LINQ基础(一)
2024-09-29 07:20:04
LINQ(Language Integrated Query,语言集成查询)，在C#语言中集成了查询语法，可以用相同的语法访问不同的数据源。
LINQ提供了不同数据源的抽象层，所以可以使用相同的语法。
这里主要介绍LINQ的核心原理和C#中支持C# LINQ查询的语言扩展。
1.语法
使用LINQ查询出来自巴西的所以世界冠军。这里可以使用List<T>类的FindAll()方法，但使用LINQ查询语法更简单
static void LINQQuery()
{
//
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);
} }
变量query只指定了LINQ查询。该查询不是通过这个赋值语句执行的，而是使用foreach循环访问查询时执行的。
2.扩展方法
编译器会转换LINQ查询，以调用方法而不是LINQ查询。LINQ为IEnumerable<T>接口提供了各种扩展方法（扩展方法在http://www.cnblogs.com/afei-24/p/6703843.html介绍到），以便用户在实现了该接口的任意集合上使用LINQ查询。
定义LINQ扩展方法的一个类是System.Linq名称空间中的Enumerable。只需要导入这个名称空间，就打开了这个类的扩展方法的作用域。下面是Where()扩展方法的实现代码：
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;
} }
}
因为Where()作为一个泛型方法，所以它可以用于包含在集合中的任意类型。实现了IEnumerable<T>接口的任意集合都支持它。
3.推迟查询的执行
前面提到，在运行期间定义LINQ查询表达式时，查询不会运行。查询在迭代数据项时才会运行。
举个例子：
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：
因为查询在迭代时才执行，所以在第一次输出后有添加项再输出，会显示又添加的项。
但在调用方法ToArray(),ToList等方法时，不会延迟执行：
static void NotDeferredQuery()
{
var names = new List<string> { "Nino", "Alberto", "Juan", "Mike", "Phil" }; var namesWithJ = (from n in names
where n.StartsWith("J")
orderby n
select n).ToList(); 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：
下面是用到的类，后续的也需要用到这些代码。
//这个类创建需要的列表
public static class Formula1
{
private static List<Racer> racers;
//返回一组赛车手
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" }));
} return racers;
} private static List<Team> teams;
//返回一组冠军车队
public static IList<Team> GetContructorChampions()
{
if (teams == null)
{
teams = new List<Team>()
{
new Team("Vanwall", ),
new Team("Cooper", , ),
new Team("Ferrari", , , , , , , , , ,
2000, , , , , , ),
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;
//返回GetChampionships类型的集合
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"
}); }
return championships;
}
}
//车手类
[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);
}
//单值属性
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; }
//多值属性，车手可能多次获得冠军，所在的车队也可能不同
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));
}
}
}
//获得冠军的车队
[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; }
} //获奖选手和年份
public class Championship
{
public int Year { get; set; }
public string First { get; set; }
public string Second { get; set; }
public string Third { get; set; }
}
最新文章
- 通过trie树单词自动补全（二）
- 黑马程序员_ C语言基础之指针（三）
- Oracle中的NVL函数
- 【原】iOS学习38网络之数据解析
- python 之ConfigParser
- 在IIS7中使用ARR(Application Request Routing)反向代理虚拟目录到Nodejs站点
- Hibernate-入门教程
- 高并发简单解决方案————redis队列缓存+mysql 批量入库(ThinkPhP)
- [原博客] POI系列(2)
- OPENCV
- [IOI1999]花店橱窗布置（DP路径记录）
- 经典switch
- spring-session 共享
- 前端Mahsup异步依赖方式不能做业务数据依赖
- Redis+TwemProxy(nutcracker)集群方案部署记录
- CentOS下防御或减轻DDoS攻击方法（转）
- js的各种正则表达式
- chrome-extension & inject.js
- 教你摆脱低级程序猿 项目中cocopads的安装使用
- Spark记录-Scala介绍