.Add(new OrderTable { OrderNum = 1001, Amount = 1234.5 });

list.Add(new OrderTable { OrderNum = 1002, Amount = 2340 });

list.Add(new OrderTable { OrderNum = 1001, Amount = 3000 });

var value = list.Where(num => num.OrderNum == 1001).ElementAt (1);

Console.WriteLine("订单号：{0}", value.Amount );

ElementAtOrDefault：是将ElementAt和DefaultIfEmpty。

List<OrderTable> list = new List<OrderTable>();

list.Add(new OrderTable { OrderNum = 1001, Amount = 1234.5 });

list.Add(new OrderTable { OrderNum = 1002, Amount = 2340 });

list.Add(new OrderTable { OrderNum = 1001, Amount = 3000 });

var value = list.Select (lis=>lis.OrderNum).ElementAtOrDefault(3);

Console.WriteLine("订单号：{0}", value);

First：返回集合中的第一个元素（Last返回最后一个）。

List<OrderTable> list = new List<OrderTable>();

list.Add(new OrderTable { OrderNum = 1001, Amount = 1234.5 });

list.Add(new OrderTable { OrderNum = 1002, Amount = 2340 });

list.Add(new OrderTable { OrderNum = 1001, Amount = 3000 });

var value = list.Select (lis=>lis.OrderNum).First ();

Console.WriteLine("订单号：{0}", value);

FirstOrDefault：返回集合中的第一个元素，如果不存在，返回默认值（LastOrDefault返回最后一个，如果不存在，返回默认值）。

List<OrderTable> list = new List<OrderTable>();

list.Add(new OrderTable { OrderNum = 1001, Amount = 1234.5 });

list.Add(new OrderTable { OrderNum = 1002, Amount = 2340 });

list.Add(new OrderTable { OrderNum = 1001, Amount = 3000 });

var value = list.Where (li=>li.OrderNum ==1003).Select(lis=>lis.OrderNum).FirstOrDefault();

Console.WriteLine("订单号：{0}", value);

Single：从一个序列中返回单个元素，或唯一满足某一特定条件的元素。（LastOrDefault返回最后一个，如果不存在，返回默认值）。

List<OrderTable> list = new List<OrderTable>();

list.Add(new OrderTable { OrderNum = 1001, Amount = 1234.5 });

list.Add(new OrderTable { OrderNum = 1002, Amount = 2340 });

list.Add(new OrderTable { OrderNum = 1001, Amount = 3000 });

var value = list.Where (li=>li.OrderNum ==1002).Select(lis=>lis.OrderNum).Single();

Console.WriteLine("订单号：{0}", value);

SingleOrDefault：从一个序列中返回单个元素，或唯一满足某一特定条件的元素。如果存在多条记录，抛出异常。

List<OrderTable> list = new List<OrderTable>();

list.Add(new OrderTable { OrderNum = 1001, Amount = 1234.5 });

list.Add(new OrderTable { OrderNum = 1002, Amount = 2340 });

list.Add(new OrderTable { OrderNum = 1001, Amount = 3000 });

var value = list.Where (li=>li.OrderNum ==1001).Select(lis=>lis.OrderNum).SingleOrDefault();

Console.WriteLine("订单号：{0}", value);

12.相等操作符：比较两个序列的元素是否相同。

SequenceEqual

int[] Arr1 = { 1, 2, 3 };

int[] Arr2 = { 1, 2, 3 };

Console.WriteLine(Arr1.SequenceEqual(Arr2))

13.量词操作符：是否存在部分或全部元素符合某个特定条件。

All：判定在集合中是否所有的值都满足特定的条件。

int[] Arr1 = { 1, 1, 1 };

Console.WriteLine(Arr1.All (arr=>arr==1));

Any：判定在集合中是否有的值满足特定的条件。

int[] Arr1 = { 1, 2, 3 };

Console.WriteLine(Arr1.Any (arr=>arr==1));

Contains：判定在集合中是否包含某个值。

int[] Arr1 = { 1, 2, 3 };

Console.WriteLine(Arr1.Contains(4));

14.分割操作符：将一个序列分割成两个或多个序列。

Skip：跳过序列的前n个元素后返回所有的元素。

int[] Arr = { 1, 2, 3,4,5,6 };

foreach (int i in Arr.Skip(2))

{

Console.WriteLine(i);

}

SkipWhile：跳过不满足条件的元素，返回余下元素的序列。

需要注意的是，这种跳出过必需排序后才能合部跳过满足条件的元素。

int[] Arr = { 1, 12, 3,4,15,6};

foreach (int i in Arr.OrderBy (nu=>nu).SkipWhile(arr=>arr<=10))

{

Console.WriteLine(i);

}

Take：返回序列的从开始到参数指定位置的元素。

int[] Arr = { 1, 2, 3,4,5,6 };

foreach (int i in Arr.Take(2))

{

Console.WriteLine(i);

}

TakeWhile：返回满足条件的元素。

需要注意的是，这种返回必需排序后才能进行操作。

int[] Arr = { 1, 12, 3,4,15,6};

foreach (int i in Arr.OrderBy (ar=>ar).TakeWhile(arr=>arr<12))

{

Console.WriteLine(i);

}

本文转自桂素伟51CTO博客，原文链接：
http://blog.51cto.com/axzxs/195102
，如需转载请自行联系原作者

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