[轉貼] Linq初級學習（標準查詢操作符的使用和調用)

2012022813:24

出處：http://www.cnblogs.com/huangcong/archive/2010/12/29/1919817.html

本文章是我最近學習Linq的一些示例,分享給和我一樣剛開始接觸Linq的朋友們作為一個參考,因為是初學,肯定有很多不足,因此希望大家友善提醒,切勿針鋒相對,我會及時改正的~~

目錄

01 投影操作符(Select,SelectMany)

02 限制操作符(Where)

03 排序操作符(OrderBy,OrderByDescending,ThenBy,ThenByDescending,Reverse)

04 聯接操作符(join,GroupJoin)

05 分組操作符(GroupBy)

06 串聯操作符(Concat)

07 聚合操作符(Aggregate,Average,Count,LongCount,Max,Min,Sum)

08 集合操作符(Distinct,Union,Intersect,Except)

09 生成操作符(Empty,Range,Repeat)

10 轉換操作符(Cast,OfType,ToArray,ToDictionary,ToList,ToLookup)

11 元素操作符(DefaultIfEmpty,ElementAt,ElementAtOrDefault,First,Last,FirstOrDefault,LastOrDefault,Single,SingleOrDefault)

12 相等操作符(SequenceEqual)

13 量詞操作符(All,Any)

14 分割操作符(Skip,SkipWhile,Take,TakeWhile)

 

 

 

01.投影操作符(Select,SelectMany)

(1)Select操作符對單個序列或者集合中的值進行投影,可以控制從序列中返回指定的列.

方法語法:

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            //創建測試數據
            User[] users = new User[]{
                new User(){ Id = 1, FirstName = "Huang", LastName = "Cong", Age = 12},
                new User(){ Id = 2, FirstName = "Yang", LastName = "Qiu", Age = 22},
                new User(){ Id = 3, FirstName = "Cai", LastName = "Zheng", Age = 18},
                new User(){ Id = 4, FirstName = "Huang", LastName = "Chao", Age = 32},
                new User(){ Id = 5, FirstName = "Teng", LastName = "Jie", Age = 12},
                new User(){ Id = 6, FirstName = "Liang", LastName = "Zhi", Age = 8}
            };

            //使用Select操作符進行投影,對序列只返回First一個列
            var query = users.Select(u => new { u.FirstName });

            //輸出
            foreach (var q in query)
                Console.WriteLine(q.FirstName);
        }
    }

    class User
    {
        public int Id { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Age { get; set; }

        public override string ToString()
        {
            return String.Format("Id:{0}\tFirstName:{1}\tLastName:{2}\tAge:{3}", this.Id, this.FirstName, this.LastName, this.Age);
        }
    }
}
查詢語法:
var query = from u in users select new { u.FirstName };
運行結果:

 

 

 (2)SelectMany操作符提供了將多個from子句組合起來的功能,它將每個對象的結果合併成單個序列.

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            //創建測試數據
            Names[] owners = 
            { new Names { FirstName = "Scott, Chris",
                Pets = new List<string>{"Yukon", "Fido"}},
             new Names { FirstName = "Jason, Steve",
                Pets = new List<string>{"Killer", "Fluffy"}},
             new Names { FirstName = "John, Joe",
                Pets = new List<string>{"Spike", "Tinkerbell"}}};

            //使用SelectMany操作符將每個對象的Pets組合成一個序列
            var query = owners.SelectMany(owner => owner.Pets);

            //對組合好的Pets進行輸出
            foreach (var q in query)
                Console.WriteLine(q);
        }
    }

    class Names
    {
        public string FirstName { get; set; }
        public List<string> Pets { get; set; }
    }
}

運行結果:

 

02.限制操作符(Where,OfType)

(1)Where是限制操作符,可以對序列中的值進行過濾.Where操作符不啟動查詢的執行.但開始對對像進行迭代的時候才將過濾器應用到數據上.

