C# 2008 学习笔记 - LINQ基础（三）- LINQ语法 （转）

http://www.cnblogs.com/sunrack/articles/1077551.html#1861535

一、LINQ查询符列表

Query Operators	Meaning in Life
from, in	Used to define the backbone for any LINQ expression, which allows you to extract a subset of data from a fitting container.
where	Used to define a restriction for which items to extract from a container.
select	Used to select a sequence from the container.
join, on, equals, into	Performs joins based on specified key. Remember, these “joins” do not need to have anything to do with data in a relational database.
orderby, ascending, descending	Allows the resulting subset to be ordered in ascending or descending order.
group, by	Yields a subset with data grouped by a specified value.

另外还有一些没有操作符号，而是以扩展函数（泛型函数）的方式提供的函数：

用不同方式产生结果集：
Reverse<>(), ToArray<>(), ToList<>()


集合操作： Distinct<>(), Union<>(),
Intersect<>()

统计函数： Count<>(),
Sum<>(), Min<>(), Max<>()

二、使用Enumerable获取Counts

为了使用这些Enumerable扩展函数，一般把LINQ查询表达式用括号括起来，先转换为IEnumerable<T>兼容的对象。

static void GetCount()
{
string[] currentVideoGames = {"Morrowind", "BioShock",
"Half Life 2: Episode 1", "The Darkness",
"Daxter", "System Shock 2"};
// Get count from the query.
int numb = (from g in currentVideoGames
where g.Length > 6
orderby g
select g).Count<string>();
// numb is the value 5.
Console.WriteLine("{0} items honor the LINQ query.", numb);
}

三、定义演示的实例

class Car
{
public string PetName = string.Empty;
public string Color = string.Empty;
public int Speed;
public string Make = string.Empty;
public override string ToString()
{
return string.Format("Make={0}, Color={1}, Speed={2}, PetName={3}",
Make, Color, Speed, PetName);
}
}

static void Main(string[] args)
{
Console.WriteLine("***** Fun with Query Expressions *****\n");
// This array will be the basis of our testing
Car[] myCars = new [] {
new Car{ PetName = "Henry", Color = "Silver", Speed = 100, Make = "BMW"},
new Car{ PetName = "Daisy", Color = "Tan", Speed = 90, Make = "BMW"},
new Car{ PetName = "Mary", Color = "Black", Speed = 55, Make = "VW"},
new Car{ PetName = "Clunker", Color = "Rust", Speed = 5, Make = "Yugo"},

new Car{ PetName = "Hank", Color = "Tan", Speed = 0, Make = "Ford"},
new Car{ PetName = "Sven", Color = "White", Speed = 90, Make = "Ford"},
new Car{ PetName = "Mary", Color = "Black", Speed = 55, Make = "VW"},
new Car{ PetName = "Zippy", Color = "Yellow", Speed = 55, Make = "VW"},
new Car{ PetName = "Melvin", Color = "White", Speed = 43, Make = "Ford"}
};
// We will call various methods here!
Console.ReadLine();
}

四、LINQ语法

基本语法

var result =  from item
                  in container

                  orderby value ascending/descending
                 
select item;

1、获取全部记录

var allCars = from c in myCars select c;

2、只获取字段名称

var names = from c in myCars select c.PetName;

这里names就是隐式类型的变量。


3、使用Enumerable.Distinct<T>()

var makes = (from c in myCars select c.Make).Distinct<string>();

4、即可以在定义的时候调用Enumberalbe扩展函数

var names = from c in myCars select c.PetName;
foreach (var n in names)
{
Console.WriteLine("Name: {0}", n);
}

也可以在兼容的数组类型上调用

var makes = from c in myCars select c.Make;
Console.WriteLine("Distinct makes:");
foreach (var m in makes.Distinct<string>())
{
Console.WriteLine("Make: {0}", m);
}

// Now get only the BMWs.
var onlyBMWs = from c in myCars where c.Make == "BMW" select c;

// Get BMWs going at least 100 mph.
var onlyFastBMWs = from c in myCars
where c.Make == "BMW" && c.Speed >= 100
select c;

5、生成新的数据类型（投影）

var makesColors = from c in myCars select new {c.Make, c.Color};

6、Reverse<T>()

var subset = (from c in myCars select c).Reverse<Car>();
foreach (Car c in subset)
{
Console.WriteLine("{0} is going
{1} MPH", c.PetName, c.Speed);
}

