Linq


1.Linq概述

列表和实体

准备数据:

public class Championship
{
public int Year { get; set; }
public string First { get; set; }
public string Second { get; set; }
public string Third { get; set; }
}

Championship

public static class Formula1
{
private static List<Racer> racers; public static IList<Racer> GetChampions()
{
if (racers == null)
{
racers = new List<Racer>();
racers.Add(new Racer("Nino", "Farina", "Italy", , , new int[] { }, new string[] { "Alfa Romeo" }));
racers.Add(new Racer("Alberto", "Ascari", "Italy", , , new int[] { , }, new string[] { "Ferrari" }));
racers.Add(new Racer("Juan Manuel", "Fangio", "Argentina", , , new int[] { , , , , }, new string[] { "Alfa Romeo", "Maserati", "Mercedes", "Ferrari" }));
racers.Add(new Racer("Mike", "Hawthorn", "UK", , , new int[] { }, new string[] { "Ferrari" }));
racers.Add(new Racer("Phil", "Hill", "USA", , , new int[] { }, new string[] { "Ferrari" }));
racers.Add(new Racer("John", "Surtees", "UK", , , new int[] { }, new string[] { "Ferrari" }));
racers.Add(new Racer("Jim", "Clark", "UK", , , new int[] { , }, new string[] { "Lotus" }));
racers.Add(new Racer("Jack", "Brabham", "Australia", , , new int[] { , , }, new string[] { "Cooper", "Brabham" }));
racers.Add(new Racer("Denny", "Hulme", "New Zealand", , , new int[] { }, new string[] { "Brabham" }));
racers.Add(new Racer("Graham", "Hill", "UK", , , new int[] { , }, new string[] { "BRM", "Lotus" }));
racers.Add(new Racer("Jochen", "Rindt", "Austria", , , new int[] { }, new string[] { "Lotus" }));
racers.Add(new Racer("Jackie", "Stewart", "UK", , , new int[] { , , }, new string[] { "Matra", "Tyrrell" }));
racers.Add(new Racer("Emerson", "Fittipaldi", "Brazil", , , new int[] { , }, new string[] { "Lotus", "McLaren" }));
racers.Add(new Racer("James", "Hunt", "UK", , , new int[] { }, new string[] { "McLaren" }));
racers.Add(new Racer("Mario", "Andretti", "USA", , , new int[] { }, new string[] { "Lotus" }));
racers.Add(new Racer("Jody", "Scheckter", "South Africa", , , new int[] { }, new string[] { "Ferrari" }));
racers.Add(new Racer("Alan", "Jones", "Australia", , , new int[] { }, new string[] { "Williams" }));
racers.Add(new Racer("Keke", "Rosberg", "Finland", , , new int[] { }, new string[] { "Williams" }));
racers.Add(new Racer("Niki", "Lauda", "Austria", , , new int[] { , , }, new string[] { "Ferrari", "McLaren" }));
racers.Add(new Racer("Nelson", "Piquet", "Brazil", , , new int[] { , , }, new string[] { "Brabham", "Williams" }));
racers.Add(new Racer("Ayrton", "Senna", "Brazil", , , new int[] { , , }, new string[] { "McLaren" }));
racers.Add(new Racer("Nigel", "Mansell", "UK", , , new int[] { }, new string[] { "Williams" }));
racers.Add(new Racer("Alain", "Prost", "France", , , new int[] { , , , }, new string[] { "McLaren", "Williams" }));
racers.Add(new Racer("Damon", "Hill", "UK", , , new int[] { }, new string[] { "Williams" }));
racers.Add(new Racer("Jacques", "Villeneuve", "Canada", , , new int[] { }, new string[] { "Williams" }));
racers.Add(new Racer("Mika", "Hakkinen", "Finland", , , new int[] { , }, new string[] { "McLaren" }));
racers.Add(new Racer("Michael", "Schumacher", "Germany", , , new int[] { , , , , , , }, new string[] { "Benetton", "Ferrari" }));
racers.Add(new Racer("Fernando", "Alonso", "Spain", , , new int[] { , }, new string[] { "Renault" }));
racers.Add(new Racer("Kimi", "Räikkönen", "Finland", , , new int[] { }, new string[] { "Ferrari" }));
racers.Add(new Racer("Lewis", "Hamilton", "UK", , , new int[] { }, new string[] { "McLaren" }));
racers.Add(new Racer("Jenson", "Button", "UK", , , new int[] { }, new string[] { "Brawn GP" }));
racers.Add(new Racer("Sebastian", "Vettel", "Germany", , , new int[] { , }, new string[] { "Red Bull Racing" }));
} return racers;
} private static List<Team> teams;
public static IList<Team> GetContructorChampions()
{
if (teams == null)
{
teams = new List<Team>()
{
new Team("Vanwall", ),
new Team("Cooper", , ),
new Team("Ferrari", , , , , , , , , , , , , , , , ),
new Team("BRM", ),
new Team("Lotus", , , , , , , ),
new Team("Brabham", , ),
new Team("Matra", ),
new Team("Tyrrell", ),
new Team("McLaren", , , , , , , , ),
new Team("Williams", , , , , , , , , ),
new Team("Benetton", ),
new Team("Renault", , ),
new Team("Brawn GP", ),
new Team("Red Bull Racing", , )
};
}
return teams;
} private static List<Championship> championships;
public static IEnumerable<Championship> GetChampionships()
{
if (championships == null)
{
championships = new List<Championship>();
championships.Add(new Championship
{
Year = ,
First = "Nino Farina",
Second = "Juan Manuel Fangio",
Third = "Luigi Fagioli"
});
championships.Add(new Championship
{
Year = ,
First = "Juan Manuel Fangio",
Second = "Alberto Ascari",
Third = "Froilan Gonzalez"
});
championships.Add(new Championship
{
Year = ,
First = "Alberto Ascari",
Second = "Nino Farina",
Third = "Piero Taruffi"
});
championships.Add(new Championship
{
Year = ,
First = "Alberto Ascari",
Second = "Juan Manuel Fangio",
