C#2008与.NET 3.5 高级程序设计读书笔记(24)-- LINQ API编程
1.LINQ to Objects
术语“LINQ to Objects”是指直接对任意 IEnumerable 或 IEnumerable<T> 集合使用 LINQ 查询，无需使用中间 LINQ 提供程序或 API，如 LINQ to SQL 或 LINQ to XML。可以使用 LINQ 来查询任何可枚举的集合，如 List<T>、Array 或 Dictionary<TKey, TValue>。该集合可以是用户定义的集合，也可以是 .NET Framework API 返回的集合。
代码
using
System;
using
System.Collections;
using
System.Linq;
namespace
NonGenericLINQ
{
public
class
Student
{
public
string
FirstName {
get
;
set
; }
public
string
LastName {
get
;
set
; }
public
int
[] Scores {
get
;
set
; }
}
class
Program
{
static
void
Main(
string
[] args)
{
ArrayList arrList
=
new
ArrayList();
arrList.Add(
new
Student
{
FirstName
=
"
Svetlana
"
, LastName
=
"
Omelchenko
"
, Scores
=
new
int
[] {
98
,
92
,
81
,
60
}
});
arrList.Add(
new
Student
{
FirstName
=
"
Claire
"
, LastName
=
"
O’Donnell
"
, Scores
=
new
int
[] {
75
,
84
,
91
,
39
}
});
arrList.Add(
new
Student
{
FirstName
=
"
Sven
"
, LastName
=
"
Mortensen
"
, Scores
=
new
int
[] {
88
,
94
,
65
,
91
}
});
arrList.Add(
new
Student
{
FirstName
=
"
Cesar
"
, LastName
=
"
Garcia
"
, Scores
=
new
int
[] {
97
,
89
,
85
,
82
}
});
var query
=
from Student student
in
arrList
where
student.Scores[
0
]
>
95
select student;
foreach
(Student s
in
query)
Console.WriteLine(s.LastName
+
"
:
"
+
s.Scores[
0
]);
//
Keep the console window open in debug mode.
Console.WriteLine(
"
Press any key to exit.
"
);
Console.ReadKey();
}
}
}
2.LINQ to DataSet
语言集成查询 (LINQ) 查询适用于实现 IEnumerable<T> 接口或 IQueryable 接口的数据源。DataTable 类不实现任何一个接口，所以如果要使用 DataTable 作为LINQ 查询的 From 子句中的源，则必须调用 AsEnumerable 方法。
代码
//
Fill the DataSet.
DataSet ds
=
new
DataSet();
ds.Locale
=
CultureInfo.InvariantCulture;
FillDataSet(ds);
DataTable orders
=
ds.Tables[
"
SalesOrderHeader
"
];
var query
=
from order
in
orders.AsEnumerable()
where
order.Field
<
bool
>
(
"
OnlineOrderFlag
"
)
==
true
select
new
{
SalesOrderID
=
order.Field
<
int
>
(
"
SalesOrderID
"
),
OrderDate
=
order.Field
<
DateTime
>
(
"
OrderDate
"
),
SalesOrderNumber
=
order.Field
<
string
>
(
"
SalesOrderNumber
"
)
};
foreach
(var onlineOrder
in
query)
{
Console.WriteLine(
"
Order ID: {0} Order date: {1:d} Order number: {2}
"
,
onlineOrder.SalesOrderID,
onlineOrder.OrderDate,
onlineOrder.SalesOrderNumber);
}
如果在应用程序设计时已知 DataSet 的架构，则建议在使用 LINQ to DataSet 时使用类型化 DataSet。类型化 DataSet 是从 DataSet 中派生的类。因此，它继承 DataSet 的所有方法、事件和属性。此外，类型化 DataSet 还提供强类型方法、事件和属性。这意味着可以按名称而不使用基于集合的方法来访问表和列。这可使查询更简单、更具可读性。
代码
var query
=
from o
in
orders
where
o.OnlineOrderFlag
==
true
select
new
{ o.SalesOrderID,
o.OrderDate,
o.SalesOrderNumber };
foreach
(var order
in
query)
{
Console.WriteLine(
"
{0}\t{1:d}\t{2}
"
,
order.SalesOrderID,
order.OrderDate,
order.SalesOrderNumber);
}
3.LINQ to XML
LINQ to XML 可以看作是一个 “better DOM” 编程模型，可以和 System.Xml.dll 程序集中的很多成员交互。
一、命名空间
System.Xml.Linq.dll 程序集定义了三个命名空间：System.Xml.Linq, System.Xml.Schema 和 System.Xml.XPath
最核心的是 System.Xml.Linq， 定义了对应 XML 文档个方面的很多类型
| Member of System.Xml.Linq |
Meaning in Life |
| XAttribute |
Represents an XML attribute on a given XML element |
| XComment |
Represents an XML comment |
| XDeclaration |
Represents the opening declaration of an XML document |
| XDocument |
Represents the entirety of an XML document |
| XElement |
Represents a given element within an XML document |
| XName/XNamespace |
Provide a very simple manner to define and reference XML namespaces |
二、编程方式创建XML文档
以前的 .NET XML编程模型需要使用很多冗长的
DOM API，而 LINQ to XML 则完全可以用与 DOM 无关的方式与 XML 文档交互。这样不但大大减少了代码行，而且这种编程模型可以直接映射到格式良好的XML文档结构。
static
void
CreateFunctionalXmlElement()
{
// A "functional" approach to build an
// XML element in memory.
