C#——LINQ技術DEMO

基本概念

LINQ（Language Integrated Query） ：LINQ(Language Integrated Query)語言集成查詢是一組用於c#和Visual Basic語言的擴展。它允許編寫C#或者Visual Basic代碼以操作內存數據的方式，查詢數據庫。

問題描述

已有Racer類和冠軍車手數據，進行以下查詢，並打印結果：

1）查詢來自英國（UK）的所有世界冠軍，並按勝利場數降序排列。

輸出結果時，可用Console.WriteLine("{0:A}", racer)或WriteLine($"{racer:A}");。

2）使用LINQ方法語法重寫這個查詢。

3）體會LINQ的推遲查詢：首先建立查詢query2，查出所有勝利場數超過25，用foreach循環並打印；然後將所有英國（UK）車手的國名改爲“United Kingdom”；再次對query2用foreach循環並打印。觀賽同一查詢的不同執行結果。

4）查出首發場數超過100，並且勝利場數超過20的車手的姓名（包含名和姓）。

5）基於現有數據，統計德國人的勝利總場數。

6）進行投影查詢，對於駕馭法拉利（Ferrari）車型奪冠的車手，輸出人名、國別與首發勝率，人名包含名和姓，首發勝率爲Wins除以Starts。 

源代碼 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Homework8
{
    public class Racer : IComparable<Racer>, IFormattable
    {
        public Racer(string firstName = null, string lastName = null, string country = null, int starts = 0, int wins = 0, IEnumerable<int> years = null, IEnumerable<string> cars = null)
        {
            this.FirstName = firstName;
            this.LastName = lastName;
            this.Country = country;
            this.Starts = starts;
            this.Wins = wins;


            var yearsList = new List<int>();
            foreach (var year in years)
            {
                yearsList.Add(year);
            }
            this.Years = yearsList.ToArray();
            var carList = new List<string>();
            foreach (var car in cars)
            {
                carList.Add(car);
            }
            this.Cars = carList.ToArray();


        }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Country { get; set; }
        public int Wins { get; set; }//奪冠場數
        public int Starts { get; set; }//首發場數
        public string[] Cars { get; private set; }//賽車手獲得冠軍那一年使用的所有車型
        public int[] Years { get; private set; }//賽車手獲得冠軍的年份

        public override string ToString()
        {
            return String.Format("{0} {1}", FirstName, LastName);
        }

        public int CompareTo(Racer other)
        {
            if (other == null) throw new ArgumentNullException("other");

            return this.LastName.CompareTo(other.LastName);
        }

        public string ToString(string format)
        {
            return ToString(format, null);
        }

        public string ToString(string format,
              IFormatProvider formatProvider)
        {
            switch (format)
            {
                case null:
                case "N":
                    return ToString();
                case "F":
                    return FirstName;
                case "L":
                    return LastName;
                case "C":
                    return Country;
                case "S":
                    return Starts.ToString();
                case "W":
                    return Wins.ToString();
                case "A":
                    return String.Format("{0} {1}, {2}; starts: {3}, wins: {4}",
                          FirstName, LastName, Country, Starts, Wins);
                default:
                    throw new FormatException(String.Format("Format {0} not supported", format));
            }
        }