Third = "Nino Farina"
});
championships.Add(new Championship
{
Year = ,
First = "Juan Manuel Fangio",
Second = "Froilan Gonzalez",
Third = "Mike Hawthorn"
});
championships.Add(new Championship
{
Year = ,
First = "Juan Manuel Fangio",
Second = "Stirling Moss",
Third = "Eugenio Castellotti"
});
championships.Add(new Championship
{
Year = ,
First = "Juan Manuel Fangio",
Second = "Stirling Moss",
Third = "Peter Collins"
});
championships.Add(new Championship
{
Year = ,
First = "Juan Manuel Fangio",
Second = "Stirling Moss",
Third = "Luigi Musso"
});
championships.Add(new Championship
{
Year = ,
First = "Mike Hawthorn",
Second = "Stirling Moss",
Third = "Tony Brooks"
});
championships.Add(new Championship
{
Year = ,
First = "Jack Brabham",
Second = "Tony Brooks",
Third = "Stirling Moss"
});
championships.Add(new Championship
{
Year = ,
First = "Jack Brabham",
Second = "Bruce McLaren",
Third = "Stirling Moss"
});
championships.Add(new Championship
{
Year = ,
First = "Phil Hill",
Second = "Wolfgang von Trips",
Third = "Stirling Moss"
});
championships.Add(new Championship
{
Year = ,
First = "Graham Hill",
Second = "Jim Clark",
Third = "Bruce McLaren"
});
championships.Add(new Championship
{
Year = ,
First = "Jim Clark",
Second = "Graham Hill",
Third = "Richie Ginther"
});
championships.Add(new Championship
{
Year = ,
First = "John Surtees",
Second = "Graham Hill",
Third = "Jim Clark"
});
championships.Add(new Championship
{
Year = ,
First = "Jim Clark",
Second = "Graham Hill",
Third = "Jackie Stewart"
});
championships.Add(new Championship
{
Year = ,
First = "Jack Brabham",
Second = "John Surtees",
Third = "Jochen Rindt"
});
championships.Add(new Championship
{
Year = ,
First = "Dennis Hulme",
Second = "Jack Brabham",
Third = "Jim Clark"
});
championships.Add(new Championship
{
Year = ,
First = "Graham Hill",
Second = "Jackie Stewart",
Third = "Dennis Hulme"
});
championships.Add(new Championship
{
Year = ,
First = "Jackie Stewart",
Second = "Jackie Ickx",
Third = "Bruce McLaren"
});
championships.Add(new Championship
{
Year = ,
First = "Jochen Rindt",
Second = "Jackie Ickx",
Third = "Clay Regazzoni"
});
championships.Add(new Championship
{
Year = ,
First = "Jackie Stewart",
Second = "Ronnie Peterson",
Third = "Francois Cevert"
});
championships.Add(new Championship
{
Year = ,
First = "Emerson Fittipaldi",
Second = "Jackie Stewart",
Third = "Dennis Hulme"
});
championships.Add(new Championship
{
Year = ,
First = "Jackie Stewart",
Second = "Emerson Fittipaldi",
Third = "Ronnie Peterson"
});
championships.Add(new Championship
{
Year = ,
First = "Emerson Fittipaldi",
Second = "Clay Regazzoni",
Third = "Jody Scheckter"
});
championships.Add(new Championship
{
Year = ,
First = "Niki Lauda",
Second = "Emerson Fittipaldi",
Third = "Carlos Reutemann"
});
championships.Add(new Championship
{
Year = ,
First = "James Hunt",
Second = "Niki Lauda",
Third = "Jody Scheckter"
});
championships.Add(new Championship
{
Year = ,
First = "Niki Lauda",
Second = "Jody Scheckter",
Third = "Mario Andretti"
});
championships.Add(new Championship
{
Year = ,
First = "Mario Andretti",
Second = "Ronnie Peterson",
Third = "Carlos Reutemann"
});
championships.Add(new Championship
{
Year = ,
First = "Jody Scheckter",
Second = "Gilles Villeneuve",
Third = "Alan Jones"
});
championships.Add(new Championship
{
Year = ,
First = "Alan Jones",
Second = "Nelson Piquet",
Third = "Carlos Reutemann"
});
championships.Add(new Championship
{
Year = ,
First = "Nelson Piquet",
Second = "Carlos Reutemann",
Third = "Alan Jones"
});
championships.Add(new Championship
{
Year = ,
First = "Keke Rosberg",
Second = "Didier Pironi",
Third = "John Watson"
});
championships.Add(new Championship
{
Year = ,
First = "Nelson Piquet",
Second = "Alain Prost",
Third = "Rene Arnoux"
});
championships.Add(new Championship
{
Year = ,
First = "Niki Lauda",
Second = "Alain Prost",
Third = "Elio de Angelis"
});
championships.Add(new Championship
{
Year = ,
First = "Alain Prost",
Second = "Michele Alboreto",
Third = "Keke Rosberg"
});
championships.Add(new Championship
{
Year = ,
First = "Alain Prost",
Second = "Nigel Mansell",
Third = "Nelson Piquet"
});
championships.Add(new Championship
{
Year = ,
First = "Nelson Piquet",
Second = "Nigel Mansell",
Third = "Ayrton Senna"
});
championships.Add(new Championship
{
Year = ,
First = "Ayrton Senna",
Second = "Alain Prost",
Third = "Gerhard Berger"
});
championships.Add(new Championship
{
Year = ,
First = "Alain Prost",
Second = "Ayrton Senna",
Third = "Riccardo Patrese"
});
championships.Add(new Championship
{
Year = ,
First = "Ayrton Senna",
Second = "Alain Prost",
Third = "Nelson Piquet"
});
championships.Add(new Championship
{
Year = ,
First = "Ayrton Senna",
Second = "Nigel Mansell",
Third = "Riccardo Patrese"
});
championships.Add(new Championship
{
Year = ,
First = "Nigel Mansell",
Second = "Riccardo Patrese",
Third = "Michael Schumacher"
});
championships.Add(new Championship
{
Year = ,
First = "Alain Prost",
