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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。
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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。
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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 to write queries。 Here are the key
points to remember：
o LINQ is an API that sits on top of other technologies such as Visual Basic objects； rela
tional databases； and XML documents。
o LINQ can work effectively only if the underlying data source technology has been opti
mized for LINQ。 Otherwise； you are left with having to load a single record set and then
manipulate that record set。
o Regardless of the data source； the techniques used to query and write LINQ are identical。
o When manipulating LINQ objects； the methods and properties associated with the
various data sources are different。 For example； when searching XML documents； you
can use XML Document Object Model （DOM） methods and properties that are not avail
able when manipulating plain…vanilla objects。
o LINQ is not just a syntax； but a series of extension methods associated with sets of data。
The methods allow for more sophisticated data pipelining and processing of
information。
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416 CH AP T E R 1 5 ■ L E A R N I N G A B OU T L I N Q
Some Things for You to Do
The following are two exercises to help you apply what you’ve learned in this chapter。
1。 The solution for finding a frequency presented in this chapter went from text to text to
calculate the statistics。 Can you think of another approach that would require minimal
changes in the interface structure？ Hint： the way the objects were parsed into objects
borrowed code from another application。 Could that other application be used somehow？
2。 You saw a LINQ query embedding another LINQ query when finding the frequency of
two numbers。 Rewrite the code to generate the frequency of all binations of single；
pairs； and triples。
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C H A P T E R 1 6
■ ■ ■
Learning About Other Visual
Basic Techniques
This last chapter in the book is about tying up loose ends。 The techniques discussed in this
chapter are those that you will use in specific situations。 This chapter covers the following
topics：
o How to use arithmetic operators to manipulate numbers
o How to overload operators
o When you might use the GoTo statement
o How to use generics constraints
o How to use nullable types
o How to use partial classes and methods
Operators
You have seen various operators used in examples throughout the book； such as the assignment
operator （a = 3）； and the logical operators （ If（ a = b））。 Visual Basic has many more arithmetic
operators that you can use to process types。 You can also define custom operators。
Using Arithmetic Operators
The subtraction （ …）； multiplication （*）； and division （/） operators are typically applicable to
only numeric values。 These operators are directly parable to the mathematical operators
you learned about in elementary school。 Let’s look at what the other arithmetic operators do。
Addition
The addition （＋） operator is used to indicate the addition of two values； like this：
a = c ＋ 1
The addition has a left…hand side and right…hand side； separated by the equal sign （=）。
On the right…hand side； the variable c is added to 1 and assigned to the variable a。
417
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418 CH AP T E R 1 6 ■ L E A R N I N G A B OU T O TH E R V IS U AL B A SI C TE C H N IQ U E S