        public static IList<Racer> GetChampions()//1950-2008年一級方程式錦標賽冠軍
        {
            IList<Racer> racers = new List<Racer>(40);
            racers.Add(new Racer("Nino", "Farina", "Italy", 33, 5, new int[] { 1950 }, new string[] { "Alfa Romeo" }));
            racers.Add(new Racer("Alberto", "Ascari", "Italy", 32, 10, new int[] { 1952, 1953 }, new string[] { "Ferrari" }));
            racers.Add(new Racer("Juan Manuel", "Fangio", "Argentina", 51, 24, new int[] { 1951, 1954, 1955, 1956, 1957 }, new string[] { "Alfa Romeo", "Maserati", "Mercedes", "Ferrari" }));
            racers.Add(new Racer("Mike", "Hawthorn", "UK", 45, 3, new int[] { 1958 }, new string[] { "Ferrari" }));
            racers.Add(new Racer("Phil", "Hill", "USA", 48, 3, new int[] { 1961 }, new string[] { "Ferrari" }));
            racers.Add(new Racer("John", "Surtees", "UK", 111, 6, new int[] { 1964 }, new string[] { "Ferrari" }));
            racers.Add(new Racer("Jim", "Clark", "UK", 72, 25, new int[] { 1963, 1965 }, new string[] { "Lotus" }));
            racers.Add(new Racer("Jack", "Brabham", "Australia", 125, 14, new int[] { 1959, 1960, 1966 }, new string[] { "Cooper", "Brabham" }));
            racers.Add(new Racer("Denny", "Hulme", "New Zealand", 112, 8, new int[] { 1967 }, new string[] { "Brabham" }));
            racers.Add(new Racer("Graham", "Hill", "UK", 176, 14, new int[] { 1962, 1968 }, new string[] { "BRM", "Lotus" }));
            racers.Add(new Racer("Jochen", "Rindt", "Austria", 60, 6, new int[] { 1970 }, new string[] { "Lotus" }));
            racers.Add(new Racer("Jackie", "Stewart", "UK", 99, 27, new int[] { 1969, 1971, 1973 }, new string[] { "Matra", "Tyrrell" }));
            racers.Add(new Racer("Emerson", "Fittipaldi", "Brazil", 143, 14, new int[] { 1972, 1974 }, new string[] { "Lotus", "McLaren" }));
            racers.Add(new Racer("James", "Hunt", "UK", 91, 10, new int[] { 1976 }, new string[] { "McLaren" }));
            racers.Add(new Racer("Mario", "Andretti", "USA", 128, 12, new int[] { 1978 }, new string[] { "Lotus" }));
            racers.Add(new Racer("Jody", "Scheckter", "South Africa", 112, 10, new int[] { 1979 }, new string[] { "Ferrari" }));
            racers.Add(new Racer("Alan", "Jones", "Australia", 115, 12, new int[] { 1980 }, new string[] { "Williams" }));
            racers.Add(new Racer("Keke", "Rosberg", "Finland", 114, 5, new int[] { 1982 }, new string[] { "Williams" }));
            racers.Add(new Racer("Niki", "Lauda", "Austria", 173, 25, new int[] { 1975, 1977, 1984 }, new string[] { "Ferrari", "McLaren" }));
            racers.Add(new Racer("Nelson", "Piquet", "Brazil", 204, 23, new int[] { 1981, 1983, 1987 }, new string[] { "Brabham", "Williams" }));
            racers.Add(new Racer("Ayrton", "Senna", "Brazil", 161, 41, new int[] { 1988, 1990, 1991 }, new string[] { "McLaren" }));
            racers.Add(new Racer("Nigel", "Mansell", "UK", 187, 31, new int[] { 1992 }, new string[] { "Williams" }));
            racers.Add(new Racer("Alain", "Prost", "France", 197, 51, new int[] { 1985, 1986, 1989, 1993 }, new string[] { "McLaren", "Williams" }));
            racers.Add(new Racer("Damon", "Hill", "UK", 114, 22, new int[] { 1996 }, new string[] { "Williams" }));
            racers.Add(new Racer("Jacques", "Villeneuve", "Canada", 165, 11, new int[] { 1997 }, new string[] { "Williams" }));
            racers.Add(new Racer("Mika", "Hakkinen", "Finland", 160, 20, new int[] { 1998, 1999 }, new string[] { "McLaren" }));
            racers.Add(new Racer("Michael", "Schumacher", "Germany", 250, 91, new int[] { 1994, 1995, 2000, 2001, 2002, 2003, 2004 }, new string[] { "Benetton", "Ferrari" }));
            racers.Add(new Racer("Fernando", "Alonso", "Spain", 132, 21, new int[] { 2005, 2006 }, new string[] { "Renault" }));
            racers.Add(new Racer("Kimi", "R?ikk?nen", "Finland", 148, 17, new int[] { 2007 }, new string[] { "Ferrari" }));
            racers.Add(new Racer("Lewis", "Hamilton", "UK", 44, 9, new int[] { 2008 }, new string[] { "McLaren" }));