Second = "Ayrton Senna",
Third = "Damon Hill"
});
championships.Add(new Championship
{
Year = ,
First = "Michael Schumacher",
Second = "Damon Hill",
Third = "Gerhard Berger"
});
championships.Add(new Championship
{
Year = ,
First = "Michael Schumacher",
Second = "Damon Hill",
Third = "David Coulthard"
});
championships.Add(new Championship
{
Year = ,
First = "Damon Hill",
Second = "Jacques Villeneuve",
Third = "Michael Schumacher"
});
championships.Add(new Championship
{
Year = ,
First = "Jacques Villeneuve",
Second = "Heinz-Harald Frentzen",
Third = "David Coulthard"
});
championships.Add(new Championship
{
Year = ,
First = "Mika Hakkinen",
Second = "Michael Schumacher",
Third = "David Coulthard"
});
championships.Add(new Championship
{
Year = ,
First = "Mika Hakkinen",
Second = "Eddie Irvine",
Third = "Heinz-Harald Frentzen"
});
championships.Add(new Championship
{
Year = ,
First = "Michael Schumacher",
Second = "Mika Hakkinen",
Third = "David Coulthard"
});
championships.Add(new Championship
{
Year = ,
First = "Michael Schumacher",
Second = "David Coulthard",
Third = "Rubens Barrichello"
});
championships.Add(new Championship
{
Year = ,
First = "Michael Schumacher",
Second = "Rubens Barrichello",
Third = "Juan Pablo Montoya"
});
championships.Add(new Championship
{
Year = ,
First = "Michael Schumacher",
Second = "Kimi Räikkönen",
Third = "Juan Pablo Montoya"
});
championships.Add(new Championship
{
Year = ,
First = "Michael Schumacher",
Second = "Rubens Barrichello",
Third = "Jenson Button"
});
championships.Add(new Championship
{
Year = ,
First = "Fernando Alonso",
Second = "Kimi Räikkönen",
Third = "Michael Schumacher"
});
championships.Add(new Championship
{
Year = ,
First = "Fernando Alonso",
Second = "Michael Schumacher",
Third = "Felipe Massa"
});
championships.Add(new Championship
{
Year = ,
First = "Kimi Räikkönen",
Second = "Lewis Hamilton",
Third = "Fernando Alonso"
});
championships.Add(new Championship
{
Year = ,
First = "Lewis Hamilton",
Second = "Felipe Massa",
Third = "Kimi Raikkonen"
});
championships.Add(new Championship
{
Year = ,
First = "Jenson Button",
Second = "Sebastian Vettel",
Third = "Rubens Barrichello"
});
championships.Add(new Championship
{
Year = ,
First = "Sebastian Vettel",
Second = "Fernando Alonso",
Third = "Mark Webber"
});
championships.Add(new Championship
{
Year = ,
First = "Sebastian Vettel",
Second = "Jenson Button",
Third = "Mark Webber"
});
}
return championships;
} private static IList<Racer> moreRacers;
private static IList<Racer> GetMoreRacers()
{
if (moreRacers == null)
{
moreRacers = new List<Racer>();
moreRacers.Add(new Racer("Luigi", "Fagioli", "Italy", starts: , wins: ));
moreRacers.Add(new Racer("Jose Froilan", "Gonzalez", "Argentina", starts: , wins: ));
moreRacers.Add(new Racer("Piero", "Taruffi", "Italy", starts: , wins: ));
moreRacers.Add(new Racer("Stirling", "Moss", "UK", starts: , wins: ));
moreRacers.Add(new Racer("Eugenio", "Castellotti", "Italy", starts: , wins: ));
moreRacers.Add(new Racer("Peter", "Collins", "UK", starts: , wins: ));
moreRacers.Add(new Racer("Luigi", "Musso", "Italy", starts: , wins: ));
moreRacers.Add(new Racer("Tony", "Brooks", "UK", starts: , wins: ));
moreRacers.Add(new Racer("Bruce", "McLaren", "New Zealand", starts: , wins: ));
moreRacers.Add(new Racer("Wolfgang von", "Trips", "Germany", starts: , wins: ));
moreRacers.Add(new Racer("Richie", "Ginther", "USA", starts: , wins: ));
moreRacers.Add(new Racer("Jackie", "Ickx", "Belgium", starts: , wins: ));
moreRacers.Add(new Racer("Clay", "Regazzoni", "Switzerland", starts: , wins: ));
moreRacers.Add(new Racer("Ronnie", "Peterson", "Sweden", starts: , wins: ));
moreRacers.Add(new Racer("Francois", "Cevert", "France", starts: , wins: ));
moreRacers.Add(new Racer("Carlos", "Reutemann", "Argentina", starts: , wins: ));
moreRacers.Add(new Racer("Gilles", "Villeneuve", "Canada", starts: , wins: ));
moreRacers.Add(new Racer("Didier", "Pironi", "France", starts: , wins: ));
moreRacers.Add(new Racer("John", "Watson", "UK", starts: , wins: ));
moreRacers.Add(new Racer("Rene", "Arnoux", "France", starts: , wins: ));
moreRacers.Add(new Racer("Elio", "de Angelis", "Italy", starts: , wins: ));
moreRacers.Add(new Racer("Michele", "Alboreto", "Italy", starts: , wins: ));
moreRacers.Add(new Racer("Gerhard", "Berger", "Austria", starts: , wins: ));
moreRacers.Add(new Racer("Riccardo", "Patrese", "Italy", starts: , wins: ));
moreRacers.Add(new Racer("David", "Coulthard", "UK", starts: , wins: ));
moreRacers.Add(new Racer("Heinz-Harald", "Frentzen", "Germany", starts: , wins: ));
moreRacers.Add(new Racer("Eddie", "Irvine", "UK", starts: , wins: ));
moreRacers.Add(new Racer("Rubens", "Barrichello", "Brazil", starts: , wins: ));
moreRacers.Add(new Racer("Juan Pablo", "Montoya", "Columbia", starts: , wins: ));
moreRacers.Add(new Racer("Felipe", "Massa", "Brazil", starts: , wins: ));
moreRacers.Add(new Racer("Mark", "Webber", "Australia", starts: , wins: ));
}
return moreRacers;
}
}

