c# Linq查詢

時間 2020-02-06

標籤 c# linq 查詢欄目 C# 简体版

原文原文鏈接

c#提供的ling查詢極大的遍歷了集合的查詢過程，且使用簡單方便，很是的有用。express

下面將分別用簡單的例子說明：ling基本查詢、延遲查詢屬性、類型篩選、複合from字句、多級排序、分組查詢、聯合查詢、合併、分頁、聚合操做符、並行linq、取消長時間運行的並行ling查詢。c#

Lambda表達式簡介：數組

 /*Lambda表達式:Lambda 表達式是一種可用於建立委託或表達式目錄樹類型的匿名函數
             表達式位於 => 運算符右側的 lambda 表達式稱爲「表達式 lambda」。
             * (input parameters) => expression 
             * 示例：*/
              delegate int del(int i);  
              static void Main(string[] args)
              { 
                   del myDelegate = x => x * x;  
                   int j = myDelegate(5); //最後j = 25  
               }

基本查詢

語句：dom

 var query = from r in listStudents where r.score < 60 orderby r.score descending select r;
 //var q2 = listStudents.Where(r => r.score < 60).OrderByDescending(r => r.score).Select(r => r) ;//或使用Lambda表達式

例子：ide

 /// <summary>
        /// 學生結構體
        /// </summary>
        struct Student
        {
            /// <summary>
            /// 姓名
            /// </summary>
            public string name;
            /// <summary>
            /// 年齡
            /// </summary>
            public int age;
            /// <summary>
            /// 班號
            /// </summary>
            public int grade;
            /// <summary>
            /// 分數
            /// </summary>
            public float score;
        }
/// <summary>
        /// linq
        /// 基本查詢
        /// </summary>
        static void Linq1()
        {
            #region 構造查詢數據
            List<Student> listStudents = new List<Student>();
            Random pRandom = new Random();
            for (int i = 1; i < 50; i++)
            {
                float sc = pRandom.Next(0, 100);
                int age = pRandom.Next(8, 13);
                int gde = pRandom.Next(1, 6);

                string name = "";
                switch (pRandom.Next(0, 6))
                {
                    case 1: name = "周xxx"; break;
                    case 2: name = "李xxx"; break;
                    case 3: name = "孫xxx"; break;
                    case 4: name = "錢xxx"; break;
                    default: name = "趙xxx"; break;
                }

                Student psdt = new Student();
                psdt.name = name;
                psdt.age = age;
                psdt.grade = gde;
                psdt.score = sc;
                listStudents.Add(psdt);
            }
            #endregion
            //從50個學生中選擇出不及格的人員名單並按分數降序排列
            var query = from r in listStudents where r.score < 60 orderby r.score descending select r;
            //var q2 = listStudents.Where(r => r.score < 60).OrderByDescending(r => r.score).Select(r => r) ;//或使用Lambda表達式
            //orderby升序即從小到大，orderby r.score descending降序即從大到小

            Console.WriteLine("不及格的人員名單：");
            foreach (Student st in query)
            {
                Console.WriteLine("***************");
                Console.WriteLine("姓名："+st.name);
                Console.WriteLine("班級："+st.grade);
                Console.WriteLine("年齡："+st.age);
                Console.WriteLine("分數："+st.score);
            }
            Console.ReadKey();
        }

View Code

延遲查詢屬性

linq查詢爲延遲查詢，只需構造一次查詢語句，能夠屢次使用。函數

例子：spa

/// <summary>
        /// Linq
        /// 延遲查詢
        /// linq查詢爲延遲查詢，只需構造一次查詢語句，能夠屢次使用
        /// </summary>
        static void Linq2()
        {
            #region 構造查詢數據
            List<string> lists = new List<string> { "Jack","Pet","Hant","Li","Kkk"};
            
            #endregion
            var query = from r in lists where r.StartsWith("J") select r;
            Console.WriteLine("第一次查詢結果：");
            foreach (string st in query)
            {
                Console.WriteLine( st);
            }

            Console.WriteLine("第二次查詢結果：");
            lists.Add("Jone");
            lists.Add("Jimi");
            lists.Add("Johu");
            foreach (string st in query)
            {
                Console.WriteLine(st);
            }

            Console.ReadKey();
            /*
             輸出結果：
             * 第一次：Jack
             * 第二次：Jack Jone Jimi Johu
             */
        }

類型篩選

利用OfType方法能夠把特定類型數據篩選出來。線程

例子：3d

 /// <summary>
        /// Linq類型篩選-ofType
        /// </summary>
        static void Linq3()
        {
            object[] pList = new object[] { 1,"one",2,"two",3,"three"};
            var query = pList.OfType<string>();
            foreach (var st in query)
            {
                Console.WriteLine(st);
            }
            Console.ReadKey();
            /*
            輸出結果：
            * one two three
            */
        }

