按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
Return String。Concat（New Object（） _
｛〃Identifier （〃； Me。Identifier； 〃） Points （〃； Me。Points； 〃）〃｝）
End Function
…………………………………………………………Page 435……………………………………………………………
CH AP T E R 1 5 ■ L E A R N I N G A B OU T L I N Q 413
Public Identifier As String
Public Points As Integer
End Class
■Note The GetHashCode（） implementation here is rudimentary。 In the source code that es with this
book； you will find a GetHashCode library class； which makes it simpler to implement GetHashCode（）。 The
source code is in the project ServerSideSpreadsheet/Devspace。Trader。mon/Automators。
Look at how GetHashCode（） and Equals（） are implemented。 Notice that the points data
member is ignored。 In the case of a customer； this is acceptable； because a customer with iden
tical identifiers but unequal points does not imply two separate customers。
Implementing Equals（） and GetHashCode（） for custom types is absolutely imperative； because
the set operations use that information to determine whether two objects are identical。 If you
don’t implement either method； the set operations will use the default implementations of
Equals（） and GetHashCode（）； which are inplete and will give you the wrong results。
The next step is to create two separate lists of customers。 In this example； both lists contain the
same valued customer。 Realize that the identical customer is not the same object instance； but
contains the same values。
Dim customers1 As Customer（） = New Customer（） ｛ _
New Customer（） With ｛。Identifier = 〃Person 1〃； 。Points = 0｝； _
New Customer（） With ｛。Identifier = 〃Person 2〃； 。Points = 10｝｝
Dim customers2 As Customer（） = New Customer（） ｛ _
New Customer（） With ｛。Identifier = 〃Person 3〃； 。Points = 0｝； _
New Customer（） With ｛。Identifier = 〃Person 2〃； 。Points = 10｝｝
To get a list of all unique customers； you can use Union（）； as follows：
Dim uniqueCustomers = customers1。Union（customers2）
Contained within the list represented by the variable uniqueCustomers will be the three
customers of the two lists。
Using LINQ in Other Contexts
So far； all of the examples in this chapter involved using LINQ and objects。 However； LINQ is
not just an object…searching technology。 It is also usable with XML and relational databases。
Using LINQ with these other data sources is not a problem； since the querying is identical。
What is a problem is getting the query to work in the first place。
Consider Figure 15…1； which illustrates the LINQ architecture。
As you can see in Figure 15…1； all programming languages can access the LINQ
library。 The data manipulated by the LINQ library es from what is called a LINQ…enabled
data source。 The examples that you’ve seen use the LINQ to objects data source。
…………………………………………………………Page 436……………………………………………………………
414 CH AP T E R 1 5 ■ L E A R N I N G A B OU T L I N Q
Figure 15…1。 LINQ architecture （based on an image in MSDN Magazine； http：//msdn。microsoft。/
msdnmag/issues/07/06/csharp30/default。aspx）
However； there is also the possibility to use a LINQ…enabled ADO connection。 The
good news is that you can use LINQ with a relational database。 The bad news is that the rela
tional database’s ADO driver must support the special LINQ characteristics。 At the time
of this writing； only the Microsoft SQL Server driver supports LINQ。 Currently； the drivers for
Microsoft Access； MySQL； and other relational databases do not support LINQ。
Consider this LINQ query：
Dim northwind As NorthwindDataContext = _
New NorthwindDataContext（）
Dim products = From p In northwind。Products _
Where p。OrderDetails。Count = 0 And p。UnitPrice 》 100 _
Select p
Notice the code in the From statement。 The data source is an object that references the rela
tional database Products table。 If a database driver is optimized for LINQ； it will understand the
LINQ query and optimize it as if it were a SQL statement。
If your database driver does not support LINQ； then you have a problem because； in theory；
you would need to download all the data from the table； and then execute the LINQ query。 That
would waste resources and is not remended。
…………………………………………………………Page 437……………………………………………………………
CH AP T E R 1 5 ■ L E A R N I N G A B OU T L I N Q 415
■Note For examples of LINQ using relational databases； see Beginning VB 2008 Databases by Vidya Vrat
Agarwal and James Huddleston （Apress； 2008）。
Let’s say that you want to execute LINQ on an XML document。 Consider the following
XML LINQ code （from http：//hookedonlinq。/LINQtoXML5MinuteOverview。ashx）。
Dim loaded As XDocument = XDocument。Load（〃C：contacts。xml〃）
' Query the data and write out a subset of contacts
Dim q = From c In loaded。Descendants（〃contact〃） _
Where CType（c。Attribute（〃contactId〃）。Value； Integer） 《 4 _
Select c。Element（〃firstName〃）。ToString（） & 〃 〃 & _
c。Element（〃lastName〃）。ToString（）
Notice how the same LINQ syntax that you’ve seen in the previous examples is used； but
the source of the data that is to be manipulated by LINQ is different。 Keep in mind that when
you are manipulating data using LINQ； you are manipulating objects that may point to XML
files； relational databases； or plain…vanilla data objects。
The Important Stuff to Remember
In this chapter； you learned about the basics of LINQ and how t