Formula1

[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; }
public int Wins { get; set; }
public int Starts { get; set; }
public IEnumerable<string> Cars { get; private set; }
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 -;
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));
}
}
}

Racer

[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; }
}

Team

Linq查询

查询来自巴西所有冠军:

var query = from r in Formula1.GetChampions()
where r.Country == "Brazil"
orderby r.Wins descending
select r; foreach (var r in query)
{
Console.WriteLine("{0:A}", r);
}

查询结果:

Ayrton Senna, Brazil; starts: , wins:
Nelson Piquet, Brazil; starts: , wins:
Emerson Fittipaldi, Brazil; starts: , wins:

from where orderby select是linq查询中预定义的关键字.

查询语句以from开头以select或group子句结尾

扩展方法
public static class StringExtension
{
public static void Foo(this string s)
{
Console.WriteLine("Foo invoked for {0}",s);
}
}

在Linq中,Linq为IEnumerable<T>接口提供了各种扩展方法,方便查询。

扩展方法必须在静态类中进行声明,定义一个静态方法,其中第一个参数定义了它扩展的类型。

对于string类,因为它是密封类,因此不能被继承,所以可以通过扩展方法来进行扩展string类。

我们扩展了string类,然后可以使用它的扩展方法:

"你好".Fo();  //Foo invoked for 你好

这种扩展方法并没有违法面向对象的原则,因为定义了它的方法,并不能访问它的私有成员.

定义Linq扩展方法的类是Enumerable,只要导入这个空间,就可以访问该类的作用域。也就是说凡是继承该类的成员都可以访问了。下面列举了Where()扩展方法的实现代码。
Where()扩展方法的this关键字类型是IEnumerable,然后Where方法就可以应用与实现Ienumerable接口的每个类型,Where()扩展方法的Func<T,bool>委托,引用类返回布尔值,参数类型为T的方法。主要是检查IEnumerable<T>源中的项是否包含在目标集合中。如果委托引用了该方法,yield return语句就将源中的项返回给目标.

public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source,Func<TSource,bool> predicate)
{
foreach(TSource item in source)
  if(predicate(item))
    yield return item;
}

最后当我们调用时,只要实现了IEnumerable接口的所有类都可以使用该扩展方法

static void ExtensionMethods()
{
var champions = new List<Racer>(Formula1.GetChampions());
IEnumerable<Racer> brazilChampions =
champions.Where(r => r.Country == "Brazil").
OrderByDescending(r => r.Wins).
Select(r => r); foreach (Racer r in brazilChampions)
{
Console.WriteLine("{0:A}", r);
}
}
推迟查询的执行