複合from子句

語句：code

 var query = from s in listStudents 
                        from n in s.name 
                        where n == '趙' orderby s.score descending 
                        select s.grade + "班-" + s.name + "，分數" + +s.score;
            /*查詢出全部姓趙的學生的班級、姓名、分數信息，並按分數由高到低排序*/

例子：

 /// <summary>
        /// linq複合from字句
        /// </summary>
        static void Linq4()
        {
            #region 構造查詢數據
            List<Student> listStudents = new List<Student>();
            Random pRandom = new Random();
            for (int i = 1; i < 50; i++)
            {
                float sc = pRandom.Next(0, 100);
                int age = pRandom.Next(8, 13);
                int gde = pRandom.Next(1, 6);

                string name = "";
                switch (pRandom.Next(0, 6))
                {
                    case 1: name = "周xxx"; break;
                    case 2: name = "李xxx"; break;
                    case 3: name = "孫xxx"; break;
                    case 4: name = "錢xxx"; break;
                    default: name = "趙xxx"; break;
                }

                Student psdt = new Student();
                psdt.name = name;
                psdt.age = age;
                psdt.grade = gde;
                psdt.score = sc;
                listStudents.Add(psdt);
            }
            #endregion
            var query = from s in listStudents 
                        from n in s.name 
                        where n == '趙' orderby s.score descending 
                        select s.grade + "班-" + s.name + "，分數" + +s.score;
            /*查詢出全部姓趙的學生的班級、姓名、分數信息，並按分數由高到低排序*/
            foreach (var t in query)
            {
                Console.WriteLine(t);
            }
            Console.ReadKey();
        }

View Code

多級排序

語句：

 //參數越靠前，優先級越高
            //先按score排序，當分數相同時再按grade排序...依次類推
            var query = from s in listStudents orderby s.score, s.grade, s.age, s.name select s;

例子：

 /// <summary>
        /// 多級排序
        /// </summary>
        static void Linq5()
        {
            #region 構造查詢數據
            List<Student> listStudents = new List<Student>();
            Random pRandom = new Random();
            for (int i = 1; i < 50; i++)
            {
                float sc = pRandom.Next(0, 100);
                int age = pRandom.Next(8, 13);
                int gde = pRandom.Next(1, 6);

                string name = "";
                switch (pRandom.Next(0, 6))
                {
                    case 1: name = "周xxx"; break;
                    case 2: name = "李xxx"; break;
                    case 3: name = "孫xxx"; break;
                    case 4: name = "錢xxx"; break;
                    default: name = "趙xxx"; break;
                }

                Student psdt = new Student();
                psdt.name = name;
                psdt.age = age;
                psdt.grade = gde;
                psdt.score = sc;
                listStudents.Add(psdt);
            }
            #endregion
            //參數越靠前，優先級越高
            //先按score排序，當分數相同時再按grade排序...依次類推
            var query = from s in listStudents orderby s.score, s.grade, s.age, s.name select s;
            foreach (Student st in query)
            {
                Console.WriteLine("***************");
                Console.WriteLine("姓名：" + st.name);
                Console.WriteLine("班級：" + st.grade);
                Console.WriteLine("年齡：" + st.age);
                Console.WriteLine("分數：" + st.score);
            }
            Console.ReadKey();
        }

View Code

分組

語句：

 //按國家分組，並選出大於2的組，造成新的集合
            var query = from r in listChampion
                        group r by r.country into g
                        orderby g.Count() descending, g.Key
                        where g.Count() >= 2
                        select
                            new { country = g.Key, count = g.Count() };

例子：

 /// <summary>
        /// 分組
        /// </summary>
        static void Linq6()
        {
            List<Champions> listChampion = new List<Champions>();
            listChampion.Add(new Champions() { name = "張**", country = "中國" });
            listChampion.Add(new Champions() { name = "趙**", country = "中國" });
            listChampion.Add(new Champions() { name = "李**", country = "中國" });
             listChampion.Add(new Champions() { name = "李**", country = "中國" });
            listChampion.Add(new Champions() { name = "Peter", country = "美國" });
            listChampion.Add(new Champions() { name = "Hune", country = "美國" });
              listChampion.Add(new Champions() { name = "Hune", country = "美國" });
            listChampion.Add(new Champions() { name = "Jack", country = "俄羅斯" });
              listChampion.Add(new Champions() { name = "Jack", country = "俄羅斯" });
            listChampion.Add(new Champions() { name = "Jimi", country = "英國" });
            //按國家分組，並選出大於2的組，造成新的集合
            var query = from r in listChampion
                        group r by r.country into g
                        orderby g.Count() descending, g.Key
                        where g.Count() >= 2
                        select
                            new { country = g.Key, count = g.Count() };
            foreach (var o in query)
            {
                Console.WriteLine("國家：{0}，冠軍數：{1}個", o.country, o.count);
            }
            Console.ReadKey();
        }