或者

var subset = from c in myCars select c;
foreach (Car c in subset.Reverse<Car>())
{
Console.WriteLine(c.ToString());
}

7、排序
默认是ascending

// Order all the cars by PetName.
var subset = from c in myCars orderby c.PetName select c;

// Now find the cars that are going less than 55 mph,
// and order by descending PetName
subset = from c in myCars
where c.Speed > 55 orderby c.PetName descending select c;

默认顺序时也可以明确指明

var subset = from c in myCars
orderby c.PetName ascending select c;

8、Enumerable.Except()

两个IEnumerable<T>兼容的对象的差集

static void GetDiff()
{
List<string> myCars = new List<String>
{ "Yugo", "Aztec", "BMW"};
List<string> yourCars = new List<String>
{ "BMW", "Saab", "Aztec" };
var carDiff =(from c in myCars select c)
.Except(from c2 in yourCars select c2);
Console.WriteLine("Here is what you don't have, but I do:");
foreach (string s in carDiff)
Console.WriteLine(s); // Prints Yugo.
}

五、使用LINQ查询结果

如果查询结果是强类型的，如string[]，List<T>等，就可以不用var类型，而是使用合适的IEnumerable<T>或IEnumerable（因为IEnumerable<T>也扩展了IEnumerable）类型。

class Program
{
static void Main(string[] args)
{
Console.WriteLine("***** LINQ Transformations *****\n");
IEnumerable<string> subset = GetStringSubset();
foreach (string item in subset)
{
Console.WriteLine(item);
}
Console.ReadLine();
}
static IEnumerable<string> GetStringSubset()
{
string[] currentVideoGames = {"Morrowind", "BioShock",
"Half Life 2: Episode 1", "The Darkness",
"Daxter", "System Shock 2"};
// Note subset is an IEnumerable<string> compatible object.
IEnumerable<string> subset = from g in currentVideoGames
where g.Length > 6
orderby g
select g;
return subset;
}
}

只有该函数原型返回类型是IEnumerable<string>，才可以使用var定义返回结果集类型。

但是在投影操作中，由于结果集合类型是隐式的，在编译时才能确定，所以这里强制规定必须使用var

// Error! Can't return a var data type!
static var GetProjectedSubset()
{
Car[] myCars = new Car[] {
new Car{ PetName = "Henry", Color = "Silver", Speed = 100, Make = "BMW"},
new Car{ PetName = "Daisy", Color = "Tan", Speed = 90, Make = "BMW"},
new Car{ PetName = "Mary", Color = "Black", Speed = 55, Make = "VW"},
new Car{ PetName = "Clunker", Color = "Rust", Speed = 5, Make = "Yugo"},
new Car{ PetName = "Melvin", Color = "White", Speed = 43, Make = "Ford"}
};

var makesColors = from c in myCars select new { c.Make, c.Color };
return makesColors; // Nope!
}

可以使用ToArray<T>()把投影结果集转化为标志的CRL数组对象：

// Return value is now an Array.
static Array GetProjectedSubset()
{
Car[] myCars = new Car[]{
new Car{ PetName = "Henry", Color = "Silver", Speed = 100, Make = "BMW"},
new Car{ PetName = "Daisy", Color = "Tan", Speed = 90, Make = "BMW"},
new Car{ PetName = "Mary", Color = "Black", Speed = 55, Make = "VW"},
new Car{ PetName = "Clunker", Color = "Rust", Speed = 5, Make = "Yugo"},
new Car{ PetName = "Melvin", Color = "White", Speed = 43, Make = "Ford"}
};
var makesColors = from c in myCars select new { c.Make, c.Color };
// Map set of anonymous objects to an Array object.
// Here were are relying on type inference of the generic
// type parameter, as we don't know the type of type!
return makesColors.ToArray();
}

然后，就可以这样使用：

Array objs = GetProjectedSubset();
foreach (object o in objs)
{
Console.WriteLine(o); // Calls ToString() on each anonymous object.
}

注意：

1、不能给ToArray<T>()指定类型，因为这里是隐式类型，到编译时才可用。
2、不能使用System.Array
的定义语法，只能使用该对象，同样因为隐式类型。
3、当需要使用投影的查询结果集时，把其转换为Array类型是必须的，当然这样会丢失强类型的好处。

分类: C# 2008