方法語法:

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Collections;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            //創建測試數據
            List<string> names = new List<string>();
            
            names.Add("Scott");
            names.Add("Dave");
            names.Add("Dave");
            names.Add("Steve");
            names.Add("Joe");

            //使用Where操作符查詢序列中以S開頭的元素
            var query = names.Where(n => n.StartsWith("S"));

            //對組合好的Pets進行輸出
            foreach (var q in query)
                Console.WriteLine(q);
        }
    }
}

查詢語法:

var query = from n in names where n.StartsWith("S") select n;

運行結果:

 

03 排序操作符(OrderBy,OrderByDescending,ThenBy,ThenByDescending,Reverse)

(1)OrderBy/OrderByDescending操作符能將序列中的返回值按照升序/降序的順序進行排列.

方法語法:

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Collections;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            //創建測試數據
            List<string> names = new List<string>();
            
            names.Add("Scott");
            names.Add("Dave");
            names.Add("Dave");
            names.Add("Steve");
            names.Add("Joe");

            //使用OrderBy操作符對序列進行排序
            var query = names.Select(n => n).OrderBy(n => n);

            Console.WriteLine("----------------------OrderBy排序----------------------");
            foreach (var q in query)
                Console.WriteLine(q);

            //使用OrderByDescending操作符對序列進行倒序
            query = names.Select(n => n).OrderByDescending(n => n);

            Console.WriteLine("----------------------OrderByDescending排序----------------------");
            foreach (var q in query)
                Console.WriteLine(q);
        }
    }
}

查詢語法:

//正序
var query = from n in names orderby n select n;

//倒序
query = from n in names orderby n descending select n;

運行結果:

 

(2)ThenBy/ThenByDescending操作符可按照次關鍵對序列進行升序/降序排列.

方法語法:

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Collections;

namespace ConsoleApplication1
{
    class Program
    {
        class User
        {
            public string Name { get; set; }
            public int Age { get; set; }
        }

        static void Main(string[] args)
        {
            //創建測試數據
            List users = new List ();

            users.Add(new User() { Name = "Scott", Age = 12 });
            users.Add(new User() { Name = "Scott", Age = 22 });
            users.Add(new User() { Name = "Doe", Age = 11 });
            users.Add(new User() { Name = "Doe", Age = 8 });
            users.Add(new User() { Name = "Scott", Age = 18 });
            users.Add(new User() { Name = "Doe", Age = 24 });

            //使用ThenBy操作符對序列再次進行排序
            var query = users.Select(u => u).OrderBy(u => u.Name).ThenBy(u => u.Age);

            Console.WriteLine("----------------------ThenBy排序----------------------");
            foreach (var q in query)
                Console.WriteLine("Name:{0}\tAge:{1}", q.Name, q.Age);

            //使用ThenByDescending操作符對序列再次進行排序
            query = users.Select(u => u).OrderBy(u => u.Name).ThenByDescending(u => u.Age);

            Console.WriteLine("----------------------ThenByDescending排序----------------------");
            foreach (var q in query)
                Console.WriteLine("Name:{0}\tAge:{1}", q.Name, q.Age);
        }
    }
}

查詢語法:

//正序
var query = from u in users orderby u.Name, u.Age select u;

//倒序
query = (from u in users orderby u.Name select u).ThenByDescending(u => u.Age);

運行結果:

 

(3)Reverse操作符只是簡單的把數據源中的數據按照相反的順序返回.(需要注意的是:Reverse在LINQ To SQL中是不支持的).

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Collections;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            //創建測試數據
            List<string> names = new List<string>();
            
            names.Add("Scott");
            names.Add("Dave");
            names.Add("Dave");
            names.Add("Steve");
            names.Add("Joe");

            //使用Reverse操作符對序列進行逆序排列
            names.Reverse();

            foreach (var q in names)
                Console.WriteLine(q);
        }
    }
}

運行結果:

 

04 聯接操作符(join,GroupJoin)

(1)join操作符類似於T-SQL中的inner join,它可以將一個數據源與另一個數據源想聯接,根據兩個數據源中相等的值進行匹配.

