Linq(1.延迟的查询执行)

Linq的概念

什么是linq？

C#集成了查询语法，可以使用相同的语法查询不同的数据源。Linq提供了不同数据源的抽象层,所以可以使用相同的语法。如：Linq可以查询数据库(EF),还可以查询XML，json等。

使用linq的好处？

使用相同的语法查询不同的数据源

学习Linq的初始数据准备

Racer.cs

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace ConsoleApplication37{    [Serializable]    public class Racer : IComparable<Racer>, IFormattable    {        public Racer(string firstName, string lastName, string country, int starts, int wins)            : this(firstName, lastName, country, starts, wins, null, null)        {        }        public Racer(string firstName, string lastName, string country, int starts, int wins, IEnumerable<int> years, IEnumerable<string> cars)        {            this.FirstName = firstName;            this.LastName = lastName;            this.Country = country;            this.Starts = starts;            this.Wins = wins;            this.Years = new List<int>(years);            this.Cars = new List<string>(cars);        }        public string FirstName { get; set; }        public string LastName { get; set; }        public string Country { get; set; }        /// <summary>        /// 胜利的次数        /// </summary>        public int Wins { get; set; }        public int Starts { get; set; }        /// <summary>        /// 赛车手获得冠军所使用的车型        /// </summary>        public IEnumerable<string> Cars { get; private set; }        /// <summary>        /// 赛车手获得冠军的年份        /// </summary>        public IEnumerable<int> Years { get; private set; }        public override string ToString()        {            return String.Format("{0} {1}", FirstName, LastName);        }        public int CompareTo(Racer other)        {            if (other == null) return -1;            return string.Compare(this.LastName, 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));            }        }    }}

Team.cs

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace ConsoleApplication37{    [Serializable]    public class Team    {        public Team(string name, params int[] years)        {            this.Name = name;            this.Years = new List<int>(years);        }        public string Name { get; private set; }        public IEnumerable<int> Years { get; private set; }    }}

Championship.cs

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace ConsoleApplication37{    public class Championship    {        public int Year { get; set; }        public string First { get; set; }        public string Second { get; set; }        public string Third { get; set; }    }}