聯合查詢-join

語句：

 //查詢出集合qSt中year等於集合qSc中year的元素並造成新的集合
            var qjoin = from r in qSt 
                        join c in qSc 
                        on r.year equals c.year 
                        select new 
                        {
                            Year = r.year, 
                            stName = r.name,
                            scName = c.name 
                        };

例子：

  /// <summary>
        /// 聯合查詢-join
        /// </summary>
        static void Linq7()
        {
            List<s> pSt = new List<s>();
            pSt.Add(new s() {year=1999,name="xxx" });
            pSt.Add(new s() { year = 2000, name = "xxx" });
            pSt.Add(new s() { year = 2001, name = "xxx" });
            pSt.Add(new s() { year = 2010, name = "xxx" });
            List<school> pSc = new List<school>();
            pSc.Add(new school() { year = 1999, name = "***" });
             pSc.Add(new school() { year = 2001, name = "***" });
            pSc.Add(new school() { year = 2002, name = "***" });
            pSc.Add(new school() { year = 2010, name = "***" });
            pSc.Add(new school() { year = 2012, name = "***" });

            var qSt = from r in pSt where r.year >= 2000 select r;
            var qSc = from r in pSc where r.year >= 2000 select r;

            //查詢出集合qSt中year等於集合qSc中year的元素並造成新的集合
            var qjoin = from r in qSt 
                        join c in qSc 
                        on r.year equals c.year 
                        select new 
                        {
                            Year = r.year, 
                            stName = r.name,
                            scName = c.name 
                        };

            foreach (var ite in qjoin)
            {
                Console.WriteLine(ite.Year + " " + ite.scName + " " + ite.stName);
                Console.WriteLine("");
            }
            Console.ReadKey();
        }

合併-zip()

.Net4.0新增，可對兩個相關的序列進行合併。

語句：

  /*若合併兩項項數不一樣，則在達到較小集合的末尾時中止*/
            var qZip = qSc.Zip(qSt, (first, second) =>string.Format("Year:{0},Name:{1}.", first.Year,second.Name));//返回值qZip爲字符串集合

例子：

 /// <summary>
        /// 合併-zip()-.Net4.0新增，可對兩個相關的序列進行合併
        /// </summary>
        static void Linq8()
        {
            List<s> pSt = new List<s>();
            pSt.Add(new s() { year = 1999, name = "一xxx" });
            pSt.Add(new s() { year = 2000, name = "二xxx" });
            pSt.Add(new s() { year = 2001, name = "三xxx" });
            pSt.Add(new s() { year = 2010, name = "四xxx" });
            pSt.Add(new s() { year = 2010, name = "五xxx" });
            List<school> pSc = new List<school>();
            pSc.Add(new school() { year = 1999, name = "1***" });
            pSc.Add(new school() { year = 2012, name = "2***" });
            pSc.Add(new school() { year = 2012, name = "3***" });
            pSc.Add(new school() { year = 2012, name = "4***" });
            pSc.Add(new school() { year = 2012, name = "5***" });

            var qSt = from r in pSt where r.year >= 2000 orderby r.year select new { Year = r.year, Name = r.name };
            var qSc = from r in pSc where r.year >= 2000 orderby r.year select new { Year = r.year, Name = r.name };
            /*若合併兩項項數不一樣，則在達到較小集合的末尾時中止*/
            var qZip = qSc.Zip(qSt, (first, second) =>string.Format("Year:{0},Name:{1}.", first.Year,second.Name));//返回值qZip爲字符串集合
           
            foreach (var ite in qZip)
            {
                Console.WriteLine(ite);
            }
            Console.ReadKey();

        }

分區（分頁）

經過Take（）和Skip（）實現只顯示部分查詢結果。

語句：

 //經過skip跳過指定數量的元素，再經過take提取固定長度元素，可實現分頁
                var qr = (from r in listStudents orderby r.score descending select r).Skip(i * pageSize).Take(5);
                //var qr2 = listStudents.OrderByDescending(r => r.score).Select(r => r).Skip(i * pageSize).Take(5);//或

例子

 /// <summary>
        /// 分區（分頁）-經過Take（）和Skip（）實現只顯示部分查詢結果
        /// </summary>
        static void Linq9()
        {
            #region 構造查詢數據
            List<Student> listStudents = new List<Student>();
            Random pRandom = new Random();
            for (int i = 1; i < 50; i++)
            {
                float sc = pRandom.Next(0, 100);
                int age = pRandom.Next(8, 13);
                int gde = pRandom.Next(1, 6);

                string name = "";
                switch (pRandom.Next(0, 6))
                {
                    case 1: name = "周xxx"; break;
                    case 2: name = "李xxx"; break;
                    case 3: name = "孫xxx"; break;
                    case 4: name = "錢xxx"; break;
                    default: name = "趙xxx"; break;
                }