            return racers;
        }
    }
    /*
    利用LINQ技術查詢Racer對象
    已有Racer類和冠軍車手數據，進行以下查詢，並打印結果：
    1）查詢來自英國（UK）的所有世界冠軍，並按勝利場數降序排列。
    輸出結果時，可用Console.WriteLine("{0:A}", racer)或WriteLine($"{racer:A}");。
    2）使用LINQ方法語法重寫這個查詢。
    3）體會LINQ的推遲查詢：首先建立查詢query2，查出所有勝利場數超過25，用foreach循環並打印；然後將所有英國（UK）車手的國名改爲“United Kingdom”；再次對query2用foreach循環並打印。觀賽同一查詢的不同執行結果。
    4）查出首發場數超過100，並且勝利場數超過20的車手的姓名（包含名和姓）。
    5）基於現有數據，統計德國人的勝利總場數。
    6）進行投影查詢，對於駕馭法拉利（Ferrari）車型奪冠的車手，輸出人名、國別與首發勝率，人名包含名和姓，首發勝率爲Wins除以Starts。 */
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("1）查詢來自英國（UK）的所有世界冠軍，並按勝利場數降序排列。");
            List<Racer> list = new List<Racer>(Racer.GetChampions());
            list.Sort((a,b) => {
                return b.Wins - a.Wins;
            });
            foreach (var racer in list)
            {
                if(racer.Country == "UK")
                    Console.WriteLine("{0:A}", racer);
            }

            Console.WriteLine("2）使用LINQ方法語法重寫這個查詢。");
            var query1 = from r in Racer.GetChampions()
                     where r.Country == "UK"
                     orderby r.Wins descending
                     select r;
            foreach (var racer in query1)
            {
                Console.WriteLine("{0:A}", racer);
            }
            Console.WriteLine("3）體會LINQ的推遲查詢：首先建立查詢query2，查出所有勝利場數超過25，用foreach循環並打印；然後將所有英國（UK）車手的國名改爲“United Kingdom”；再次對query2用foreach循環並打印。觀賽同一查詢的不同執行結果。");
            var query2 = from r in Racer.GetChampions()
                         where r.Wins > 25
                         orderby r.Wins descending
                         select r;
            foreach (var racer in query2)
            {
                Console.WriteLine("{0:A}", racer);
                if (racer.Country == "UK")
                    racer.Country = "United Kingdom";
            }
            foreach (var racer in query2)
            {
                Console.WriteLine("{0:A}", racer);
            }
            Console.WriteLine("4）查出首發場數超過100，並且勝利場數超過20的車手的姓名（包含名和姓）。");

            var query3 = from r in Racer.GetChampions()
                         where r.Starts >100 
                         where r.Wins > 20
                         orderby r.Wins descending
                         select r.FirstName + " " + r.LastName;
            foreach (var racer in query3)
            {
                Console.WriteLine("{0:A}", racer);
            }
            Console.WriteLine("5）基於現有數據，統計德國人的勝利總場數。");

            var query4 = from r in Racer.GetChampions()
                         where r.Country == "Germany"
                         orderby r.Wins descending
                         select r.Wins;
            Console.WriteLine(query4.Sum());
            Console.WriteLine("6）進行投影查詢，對於駕馭法拉利（Ferrari）車型奪冠的車手，輸出人名、國別與首發勝率，人名包含名和姓，首發勝率爲Wins除以Starts。");

            var query5 = from r in Racer.GetChampions()
                         from c in r.Cars
                         where c == "Ferrari"
                         orderby r.Wins descending
                         select new { name = r.FirstName + " " + r.LastName , Country=r.Country, rate=(double)r.Wins/r.Starts };
            foreach (var racer in query5)
            {
                Console.WriteLine("{0:A}", racer);
            }
        }
    }
}

運行結果

參考文章

https://www.pianshen.com/article/159246876/

https://www.cnblogs.com/hackpig/archive/2010/02/15/1668448.html

https://www.cnblogs.com/chquwa/p/3792901.html

https://blog.csdn.net/liujunjie612/article/details/46639417