XElement inventory =
new XElement( " Inventory " ,
new XElement( " Car " , new XAttribute( " ID " , " 1 " ),
new XElement( " Color " , " Green " ),
new XElement( " Make " , " BMW " ),
new XElement( " PetName " , " Stan " )
)
);
// Call ToString() on our XElement.
Console.WriteLine(inventory);
}
{
// A "functional" approach to build an
// XML element in memory.
XElement inventory =
new XElement( " Inventory " ,
new XElement( " Car " , new XAttribute( " ID " , " 1 " ),
new XElement( " Color " , " Green " ),
new XElement( " Make " , " BMW " ),
new XElement( " PetName " , " Stan " )
)
);
// Call ToString() on our XElement.
Console.WriteLine(inventory);
}
在内存中创建XML文档
static
void
CreateFunctionalXmlDoc()
{
XDocument inventoryDoc =
new XDocument(
new XDeclaration( " 1.0 " , " utf-8 " , " yes " ),
new XComment( " Current Inventory of AutoLot " ),
new XElement( " Inventory " ,
new XElement( " Car " , new XAttribute( " ID " , " 1 " ),
new XElement( " Color " , " Green " ),
new XElement( " Make " , " BMW " ),
new XElement( " PetName " , " Stan " )
),
new XElement( " Car " , new XAttribute( " ID " , " 2 " ),
new XElement( " Color " , " Pink " ),
new XElement( " Make " , " Yugo " ),
new XElement( " PetName " , " Melvin " )
)
)
);
// Display the document and save to disk.
Console.WriteLine(inventoryDoc);
inventoryDoc.Save( " SimpleInventory.xml " );
}
{
XDocument inventoryDoc =
new XDocument(
new XDeclaration( " 1.0 " , " utf-8 " , " yes " ),
new XComment( " Current Inventory of AutoLot " ),
new XElement( " Inventory " ,
new XElement( " Car " , new XAttribute( " ID " , " 1 " ),
new XElement( " Color " , " Green " ),
new XElement( " Make " , " BMW " ),
new XElement( " PetName " , " Stan " )
),
new XElement( " Car " , new XAttribute( " ID " , " 2 " ),
new XElement( " Color " , " Pink " ),
new XElement( " Make " , " Yugo " ),
new XElement( " PetName " , " Melvin " )
)
)
);
// Display the document and save to disk.
Console.WriteLine(inventoryDoc);
inventoryDoc.Save( " SimpleInventory.xml " );
}
三、使用 LINQ 查询创建XML文档
static
void
CreateXmlDocFromArray()
{
// Create an anonymous array of anonymous types.
var data = new [] {
new { PetName = " Melvin " , ID = 10 },
new { PetName = " Pat " , ID = 11 },
new { PetName = " Danny " , ID = 12 },
new { PetName = " Clunker " , ID = 13 }
};
// Now enumerate over the array to build
// an XElement.
XElement vehicles =
new XElement( " Inventory " ,
from c in data
select new XElement( " Car " ,
new XAttribute( " ID " , c.ID),
new XElement( " PetName " , c.PetName)
)
);
Console.WriteLine(vehicles);
}
{
// Create an anonymous array of anonymous types.
var data = new [] {
new { PetName = " Melvin " , ID = 10 },
new { PetName = " Pat " , ID = 11 },
new { PetName = " Danny " , ID = 12 },
new { PetName = " Clunker " , ID = 13 }
};
// Now enumerate over the array to build
// an XElement.
XElement vehicles =
new XElement( " Inventory " ,
from c in data
select new XElement( " Car " ,
new XAttribute( " ID " , c.ID),
new XElement( " PetName " , c.PetName)
)
);
Console.WriteLine(vehicles);
}
四、加载和解析XML内容
static
void
LoadExistingXml()
{
// Build an XElement from string.
string myElement =
@" <Car ID ='3'>
<Color>Yellow</Color>
<Make>Yugo</Make>
</Car> " ;
XElement newElement = XElement.Parse(myElement);
Console.WriteLine(newElement);
Console.WriteLine();
// Load the SimpleInventory.xml file.