在运行期间定义查询表达式时,查询不会运行,查询会在迭代数据项时运行

2.标准查询

筛选

使用where子句可以合并多个表达式

var racers = from r in Formula1.GetChampions()
where r.Wins > && (r.Country == "Brazil" || r.Country == "Austria")
select r; foreach (var r in racers)
{
Console.WriteLine("{0:A}", r);
}

不是所有的查询都用linq语句,高级查询可以使用扩展方法

var racers = Formula1.GetChampions().Where(r => r.Wins >  && (r.Country == "Brazil" || r.Country == "Austria")).Select(r => r);
用索引筛选

在Where方法重载中,可以传递第2个参数——索引,扩展方法Where方法中,含有参数是Func<TSource, int, bool> predicate的委托,第2个类型是索引

var racers = Formula1.GetChampions().
Where((r, index) => r.LastName.StartsWith("A") && index % != );
foreach (var r in racers)
{
Console.WriteLine("{0:A}", r);
}
类型筛选

OfType扩展方法基于类型的筛选,通过指定的类型返回

object[] data = { "one", , , "four", "five",  };
var query = data.OfType<int>();
foreach (var s in query)
{
Console.WriteLine(s);// 2 3 6
} var query2 = data.OfType<string>();
foreach (var s in query2)
{
Console.WriteLine(s);// one four five
}
复合from子句

如果需要对对象的成员进行筛选,则需要使用复合from子句,这种方式类似于循环里面套循环

var ferrariDrivers = from r in Formula1.GetChampions()
from c in r.Cars
where c == "Ferrari"
orderby r.LastName
select r.FirstName + " " + r.LastName; foreach (var racer in ferrariDrivers)
{
Console.WriteLine(racer);
}

C#把复合的from子句和linq查询转换为SelectMany扩展方法,SelectMany()方法可用于迭代序列的序列。

public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector);
var ferrariDrivers2 = Formula1.GetChampions().SelectMany(r => r.Cars, (r, c) => new { Racer = r, Car = c }).Where(r => r.Car == "Ferrari").OrderBy(r => r.Racer.FirstName).Select(r => r.Racer.FirstName + "" + r.Racer.LastName);
排序

使用orderby子句对序列进行排序,OrderBy()和OrderByDescending方法返回IOderEnumerable<TSource>,linq排序时多个关键字,逗号隔开.

 var racers2 = from r in Formula1.GetChampions()
where r.Country == "Brazil"
orderby r.Wins descending
select r;
var racers = (from r in Formula1.GetChampions()
orderby r.Country, r.LastName, r.FirstName
select r).Take(); foreach (var racer in racers)
{
Console.WriteLine("{0}: {1}, {2}", racer.Country, racer.LastName, racer.FirstName);
}
var racers2 = Formula1.GetChampions().OrderBy(r => r.Country).ThenBy(r => r.LastName).ThenBy(r => r.FirstName).Take();

使用ThenBy和ThenByDescending同样可以实现排序.Take方法提取了排序的前10个元素。

分组

要根据一个关键字值对查询结果分组,可以使用group子句。

var countries = from r in Formula1.GetChampions()
group r by r.Country into g
orderby g.Count() descending, g.Key
where g.Count() >=
select new
{
Country = g.Key,
Count = g.Count()
}; foreach (var item in countries)
{
Console.WriteLine("{0, -10} {1}", item.Country, item.Count);
}

代码中的标识符g,用来访问分组的结果信息。

要用扩展方法执行相同的操作。把groupby 子句解析为GroupBy()方法.

var countries2 = Formula1.GetChampions().
GroupBy(r => r.Country).
OrderByDescending(g => g.Count()).
ThenBy(g => g.Key).
Where(g => g.Count() >= ).
Select(g => new { Country = g.Key, Count = g.Count() });
对嵌套的对象分组

如果分组的对象应包含嵌套的序列,就可以改变select子句的创建的匿名类型

var countries = from r in Formula1.GetChampions()
group r by r.Country into g
orderby g.Count() descending, g.Key
where g.Count() >=
select new
{
Country = g.Key,
Count = g.Count(),
Racers = from r1 in g
orderby r1.LastName
select r1.FirstName + " " + r1.LastName
};
foreach (var item in countries)
{
Console.WriteLine("{0, -10} {1}", item.Country, item.Count);
foreach (var name in item.Racers)
{
Console.Write("{0}; ", name);
}
Console.WriteLine();
}
内连接

使用join子句可以根据特定的条件合并两个数据源,但之前要获得两个要连接的列表。

var racers = from r in Formula1.GetChampions()
from y in r.Years
select new
{
Year = y,
Name = r.FirstName + " " + r.LastName
}; var teams = from t in Formula1.GetContructorChampions()
from y in t.Years
select new
{
Year = y,
Name = t.Name
};
var racersAndTeams =
(from r in racers
join t in teams on r.Year equals t.Year
orderby t.Year
select new
{
Year = r.Year,
Champion = r.Name,
Constructor = t.Name
}).Take(); Console.WriteLine("Year World Champion\t Constructor Title");
foreach (var item in racersAndTeams)
{
Console.WriteLine("{0}: {1,-20} {2}",
item.Year, item.Champion, item.Constructor);
}
左外连接