                Student psdt = new Student();
                psdt.name = name;
                psdt.age = age;
                psdt.grade = gde;
                psdt.score = sc;
                listStudents.Add(psdt);
            }
            #endregion

            int pageSize = 5;
            int numofPage = (int)Math.Ceiling(listStudents.Count /(double)pageSize);

            for (int i = 0; i < numofPage; i++)
            {
                Console.WriteLine("第{0}頁", i);

                //經過skip跳過指定數量的元素，再經過take提取固定長度元素，可實現分頁
                var qr = (from r in listStudents orderby r.score descending select r).Skip(i * pageSize).Take(5);
                //var qr2 = listStudents.OrderByDescending(r => r.score).Select(r => r).Skip(i * pageSize).Take(5);//或
                foreach (var ite in qr)
                {
                    Console.WriteLine(ite.name);
                    Console.WriteLine(ite.score);
                }
                Console.WriteLine("");
            }
            Console.ReadKey();
        }

View Code

並行linq

並行查詢能夠分解查詢的工做，使其分佈在多個線程上。當pc擁有多個cpu時，能夠看到並行查詢帶來的改進效果。並行LINQ適用於大型的集合查詢，並擁有必定的優點。使用

System.Collections.Concurrent.Partitioner.Create
能夠手動建立分區器。

語法：

var sum2 = (from x2 in data.AsParallel() where x2 > 20 select x2).Sum();//並行查詢

//var sum3 = data.AsParallel().Where(x3 => x3 > 20).Sum();//或並行查詢（Lambda表達式）

例子：

 /// <summary>
        /// 並行linq
        /// </summary>
        static void Linq11()
        {
            Console.WriteLine("開始構造大數組...");
            //構造大數組
            const int count = 100000000;
            var data = new int[count];
            var r = new Random();
            for (int i = 0; i < count; i++)
            {
                data[i] = r.Next(40);
            }
            Console.WriteLine("開始計算...");
            var st = System.DateTime.Now;
            var sum = (from x in data where x > 20 select x).Sum();//常規linq-耗時1.8641s
            var st2 = System.DateTime.Now;
            var sum2 = (from x2 in data.AsParallel() where x2 > 20 select x2).Sum();//並行查詢-耗時0.6620s

            //var sum3 = data.AsParallel().Where(x3 => x3 > 20).Sum();//或並行查詢----x3 => x3 > 20（Lambda表達式）
            var st3 = System.DateTime.Now;

            /*Partitioner.Create
             手動建立分區器
             * Create具備多個重載，可依據需求進行分區
             */
            var sum4 = (from c in System.Collections.Concurrent.Partitioner.Create(data, true).AsParallel() where c > 20 select c).Sum();

            var dt1 = st2 - st;
            var dt2 = st3 - st2;
            Console.WriteLine("常規linq耗時：{0}s",dt1.TotalSeconds.ToString());
            Console.WriteLine("並行linq耗時：{0}s", dt2.TotalSeconds.ToString());
            Console.ReadKey();
        }

輸出結果：

能夠粗魯的認爲並行linq對於大集合的查詢是優點比較明顯的。

取消長時間運行的並行linq查詢

對於並行ling而言，能夠設置能夠利用System.Threading.CancellationTokenSource設置取消操做。

語法：

 //經過.WithCancellation(cts.Token)設置
                    var sum = (from x in data.AsParallel().WithCancellation(cts.Token) where x < 20 select x).Average();

例子：

 /// <summary>
        /// 取消長時間運行的並行linq查詢
        /// </summary>
        static void Linq12()
        {
            //構造大數組
            const int count = 100000000;
            var data = new int[count];
            var r = new Random();
            for (int i = 0; i < count; i++)
            {
                data[i] = r.Next(40);
            }

            var cts = new System.Threading.CancellationTokenSource();

            new System.Threading.Thread(() =>
            {
                try
                {
                    //經過.WithCancellation(cts.Token)設置
                    var sum = (from x in data.AsParallel().WithCancellation(cts.Token) where x < 20 select x).Average();
                    Console.WriteLine("sum:{0}", sum);
                }
                catch(OperationCanceledException ex)
                {
                    Console.WriteLine(ex.Message);
                }
            }).Start();

            Console.WriteLine("計算開始...");
            Console.WriteLine("取消請按n！");
            var rk = Console.Read();
            if(rk=='n'||rk=='N')
            {
                cts.Cancel();
            }

        }

至此，對於LINQ已經有了一個基本的瞭解。

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