方法語法:

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Collections;

namespace ConsoleApplication1
{
    class Program
    {
        class User
        {
            public string Name { set; get; }
            public int RoleId { set; get; }
        }

        class Role 
        {
            public int Id { get; set; }
            public string Name { get; set; }
        }

        static void Main(string[] args)
        {
            //創建測試數據
            List users = new List ();
            List roles = new List ();

            roles.Add(new Role() { Id = 1, Name = "Admin" });
            roles.Add(new Role() { Id = 2, Name = "Guest" });

            users.Add(new User { RoleId = 1, Name = "Scott" });
            users.Add(new User { RoleId = 1, Name = "Dave" });
            users.Add(new User { RoleId = 2, Name = "Steve" });
            users.Add(new User { RoleId = 2, Name = "Joe" });

            //查詢出ID為1的角色下的所有用戶
            var query = users.Join(roles, u => u.RoleId, r => r.Id, (u, r) => new { UserName = u.Name, RoleId = r.Id, RoleName = r.Name }).Where(q => q.RoleId == 1);

            foreach (var q in query)
                Console.WriteLine("UserName:{0}\tRoleId:{1}\tRoleName:{2}", q.UserName, q.RoleId, q.RoleName);
        }
    }
}

查詢語法:

var query = from r in roles
                   join u in users on r.Id equals u.RoleId
                   where r.Id == 1
                   select new { RoleId = r.Id, RoleName = r.Name, UserName = u.Name };

運行結果:

 

(2)GroupJoin操作符類似於T-SQL中的INNER JOIN, LEFT OUTER JOIN,返回的是一個層級的結果集

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Collections;

namespace ConsoleApplication1
{
    class Program
    {
        class User
        {
            public string Name { set; get; }
            public int RoleId { set; get; }
        }

        class Role 
        {
            public int Id { get; set; }
            public string Name { get; set; }
        }

        static void Main(string[] args)
        {
            //創建測試數據
            List users = new List ();
            List roles = new List ();

            roles.Add(new Role() { Id = 1, Name = "Admin" });
            roles.Add(new Role() { Id = 2, Name = "Guest" });

            users.Add(new User { RoleId = 1, Name = "Scott" });
            users.Add(new User { RoleId = 1, Name = "Dave" });
            users.Add(new User { RoleId = 2, Name = "Steve" });
            users.Add(new User { RoleId = 2, Name = "Joe" });

            //查詢所有角色下的用戶
            var query = roles.GroupJoin(users, r => r.Id, u => u.RoleId, (role, user) => new { RoleId = role.Id, RoleName = role.Name, Users = user.Where(u => u.RoleId == role.Id) });

            foreach (var role in query)
            {
                Console.WriteLine("RoleId:{0} RoleName:{1}", role.RoleId, role.RoleName);
                foreach (var user in role.Users)
                {
                    Console.WriteLine("  "+user.Name);
                }
            }

        }
    }
}

運行結果:

 

05 分組操作符(GroupBy)

(1)GroupBy分組操作符是根據一個指定的特定值將序列中的值或元素進行分組.

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Collections;

namespace ConsoleApplication1
{
    class Program
    {
        class User
        {
            public string Name { set; get; }
            public int RoleId { set; get; }
        }

        static void Main(string[] args)
        {
            //創建測試數據
            List users = new List ();
            users.Add(new User { RoleId = 1, Name = "Scott" });
            users.Add(new User { RoleId = 1, Name = "Dave" });
            users.Add(new User { RoleId = 2, Name = "Steve" });
            users.Add(new User { RoleId = 2, Name = "Joe" });

            //按RoleId對所有用戶進行分組
            var query = users.GroupBy(u => u.RoleId);

            foreach (var o in query)
            {
                Console.WriteLine("RoleId:{0}:", o.Key);
                foreach (var user in o)
                {
                    Console.WriteLine("  Name:{0}", user.Name);
                }
            }
        }
    }
}

運行結果:

 

06 串聯操作符(Concat)