左外连接返回左边序列中的全部元素,即使他们在右边的序列中没有匹配的元素,左外连接用join子句和DefaultIfEmpty方法定义

var racers = from r in Formula1.GetChampions()
from y in r.Years
select new
{
Year = y,
Name = r.FirstName + " " + r.LastName
}; var teams = from t in Formula1.GetContructorChampions()
from y in t.Years
select new
{
Year = y,
Name = t.Name
}; var racersAndTeams =
(from r in racers
join t in teams on r.Year equals t.Year into rt
from t in rt.DefaultIfEmpty()
orderby r.Year
select new
{
Year = r.Year,
Champion = r.Name,
Constructor = t == null ? "no constructor championship" : t.Name
}).Take(); Console.WriteLine("Year Champion\t\t Constructor Title");
foreach (var item in racersAndTeams)
{
Console.WriteLine("{0}: {1,-20} {2}",
item.Year, item.Champion, item.Constructor);
}
组连接

左外连接使用了组连接和into子句,它有一部分语法和组连接相同,只不过组连接不使用DefaultIfEmpty方法,使用组连接时,可以连接两个独立的序列,对于其中一个序列中的某个元素,另一个序列中存在对应的一个项列表。

var racers = Formula1.GetChampionships()
.SelectMany(cs => new List<RacerInfo>()
{
new RacerInfo {
Year = cs.Year,
Position = ,
FirstName = cs.First.FirstName(),
LastName = cs.First.LastName()
},
new RacerInfo {
Year = cs.Year,
Position = ,
FirstName = cs.Second.FirstName(),
LastName = cs.Second.LastName()
},
new RacerInfo {
Year = cs.Year,
Position = ,
FirstName = cs.Third.FirstName(),
LastName = cs.Third.LastName()
}
}); var q = (from r in Formula1.GetChampions()
join r2 in racers on
new
{
FirstName = r.FirstName,
LastName = r.LastName
}
equals
new
{
FirstName = r2.FirstName,
LastName = r2.LastName
}
into yearResults
select new
{
FirstName = r.FirstName,
LastName = r.LastName,
Wins = r.Wins,
Starts = r.Starts,
Results = yearResults
}); foreach (var r in q)
{
Console.WriteLine("{0} {1}", r.FirstName, r.LastName);
foreach (var results in r.Results)
{
Console.WriteLine("{0} {1}", results.Year, results.Position);
}
}
集合操作

扩展方法Distinct()、Union()、Intersect()和Except()都是集合操作。

Func<string, IEnumerable<Racer>> racersByCar =
car => from r in Formula1.GetChampions()
from c in r.Cars
where c == car
orderby r.LastName
select r; Console.WriteLine("World champion with Ferrari and McLaren");
foreach (var racer in racersByCar("Ferrari").Intersect(racersByCar("McLaren")))
{
Console.WriteLine(racer);
}
合并

Zip方法是.NET4中的,允许把2个相关的序列合并为一个

var racers = Formula1.GetChampionships().SelectMany(cs => new List<RacerInfo>()
{
new RacerInfo {
Year = cs.Year,
Position = ,
FirstName = cs.First.FirstName(),
LastName = cs.First.LastName()
},
new RacerInfo {
Year = cs.Year,
Position = ,
FirstName = cs.Second.FirstName(),
LastName = cs.Second.LastName()
},
new RacerInfo {
Year = cs.Year,
Position = ,
FirstName = cs.Third.FirstName(),
LastName = cs.Third.LastName()
}
}); var nonChampions = racers.Select(r =>
new
{
FirstName = r.FirstName,
LastName = r.LastName
}).Except(Formula1.GetChampions().Select(r =>
new
{
FirstName = r.FirstName,
LastName = r.LastName
})); foreach (var r in nonChampions)
{
Console.WriteLine("{0} {1}", r.FirstName, r.LastName);
}
分区

扩展方法Take()和Skip()等可用于分页

var racerNames = from r in Formula1.GetChampions()
where r.Country == "Italy"
orderby r.Wins descending
select new
{
Name = r.FirstName + " " + r.LastName
}; var racerNamesAndStarts = from r in Formula1.GetChampions()
where r.Country == "Italy"
orderby r.Wins descending
select new
{
LastName = r.LastName,
Starts = r.Starts
};
聚合操作符

聚合操作符返回一个值(Count()、Sum()、Min()、Max()、Average()和Aggregate())

int pageSize = ;

      int numberPages = (int)Math.Ceiling(Formula1.GetChampions().Count() /
(double)pageSize); for (int page = ; page < numberPages; page++)
{
Console.WriteLine("Page {0}", page); var racers =
(from r in Formula1.GetChampions()
orderby r.LastName, r.FirstName
select r.FirstName + " " + r.LastName).
Skip(page * pageSize).Take(pageSize); foreach (var name in racers)
{
Console.WriteLine(name);
}
Console.WriteLine();
}
var countries = (from c in
from r in Formula1.GetChampions()
group r by r.Country into c
select new
{
Country = c.Key,
Wins = (from r1 in c
select r1.Wins).Sum()
}
orderby c.Wins descending, c.Country
select c).Take(); foreach (var country in countries)
{
Console.WriteLine("{0} {1}", country.Country, country.Wins);
}
转换操作符

linq查询可以推迟到访问数据项时再执行。在迭代中使用查询时,查询会执行。使用转换操作符会立即执行查询。

var list = new System.Collections.ArrayList(Formula1.GetChampions() as System.Collections.ICollection);

      var query = from r in list.Cast<Racer>()
where r.Country == "USA"
orderby r.Wins descending
select r;
foreach (var racer in query)
{
Console.WriteLine("{0:A}", racer);
}
生成操作符

