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。
{
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
{
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);
}
}
{
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
in container
orderby value ascending/descending
select item;
1、获取全部记录
2、只获取字段名称
这里names就是隐式类型的变量。
3、使用Enumerable.Distinct<T>()
4、即可以在定义的时候调用Enumberalbe扩展函数
foreach (var n in names)
{
Console.WriteLine("Name: {0}", n);
}
也可以在兼容的数组类型上调用
Console.WriteLine("Distinct makes:");
foreach (var m in makes.Distinct<string>())
{
Console.WriteLine("Make: {0}", m);
}
var onlyBMWs = from c in myCars where c.Make == "BMW" select c;
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）
6、Reverse<T>()
foreach (Car c in subset)
{
Console.WriteLine("{0} is going {1} MPH", c.PetName, c.Speed);
}
或者
foreach (Car c in subset.Reverse<Car>())
{
Console.WriteLine(c.ToString());
}
7、排序
默认是ascending
var subset = from c in myCars orderby c.PetName select c;
// 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
orderby c.PetName ascending select c;
8、Enumerable.Except()
两个IEnumerable<T>兼容的对象的差集
{
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。
{
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
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：
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：
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。