(1)Concat操作符可以將兩個對像聯接在一起

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Collections;

namespace ConsoleApplication1
{
    class Program
    {
        class User
        {
            public string Name { set; get; }
            public int RoleId { set; get; }
        }

        class Role 
        {
            public int Id { get; set; }
            public string Name { get; set; }
        }

        static void Main(string[] args)
        {
            //創建測試數據
            List users = new List ();
            List roles = new List ();

            roles.Add(new Role() { Id = 1, Name = "Admin" });
            roles.Add(new Role() { Id = 2, Name = "Guest" });

            users.Add(new User { RoleId = 1, Name = "Scott" });
            users.Add(new User { RoleId = 1, Name = "Dave" });
            users.Add(new User { RoleId = 2, Name = "Steve" });
            users.Add(new User { RoleId = 2, Name = "Joe" });

            //按所有的用戶名稱和角色名稱聯接起來
            var query = users.Select(u => u.Name).Concat(roles.Select(r => r.Name));

            foreach (var q in query)
            {
                Console.WriteLine(q);
            }
        }
    }
}

運行結果:

 

07 聚合操作符(Aggregate,Average,Count,LongCount,Max,Min,Sum)

(1)聚合函數是在序列上執行特定的運算後返回單個值.共有7個聚合查詢操作符:
　 Aggregate:將序列中的值進行累積並返回結果.
    Average:計算一個數值序列的平均值.
    Count,LongCount:計算一個集合中元素的個數.但是前者返回的是Int32類型的值,後者為Int64
    Max,Min:分別是返回序列中的最大值和最小值
    Sum:返回集合中數值的總和

代碼 
//-----------------------------------------------------------
// All Rights Reserved , Copyright (C) 2010 ,黃聰 , Ltd .
//-----------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using System.Collections;

namespace ConsoleApplication1
{
    class Program
    {
        class User
        {
            public string Name { set; get; }
            public int Age { set; get; }
        }

        static void Main(string[] args)
        {
            //創建測試數據
            List users = new List ();

            users.Add(new User { Age = 12, Name = "Scott" });
            users.Add(new User { Age = 22, Name = "Dave" });
            users.Add(new User { Age = 18, Name = "Steve" });
            users.Add(new User { Age = 24, Name = "Joe" });

            string Names = "Scott,Dave,Steve,Joe";
            string[] name = Names.Split(',');
            Console.WriteLine("使用Aggregate將名稱組合起來:");
            string query1 = name.Aggregate((n, next) => n + " " + next);
            Console.WriteLine(query1);
            Console.WriteLine("------------------------------------------------");

            Console.WriteLine("使用Average求出所有用戶年齡的平均數:");
            var query2 = users.Select(u => u.Age).Average();
            Console.WriteLine(query2);
            Console.WriteLine("------------------------------------------------");

            Console.WriteLine("使用Count求出總共有多少個用戶:");
            var query3 = users.Count();
            Console.WriteLine(query3);
            Console.WriteLine("------------------------------------------------");

            Console.WriteLine("使用LongCount求出所有用戶年齡的平均數:");
            var query4 = users.LongCount();
            Console.WriteLine(query4);
            Console.WriteLine("------------------------------------------------");

            Console.WriteLine("使用Max求出所有用戶年齡的最大數:");
            var query5 = users.Select(u => u.Age).Max();
            Console.WriteLine(query5);
            Console.WriteLine("------------------------------------------------");

            Console.WriteLine("使用Min求出所有用戶年齡的最小數:");
            var query6 = users.Select(u => u.Age).Min();
            Console.WriteLine(query6);
            Console.WriteLine("------------------------------------------------");

            Console.WriteLine("使用Sum求出所有用戶年齡的總和:");
            var query7 = users.Select(u => u.Age).Sum();
            Console.WriteLine(query7);
            Console.WriteLine("------------------------------------------------");
        }
    }
}

運行結果:

 

08 集合操作符(Distinct,Union,Intersect,Except)

(1)Distinct操作符可以刪除集合中重複的值,並返回該集合中不相同的元素.

代碼 
//-----------------------------------------------------------
// All Ri
    

    


    






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