生成操作符Range()、Empty()和Repear()不是扩展方法。是返回序列的静态方法.

var values=Enumerable.Range(,);
foreach(var item in values)
{
Console.Write("{0}",item);
}

输出1到20的20个数字。

3.并行Linq

Linq中的ParallelEnumerable类可以分解查询的工作。使其分布在多个线程上。ParallelEnumerable类的大多数扩展方法是ParallelQuery<TSource>类的扩展。

并行查询

对于ParallelEnumerable类,查询是分区的,以便多个线程可以同时处理该查询。集合可以分多个部分,其中每个部分由不同的线程处理,以筛选其余项,完成分区的工作后,就需要合并,获得所有部分的总和。

   static IEnumerable<int> SampleData()
{
const int arraySize = ;
var r = new Random();
return Enumerable.Range(, arraySize).Select(x => r.Next()).ToList();
} static void IntroParallel()
{
var data = SampleData(); var watch = new Stopwatch(); watch.Start();
var q1 = (from x in data
where Math.Log(x) <
select x).Average();
watch.Stop();
Console.WriteLine("sync {0}, result: {1}", watch.ElapsedMilliseconds, q1); watch.Reset();
watch.Start();
var q2 = (from x in Partitioner.Create(data).AsParallel()
where Math.Log(x) <
select x).Average();
watch.Stop();
Console.WriteLine("async {0}, result: {1}", watch.ElapsedMilliseconds, q2);
}
分区器

可以调用WithExecutionMode()和WithDegreeOfParallelism()方法,来影响并行机制。对于WithExecutionMode()方法可以传递ParallelExecutionMode的一个Default值或者ForceParallelism值。默认情况下,并行Linq避免使用系统开销很高的并行机制。对于WithDegreeOfParallelism()方法可以传递一个整数值,以指定应并行运行的最大任务数。查询不应使用全部CPU

var res = (from x in data.AsParallel().WithCancellation(cts.Token)
where Math.Log(x) <
select x).Average();
取消

.NET提供了一个标准方式。来取消长时间运行的任务。要取消长时间运行的查询。可以给查询添加WithCancellation()方法,并传递一个CancellationToken令牌作为参数。CancellationToken令牌从CancellationTokenSource类中创建,该查询在单独的线程中运行。在该线程中。捕获一个异常。如果取消查询就触发这个异常。在主线程中,调用CancellationTokenSource类的Cancel()方法可以取消任务

 var cts = new CancellationTokenSource();

            Task.Factory.StartNew(() =>
{
try
{
var res = (from x in data.AsParallel().WithCancellation(cts.Token)
where Math.Log(x) <
select x).Average();
Console.WriteLine("query finished, result: {0}", res);
}
catch (OperationCanceledException ex)
{
Console.WriteLine(ex.Message);
}
}); Console.WriteLine("query started");
Console.Write("cancel? ");
string input = Console.ReadLine();
if (input.ToLower().Equals("y"))
{
cts.Cancel();
Console.WriteLine("sent a cancel");
}

cts.Cancel();

4.表达式树

我个人觉得理解某种程序扩展是很有意义的。

比如:

学会ASP.NET WebForm的服务器控件,就学会了ASP.NET用户控件,

学会ASP.NET MVC的@Html的扩展,就学会了HTMLHelper的自定义扩展,

现在学会了Linq表达式,就得学会Linq表达式树的扩展.

表达式树用于表示针对数据源的结构化查询,这些数据源实现 IQueryable<T>

以下部分代码来自世界中心的专栏

首先我们看看这块代码:

ParameterExpression exp1 = Expression.Parameter(typeof(int), "a");
ParameterExpression exp2 = Expression.Parameter(typeof(int), "b");
//表达式主体的构建
BinaryExpression exp = Expression.Add(exp1, exp2);
//表达式树的构建(如下定义,表达式的类型为Lambda
//lambda表达式的类型为Func<int, int, int>)
var lambda = Expression.Lambda<Func<int, int, int>>(exp, exp1, exp2);

Expression介绍

Expression.Parameter用于构建表达式树的参数.

Expression.Parameter(typeof(int), "a")构建了表达式树的参数a,数据类型为int
委托表达式:(a,b)=>a+b 中的参数a已经构建好了。
既然参数a构建好了,那么b就同样构建出来了,现在就有了下面的代码

ParameterExpression exp1 = Expression.Parameter(typeof(int), "a");
ParameterExpression exp2 = Expression.Parameter(typeof(int), "b");
构建了委托表达式:(a,b)=>a+b 中的参数(a,b)

接下来构建表达式a+b,即构建BinaryExpression 对象

BinaryExpression 表示具有二进制运算符的表达式。

BinaryExpression exp = Expression.Add(exp1, exp2);构建了表达式a+b

Expression.Lambda()方法用于构建lamdba表达式,构建lamdba表达式的方法很多。我们就先了解

var lambda = Expression.Lambda<Func<int, int, int>>(exp, exp1, exp2);