Formula1.cs

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace ConsoleApplication37{    public static class Formula1    {        private static List<Racer> racers;        /// <summary>        /// 返回一组赛车手，这个列表包含1950-2011年之间所有冠军        /// </summary>        /// <returns></returns>        public static IList<Racer> GetChampions()        {            if (racers == null)            {                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", 287, 91, new int[] { 1994, 1995, 2000, 2001, 2002, 2003, 2004 }, new string[] { "Benetton", "Ferrari" }));                racers.Add(new Racer("Fernando", "Alonso", "Spain", 177, 27, new int[] { 2005, 2006 }, new string[] { "Renault" }));                racers.Add(new Racer("Kimi", "Räikkönen", "Finland", 148, 17, new int[] { 2007 }, new string[] { "Ferrari" }));                racers.Add(new Racer("Lewis", "Hamilton", "UK", 90, 17, new int[] { 2008 }, new string[] { "McLaren" }));                racers.Add(new Racer("Jenson", "Button", "UK", 208, 12, new int[] { 2009 }, new string[] { "Brawn GP" }));                racers.Add(new Racer("Sebastian", "Vettel", "Germany", 81, 21, new int[] { 2010, 2011 }, new string[] { "Red Bull Racing" }));            }            return racers;        }        private static List<Team> teams;        /// <summary>        /// 返回所有车队冠军列表        /// </summary>        /// <returns></returns>        public static IList<Team> GetContructorChampions()        {            if (teams == null)            {                teams = new List<Team>()                {                    new Team("Vanwall", 1958),                    new Team("Cooper", 1959, 1960),                    new Team("Ferrari", 1961, 1964, 1975, 1976, 1977, 1979, 1982, 1983, 1999, 2000, 2001, 2002, 2003, 2004, 2007, 2008),                    new Team("BRM", 1962),                    new Team("Lotus", 1963, 1965, 1968, 1970, 1972, 1973, 1978),                    new Team("Brabham", 1966, 1967),                    new Team("Matra", 1969),                    new Team("Tyrrell", 1971),                    new Team("McLaren", 1974, 1984, 1985, 1988, 1989, 1990, 1991, 1998),                    new Team("Williams", 1980, 1981, 1986, 1987, 1992, 1993, 1994, 1996, 1997),                    new Team("Benetton", 1995),                    new Team("Renault", 2005, 2006 ),                    new Team("Brawn GP", 2009),                    new Team("Red Bull Racing", 2010, 2011)                };            }            return teams;        }        private static List<Championship> championships;        public static IEnumerable<Championship> GetChampionships()        {            if (championships == null)            {                championships = new List<Championship>();                championships.Add(new Championship                {                    Year = 1950,                    First = "Nino Farina",                    Second = "Juan Manuel Fangio",                    Third = "Luigi Fagioli"                });                championships.Add(new Championship                {                    Year = 1951,                    First = "Juan Manuel Fangio",                    Second = "Alberto Ascari",                    Third = "Froilan Gonzalez"                });                championships.Add(new Championship                {                    Year = 1952,                    First = "Alberto Ascari",                    Second = "Nino Farina",                    Third = "Piero Taruffi"                });                championships.Add(new Championship                {                    Year = 1953,                    First = "Alberto Ascari",                    Second = "Juan Manuel Fangio",                    Third = "Nino Farina"                });                championships.Add(new Championship                {                    Year = 1954,                    First = "Juan Manuel Fangio",                    Second = "Froilan Gonzalez",                    Third = "Mike Hawthorn"                });                championships.Add(new Championship                {                    Year = 1955,                    First = "Juan Manuel Fangio",                    Second = "Stirling Moss",                    Third = "Eugenio Castellotti"                });                championships.Add(new Championship                {                    Year = 1956,                    First = "Juan Manuel Fangio",                    Second = "Stirling Moss",                    Third = "Peter Collins"                });                championships.Add(new Championship                {                    Year = 1957,                    First = "Juan Manuel Fangio",                    Second = "Stirling Moss",                    Third = "Luigi Musso"                });                championships.Add(new Championship                {                    Year = 1958,                    First = "Mike Hawthorn",                    Second = "Stirling Moss",                    Third = "Tony Brooks"                });                championships.Add(new Championship                {                    Year = 1959,                    First = "Jack Brabham",                    Second = "Tony Brooks",                    Third = "Stirling Moss"                });                championships.Add(new Championship                {                    Year = 1960,                    First = "Jack Brabham",                    Second = "Bruce McLaren",                    Third = "Stirling Moss"                });                championships.Add(new Championship                {                    Year = 1961,                    First = "Phil Hill",                    Second = "Wolfgang von Trips",                    Third = "Stirling Moss"                });                championships.Add(new Championship                {                    Year = 1962,                    First = "Graham Hill",                    Second = "Jim Clark",                    Third = "Bruce McLaren"                });                championships.Add(new Championship                {                    Year = 1963,                    First = "Jim Clark",                    Second = "Graham Hill",                    Third = "Richie Ginther"                });                championships.Add(new Championship                {                    Year = 1964,                    First = "John Surtees",                    Second = "Graham Hill",                    Third = "Jim Clark"                });                championships.Add(new Championship                {                    Year = 1965,                    First = "Jim Clark",                    Second = "Graham Hill",                    Third = "Jackie Stewart"                });                championships.Add(new Championship                {                    Year = 1966,                    First = "Jack Brabham",                    Second = "John Surtees",                    Third = "Jochen Rindt"                });                championships.Add(new Championship                {                    Year = 1967,                    First = "Dennis Hulme",                    Second = "Jack Brabham",                    Third = "Jim Clark"                });                