XDocument myDoc = XDocument.Load( " SimpleInventory.xml " );
Console.WriteLine(myDoc);
}
{
// Build an XElement from string.
string myElement =
@" <Car ID ='3'>
<Color>Yellow</Color>
<Make>Yugo</Make>
</Car> " ;
XElement newElement = XElement.Parse(myElement);
Console.WriteLine(newElement);
Console.WriteLine();
// Load the SimpleInventory.xml file.
XDocument myDoc = XDocument.Load( " SimpleInventory.xml " );
Console.WriteLine(myDoc);
}
五、遍历内存中的XML文档
XML示例：
代码
<?
xml version="1.0" encoding="utf-8"
?>
<
Inventory
>
<
Car
carID
="0"
>
<
Make
>
Ford
</
Make
>
<
Color
>
Blue
</
Color
>
<
PetName
>
Chuck
</
PetName
>
</
Car
>
<
Car
carID
="1"
>
<
Make
>
VW
</
Make
>
<
Color
>
Silver
</
Color
>
<
PetName
>
Mary
</
PetName
>
</
Car
>
<
Car
carID
="2"
>
<
Make
>
Yugo
</
Make
>
<
Color
>
Pink
</
Color
>
<
PetName
>
Gipper
</
PetName
>
</
Car
>
<
Car
carID
="55"
>
<
Make
>
Ford
</
Make
>
<
Color
>
Yellow
</
Color
>
862 CHAPTER 24 n PROGRAMMING WITH THE LINQ APIS
<
PetName
>
Max
</
PetName
>
</
Car
>
<
Car
carID
="98"
>
<
Make
>
BMW
</
Make
>
<
Color
>
Black
</
Color
>
<
PetName
>
Zippy
</
PetName
>
</
Car
>
</
Inventory
>
加载
static
void
Main(
string
[] args)
{
Console.WriteLine( " ***** Fun with LINQ to XML *****\n " );
// Load the Inventory.xml document into memory.
XElement doc = XElement.Load( " Inventory.xml " );
// We will author each of these next
PrintAllPetNames(doc);
Console.WriteLine();
GetAllFords(doc);
Console.ReadLine();
}
{
Console.WriteLine( " ***** Fun with LINQ to XML *****\n " );
// Load the Inventory.xml document into memory.
XElement doc = XElement.Load( " Inventory.xml " );
// We will author each of these next
PrintAllPetNames(doc);
Console.WriteLine();
GetAllFords(doc);
Console.ReadLine();
}
遍历
static
void
PrintAllPetNames(XElement doc)
{
var petNames = from pn in doc.Descendants( " PetName " )
select pn.Value;
foreach (var name in petNames)
Console.WriteLine( " Name: {0} " , name);
}
{
var petNames = from pn in doc.Descendants( " PetName " )
select pn.Value;
foreach (var name in petNames)
Console.WriteLine( " Name: {0} " , name);
}
查询
static
void
GetAllFords(XElement doc)
{
var fords = from c in doc.Descendants( " Make " )
where c.Value == " Ford "
select c;
foreach (var f in fords)
Console.WriteLine( " Name: {0} " , f);
}
{
var fords = from c in doc.Descendants( " Make " )
where c.Value == " Ford "
select c;
foreach (var f in fords)
Console.WriteLine( " Name: {0} " , f);
}
六、修改 XML文档
static
void
AddNewElements(XElement doc)
{
// Add 5 new purple Fords to the incoming document.
for ( int i = 0 ; i < 5 ; i ++ )
{
// Create a new XElement
XElement newCar =
new XElement( " Car " , new XAttribute( " ID " , i + 1000 ),
new XElement( " Color " , " Green " ),
new XElement( " Make " , " Ford " ),
new XElement( " PetName " , "" )
);
// Add to doc.
doc.Add(newCar);
}
// Show the updates.
Console.WriteLine(doc);
}
{
// Add 5 new purple Fords to the incoming document.
for ( int i = 0 ; i < 5 ; i ++ )
{
// Create a new XElement
XElement newCar =
new XElement( " Car " , new XAttribute( " ID " , i + 1000 ),
new XElement( " Color " , " Green " ),
new XElement( " Make " , " Ford " ),
new XElement( " PetName " , "" )
);
// Add to doc.
doc.Add(newCar);
}
// Show the updates.
Console.WriteLine(doc);
}