这是当前Lamdba方法的定义:

public static Expression<TDelegate> Lambda<TDelegate>(Expression body, params ParameterExpression[] parameters);

Expression是表达式树的主体,凡是继承该类的都可以使用,BinaryExpression 就是继承了该类。

params ParameterExpression[] parameters用于传递多个参数值。注意Lamdba方法的第一个参数是表达式主体,第2个是参数列表

这里再重复一下另外一个问题。

当一个方法的参数列表超过3个的时候,需要考虑使用params或者对象进行传递数据.通常一个方法参数列表超过3个,那么第4个参数类型应该为params object[] objs,第4个参数需要使用params

最后这块代码:

ParameterExpression exp1 = Expression.Parameter(typeof(int), "a");
ParameterExpression exp2 = Expression.Parameter(typeof(int), "b");
//表达式主体的构建
BinaryExpression exp = Expression.Add(exp1, exp2);
//表达式树的构建(如下定义,表达式的类型为Lambda
//lambda表达式的类型为Func<int, int, int>)
var lambda = Expression.Lambda<Func<int, int, int>>(exp, exp1, exp2);

构建了(a,b)=>a+b表达式,表达式返回类型为:Func<int,int,int>,这个由调用Lambda方法的类型约束决定。

接下来,考考你吧:

ParameterExpression exp3 = Expression.Parameter(typeof(Person), "p");
var property = Expression.Property(exp3, "Name");
var lambda2 = Expression.Lambda<Func<Person, string>>(property, exp3);

这块代码构建了什么表达式:

Expression.Property(exp3, "Name")顾名思义就是给参数p提供了一个属性Name;

那么构建后的表达式应该是:

p=>p.Name

接下来看看Where扩展方法中的表达式构建:

ParameterExpression exp3 = Expression.Parameter(typeof(Person), "p");
var property = Expression.Property(exp3, "Name");
var lambda2 = Expression.Lambda<Func<Person, string>>(property, exp3); //p=>p.Name == "daisy"
List<Person> persons = new List<Person>()
{ new Person(){ Name = "daisy", age = },
new Person(){ Name = "daisy", age = },
new Person(){Name="dom", age=},
new Person(){Name="caren", age=}};
var compareExp = simpleCompare<Person>("Name", "daisy");
var daisys = persons.Where(compareExp).ToList();
foreach (var item in daisys)
{
Console.WriteLine("Name: " + item.Name + " Age: " + item.age);
}
Console.ReadKey();
public static Func<TSource, bool> simpleCompare<TSource>(string property, object value)
{
var type = typeof(TSource); //解析传入的对象类型,通过该类型指定表达式的参数类型为该对象类型
var pe = Expression.Parameter(type, "p");//构建表达式参数
var propertyReference = Expression.Property(pe, property);//构建表达式的对象属性,属性名为value
var constantReference = Expression.Constant(value);//构建一个常数,该常数不是表达式中的参数
var binaryExpression = Expression.Equal(propertyReference, constantReference);//构建匹配的表达式 p.Name==value //compile 是表达式的一个接口,生成该lambda表达式树对的委托
var res = Expression.Lambda<Func<TSource, bool>>(binaryExpression, pe).Compile();//构建查询表达式:第一个参数为匹配表达式binaryExpression,第2个参数为对象
// p=>p.Name==Value表达式构建成功
return res;
}
//最后在调用时:persons.Where(compareExp).ToList();表现为:persons.Where(p=>p.Name==value).ToList();的形式

5.Linq提供程序

LINQ 提供程序为特定的数据源实现了标准的查询操作符。

LINQ 提供程序也许会实现LINQ 定义的更多扩展方法,但至少要实现标准操作符。

LINQ to XML 不仅实现了专门用于XML 的方法,还实现了其他方法,例如System.Xml.Linq 命名空间的Extensions 类定义的方法Elements()、 
Descendants 和Ancestors。 
LINQ 提供程序的实现方案是根据命名空间第一个参数的类型来选择的。

实现扩展方法的类的命名空间必须是打开的,否则扩展类就不在作用域内。

在LINQ to Objects 中定义的Wherer()方法参数和在LINQ to SQL中定义的Wherer()方法参数是不同的。 
LINQ to Objects 中的Wherer()方法是用Enumerable 类定义的: 
public static IEnumerable <TSource> Where <TSource> (this IEnumerable <TSource> source, Func <TSource, bool> predicate); 
在 System.Linq 命名空间中,还有另一个类实现了操作符Where。这个实现代码由LINQ to SQL 使用, 
这些代码在类Queryable 中: 
public static IQueryable <TSource> Where <TSource> (this IQueryable <TSource> source, Expression < Func <TSource, bool> > predicate); 
这两个类都在System.Linq 命名空间的System.Core 程序集中实现。

那么,它是如何定义的?使用了什么方法?

无论是用Func<TSource, bool>参数传送,还是用Expression< Func<TSource,bool>>参数传送,

λ表达式都是相同的。只是编译器的操作是不同的,它根据source 参数来选择。

编译器根据其参数选择最匹配的方法。

在LINQ to SQL 中定义的DataContext 类的GetTable()方法返回IQueryable<TSource>,因此LINQ to SQL 使用类Queryable 的Wherer()方法。 
LINQ to SQL 提供程序使用表达式树,实现了接口IQueryable 和IQueryProvider。

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