championships.Add(new Championship                {                    Year = 1968,                    First = "Graham Hill",                    Second = "Jackie Stewart",                    Third = "Dennis Hulme"                });                championships.Add(new Championship                {                    Year = 1969,                    First = "Jackie Stewart",                    Second = "Jackie Ickx",                    Third = "Bruce McLaren"                });                championships.Add(new Championship                {                    Year = 1970,                    First = "Jochen Rindt",                    Second = "Jackie Ickx",                    Third = "Clay Regazzoni"                });                championships.Add(new Championship                {                    Year = 1971,                    First = "Jackie Stewart",                    Second = "Ronnie Peterson",                    Third = "Francois Cevert"                });                championships.Add(new Championship                {                    Year = 1972,                    First = "Emerson Fittipaldi",                    Second = "Jackie Stewart",                    Third = "Dennis Hulme"                });                championships.Add(new Championship                {                    Year = 1973,                    First = "Jackie Stewart",                    Second = "Emerson Fittipaldi",                    Third = "Ronnie Peterson"                });                championships.Add(new Championship                {                    Year = 1974,                    First = "Emerson Fittipaldi",                    Second = "Clay Regazzoni",                    Third = "Jody Scheckter"                });                championships.Add(new Championship                {                    Year = 1975,                    First = "Niki Lauda",                    Second = "Emerson Fittipaldi",                    Third = "Carlos Reutemann"                });                championships.Add(new Championship                {                    Year = 1976,                    First = "James Hunt",                    Second = "Niki Lauda",                    Third = "Jody Scheckter"                });                championships.Add(new Championship                {                    Year = 1977,                    First = "Niki Lauda",                    Second = "Jody Scheckter",                    Third = "Mario Andretti"                });                championships.Add(new Championship                {                    Year = 1978,                    First = "Mario Andretti",                    Second = "Ronnie Peterson",                    Third = "Carlos Reutemann"                });                championships.Add(new Championship                {                    Year = 1979,                    First = "Jody Scheckter",                    Second = "Gilles Villeneuve",                    Third = "Alan Jones"                });                championships.Add(new Championship                {                    Year = 1980,                    First = "Alan Jones",                    Second = "Nelson Piquet",                    Third = "Carlos Reutemann"                });                championships.Add(new Championship                {                    Year = 1981,                    First = "Nelson Piquet",                    Second = "Carlos Reutemann",                    Third = "Alan Jones"                });                championships.Add(new Championship                {                    Year = 1982,                    First = "Keke Rosberg",                    Second = "Didier Pironi",                    Third = "John Watson"                });                championships.Add(new Championship                {                    Year = 1983,                    First = "Nelson Piquet",                    Second = "Alain Prost",                    Third = "Rene Arnoux"                });                championships.Add(new Championship                {                    Year = 1984,                    First = "Niki Lauda",                    Second = "Alain Prost",                    Third = "Elio de Angelis"                });                championships.Add(new Championship                {                    Year = 1985,                    First = "Alain Prost",                    Second = "Michele Alboreto",                    Third = "Keke Rosberg"                });                championships.Add(new Championship                {                    Year = 1986,                    First = "Alain Prost",                    Second = "Nigel Mansell",                    Third = "Nelson Piquet"                });                championships.Add(new Championship                {                    Year = 1987,                    First = "Nelson Piquet",                    Second = "Nigel Mansell",                    Third = "Ayrton Senna"                });                championships.Add(new Championship                {                    Year = 1988,                    First = "Ayrton Senna",                    Second = "Alain Prost",                    Third = "Gerhard Berger"                });                championships.Add(new Championship                {                    Year = 1989,                    First = "Alain Prost",                    Second = "Ayrton Senna",                    Third = "Riccardo Patrese"                });                championships.Add(new Championship                {                    Year = 1990,                    First = "Ayrton Senna",                    Second = "Alain Prost",                    Third = "Nelson Piquet"                });                championships.Add(new Championship                {                    Year = 1991,                    First = "Ayrton Senna",                    Second = "Nigel Mansell",                    Third = "Riccardo Patrese"                });                championships.Add(new Championship                {                    Year = 1992,                    First = "Nigel Mansell",                    Second = "Riccardo Patrese",                    Third = "Michael Schumacher"                });                championships.Add(new Championship                {                    Year = 1993,                    First = "Alain Prost",                    Second = "Ayrton Senna",                    Third = "Damon Hill"                });                championships.Add(new Championship                {                    Year = 1994,                    First = "Michael Schumacher",                    Second = "Damon Hill",                    Third = "Gerhard Berger"                });                championships.Add(new Championship                {                    Year = 1995,                    First = "Michael Schumacher",                    Second = "Damon Hill",                    Third = "David Coulthard"                });                championships.Add(new Championship                {                    Year = 1996,                    First = "Damon Hill",                    Second = "Jacques Villeneuve",                    Third = "Michael Schumacher"                });                championships.Add(new Championship                {                    Year = 1997,                    First = "Jacques Villeneuve",                    Second = "Heinz-Harald Frentzen",                    Third = "David Coulthard"                });                championships.Add(new Championship                {                    Year = 1998,                    First = "Mika Hakkinen",                    Second = "Michael Schumacher",                    Third = "David Coulthard"                });                championships.Add(new Championship                {                    Year = 1999,                    First = "Mika Hakkinen",                    Second = "Eddie Irvine",                    Third = "Heinz-Harald Frentzen"                });                championships.Add(new Championship                {                    Year = 2000,                    First = "Michael Schumacher",                    Second = "Mika Hakkinen",                    Third = "David Coulthard"                });                championships.Add(new Championship                {                    Year = 2001,                    First = "Michael Schumacher",                    Second = "David Coulthard",                    Third = "Rubens Barrichello"                });                championships.Add(new Championship                {                    Year = 2002,                    First = "Michael Schumacher",                    Second = "Rubens Barrichello",                    Third = "Juan Pablo Montoya"                });                championships.Add(new Championship                {                    Year = 2003,                    First = "Michael Schumacher",                    Second = "Kimi Räikkönen",                    Third = "Juan Pablo Montoya"                });                championships.Add(new Championship                {                    Year = 2004,                    First = "Michael Schumacher",                    Second = "Rubens Barrichello",                    Third = "Jenson Button"                });                championships.Add(new Championship                {                    Year = 2005,                    First = "Fernando Alonso",                    Second = "Kimi Räikkönen",                    Third = "Michael Schumacher"                });                championships.Add(new Championship                {                    Year = 2006,                    First = "Fernando Alonso",                    Second = "Michael Schumacher",                    Third = "Felipe Massa"                });                championships.Add(new Championship                {                    Year = 2007,                    First = "Kimi Räikkönen",                    Second = "Lewis Hamilton",                    Third = "Fernando Alonso"                });                championships.Add(new Championship                {                    Year = 2008,                    First = "Lewis Hamilton",                    Second = "Felipe Massa",                    Third = "Kimi Raikkonen"                });                championships.Add(new Championship                {                    Year = 2009,                    First = "Jenson Button",                    Second = "Sebastian Vettel",                    Third = "Rubens Barrichello"                });                championships.Add(new Championship                {                    Year = 2010,                    First = "Sebastian Vettel",                    Second = "Fernando Alonso",                    Third = "Mark Webber"                });                championships.Add(new Championship                {                    Year = 2011,                    First = "Sebastian Vettel",                    Second = "Jenson Button",                    Third = "Mark Webber"                });            }            return championships;        }        private static IList<Racer> moreRacers;        /// <summary>        /// 返回一组赛车手，这个列表包含没有获得过冠军的赛车手        /// </summary>        /// <returns></returns>        private static IList<Racer> GetMoreRacers()        {            if (moreRacers == null)            {                moreRacers = new List<Racer>();                moreRacers.Add(new Racer(lastName:"Luigi", firstName:"Fagioli",country: "Italy", starts: 7, wins: 1));                moreRacers.Add(new Racer("Jose Froilan", "Gonzalez", "Argentina", starts: 26, wins: 2));                moreRacers.Add(new Racer("Piero", "Taruffi", "Italy", starts: 18, wins: 1));                moreRacers.Add(new Racer("Stirling", "Moss", "UK", starts: 66, wins: 16));                moreRacers.Add(new Racer("Eugenio", "Castellotti", "Italy", starts: 14, wins: 0));                moreRacers.Add(new Racer("Peter", "Collins", "UK", starts: 32, wins: 3));                moreRacers.Add(new Racer("Luigi", "Musso", "Italy", starts: 24, wins: 1));                moreRacers.Add(new Racer("Tony", "Brooks", "UK", starts: 38, wins: 6));                moreRacers.Add(new Racer("Bruce", "McLaren", "New Zealand", starts: 100, wins: 4));                moreRacers.Add(new Racer("Wolfgang von", "Trips", "Germany", starts: 27, wins: 2));                moreRacers.Add(new Racer("Richie", "Ginther", "USA", starts: 52, wins: 1));                moreRacers.Add(new Racer("Jackie", "Ickx", "Belgium", starts: 116, wins: 8));                moreRacers.Add(new Racer("Clay", "Regazzoni", "Switzerland", starts: 132, wins: 5));                moreRacers.Add(new Racer("Ronnie", "Peterson", "Sweden", starts: 123, wins: 10));                moreRacers.Add(new Racer("Francois", "Cevert", "France", starts: 46, wins: 1));                moreRacers.Add(new Racer("Carlos", "Reutemann", "Argentina", starts: 146, wins: 12));                moreRacers.Add(new Racer("Gilles", "Villeneuve", "Canada", starts: 67, wins: 6));                moreRacers.Add(new Racer("Didier", "Pironi", "France", starts: 70, wins: 3));                moreRacers.Add(new Racer("John", "Watson", "UK", starts: 152, wins: 5));                moreRacers.Add(new Racer("Rene", "Arnoux", "France", starts: 149, wins: 7));                moreRacers.Add(new Racer("Elio", "de Angelis", "Italy", starts: 108, wins: 2));                moreRacers.Add(new Racer("Michele", "Alboreto", "Italy", starts: 194, wins: 5));                moreRacers.Add(new Racer("Gerhard", "Berger", "Austria", starts: 210, wins: 10));                moreRacers.Add(new Racer("Riccardo", "Patrese", "Italy", starts: 256, wins: 6));                moreRacers.Add(new Racer("David", "Coulthard", "UK", starts: 246, wins: 13));                moreRacers.Add(new Racer("Heinz-Harald", "Frentzen", "Germany", starts: 156, wins: 3));                moreRacers.Add(new Racer("Eddie", "Irvine", "UK", starts: 147, wins: 4));                moreRacers.Add(new Racer("Rubens", "Barrichello", "Brazil", starts: 322, wins: 11));                moreRacers.Add(new Racer("Juan Pablo", "Montoya", "Columbia", starts: 94, wins: 7));                moreRacers.Add(new Racer("Felipe", "Massa", "Brazil", starts: 152, wins: 11));                moreRacers.Add(new Racer("Mark", "Webber", "Australia", starts: 176, wins: 7));            }            return moreRacers;        }    }}

Linq的简单查询

linq查询所有来自巴西的冠军赛车手，按夺冠次数排序

        static void Main(string[] args)        {            //得到所有冠军赛车手            IList<Racer> champions_racers = Formula1.GetChampions();            //linq查询所有来自巴西的冠军赛车手，按夺冠次数排序            var query = from r in champions_racers where r.Country == "Brazil" orderby r.Wins descending select r;            foreach (var item in query)            {                Console.WriteLine(item.ToString("A"));            }        }

语法介绍：
1. 必须以from开头，select或者group结尾
2. 两个子句之间可以使用任何where、join、orderby、desceding、left和其他from等子句
3. Linq语句最终被转换为调用IEnumerable<T>的扩展方法，在System.Linq.Enumerable静态类中定义了N多扩展。所以只要继承与IEnumerable的类都支持Linq查询
4. 上续代码不是通过赋值语句执行查询的，而是通过foreach遍历才执行查询的，具体可见下节延迟的查询执行。

延迟执行查询

上节提到，Linq的执行不是在Linq的赋值语句执行，而是在访问结果时执行。

查询一组集合中以J字母开头的元素并进行升序排列。

        static void DeferredQuery()        {            var names = new List<string> { "Nino", "Alberto", "Juan", "Mike", "Phil" };            var namesWithJ = from n in names                             where n.StartsWith("J")                             orderby n                             select n;            Console.WriteLine("First iteration");            foreach (string name in namesWithJ)            {                Console.WriteLine(name);            }            Console.WriteLine();            names.Add("John");            names.Add("Jim");            names.Add("Jack");            names.Add("Denny");            Console.WriteLine("Second iteration");            foreach (string name in namesWithJ)            {                Console.WriteLine(name);            }        }

输出：

从代码理解可以看到，两次遍历的结果不一样，说明执行并不是在Linq的定义语句执行，而是在foreach执行。

换成如下，两次执行结果就一样了。

  var namesWithJ = (from n in names                             where n.StartsWith("J")                             orderby n                             select n).ToList();