Download Introduction to LINQ

Visual Basic 2010 How to Program
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© 1992-2011 by Pearson Education, Inc.
All Rights Reserved.
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Large amounts of data are often stored in a database—an
organized collection of data.
A database management system (DBMS) provides
mechanisms for storing, organizing, retrieving and
modifying data contained in the database.
Today’s most popular database systems are relational
databases.
A language called Structured Query Language (SQL)—
pronounced “sequel”—is an international standard used
with relational databases to perform queries (that is, to
request information that satisfies given criteria) and to
manipulate data.
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For years, programs that accessed a relational database
passed SQL queries as Strings to the database
management system then processed the results.
A logical extension of querying and manipulating data in
databases is to perform similar operations on any sources of
data, such as arrays, collections (like the Items collection
of a ListBox) and files.
Microsoft developed LINQ (Language Integrated Query) to
enable you to write query expressions similar to SQL
queries that retrieve information from a wide variety of data
sources—not just relational databases—using a common
syntax that is built into Visual Basic.
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This enables you to take advantage of the IDE’s
IntelliSense feature when writing LINQ queries—
IntelliSense is not provided for SQL queries.
We use LINQ to Objects in this chapter to query the
contents of arrays, selecting elements that satisfy a set
of conditions—this is known as filtering.
We also use LINQ to Objects to perform common array
manipulations such as sorting an array.
Figure 11.1 shows the types of LINQ queries we cover
in this book and how we use them.
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LINQ allows you to look at collections of data, extract
information and manipulate data.
In a LINQ query, you might want to locate all Employees
whose salaries are in a specific range (Section 11.3).
To respond to that query, LINQ has to iterate over the data,
looking at each item to see if the Employee’s salary is in
range and, if so, selecting that item.
You might also manipulate the data—for example, for each
Employee in the result of the preceding query, you could
increase the Employee’s base salary by 4%.
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LINQ to Objects works with objects that implement the
IEnumerable interface, which enables a program to iterate
over a collection of data.
In fact, the For Each…Next statement is specifically
used to iterate over IEnumerable objects, such as
arrays—arrays implement the IEnumerable interface.
Any object of a class that implements the IEnumerable
interface has an is-a relationship with IEnumerable—
this is true of collections like the Items collection in a
ListBox and other collections that we introduce in the
online Chapter 25.
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Figure 11.2 demonstrates querying the array of Integers
named values (declared in line 9) using LINQ.
Repetition statements that filter arrays focus on the process
of getting the results—iterating through the elements and
checking whether they satisfy the desired criteria.
In contrast, LINQ specifies the conditions that elements
must satisfy, not the steps necessary to get the results.
The query in lines 20–22 specifies that the results should
consist of all the Integers in the values array that are
greater than 4.
It does not specify how those results are obtained—
nevertheless, the compiler generates all the necessary code.
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Our first LINQ query begins with a From clause (line 20), which
specifies a range variable (value) and the data source to query
(the array values).
The range variable represents each item in the data source, much
like the control variable in a For Each…Next statement.
Introducing the range variable in the From clause at the
beginning of the query allows the IDE to provide IntelliSense
while you write the rest of the query—the IDE knows the type of
the range variable (via local type inference), so it can display the
methods and properties of the object.
If the condition in the Where clause (line 21) evaluates to True,
the element is selected—that is, it’s included in the collection of
Integers that represents the query results.
Here, the Integers in the array are included only if they’re
greater than 4.
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For each item in the data source, the Select clause (line 22)
determines what value appears in the results.
In this case, it’s the Integer that the range variable
currently represents.
The Select clause is usually placed at the end of the
query for clarity, though it may be placed after the From
clause and before other clauses, or omitted.
If omitted, the range variable is implicitly selected.
Later, you’ll see that the Select clause can transform the
selected items—for example, Select value * 2 in this
example would have multiplied each selected value in the
result by 2.
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Displaying the Results of a LINQ Query
◦ Lines 27–29 display the query results using a For
Each…Next statement.
◦ A LINQ query returns an IEnumerable object.
◦ Therefore, you can use a For Each…Next statement to
iterate over the results of any LINQ query.
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Sorting LINQ Query Results
◦ The LINQ query in lines 33–35 selects the elements of the array
values and returns an IEnumerable object containing a sorted copy
of the elements.
◦ The Order By clause (line 34) sorts the query results in ascending order.
◦ The LINQ queries in lines 46–48 and 60–63 use the Descending
modifier in the Order By clause to sort query results in descending
order.
◦ An Ascending modifier also exists but is rarely used, because it’s the
default.
◦ You can use the Order By clause only for values that can be compared
to one another.
◦ The Order By clause supports values of any type that implements the
interface IComparable, such as the primitive numeric types and
String.
◦ Such types provide a CompareTo method.
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Chaining LINQ Queries
◦ The query in lines 46–48 shows that you can chain LINQ
queries by having one query operate on the results of another.
◦ In this case, the query sorts filtered (the result of the query
in lines 20–22) in descending order.
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Combining the Where and Order By Clauses in a
LINQ Query
◦ The query in lines 60–63 uses the Where and Order By
clauses to filter the elements of array values, looking for all
values greater than 4, and sorts the results in descending order.
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LINQ is not limited to querying arrays of primitive
types.
It can be used with most data types.
It cannot be used when a query does not have a defined
meaning—for example, you cannot use Order By on
objects that cannot be compared to one another to
determine sorting order.
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Figure 11.3 presents class Employee and Fig. 11.4
uses LINQ to perform the following operations on an
array of Employee objects:
◦ Select all Employees with salaries in the range $4000–6000.
◦ Sort Employees by LastName then by FirstName for
Employees that have the same LastName.
◦ Obtain the first Employee in the sorted collection of
Employees.
◦ Select the unique LastNames.
◦ Select just the FirstName and LastName from each
Employee object.
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© 1992-2011 by Pearson Education, Inc.
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© 1992-2011 by Pearson Education, Inc.
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© 1992-2011 by Pearson Education, Inc.
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Filtering Employees Based on MonthlySalary
◦ Lines 28–29 of Fig. 11.4 shows a Where clause that accesses
the properties of the range variable employee.
◦ In this example, the compiler infers that the range variable is of
type Employee because employees was declared as an
array of Employees (lines 8–15).
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Any Boolean expression can be used in a Where
clause.
Lines 28–29 use AndAlso to combine conditions.
Here, only employees that have a salary between
$4,000 and $6,000 per month, inclusive, are included in
the query result.
Lines 36–39 display the query results.
Each element in the result is an Employee object, so
when each element is implicitly converted to a
String (line 38), Employee’s ToString method
is called implicitly.
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Sorting Employees by LastName and FirstName
◦ Line 44 uses an Order By clause to sort the Employees in
the result according to multiple properties specified in a
comma-separated list.
◦ In this query, the employees are sorted alphabetically by last
name.
◦ Each group of Employees that have the same last name is
then sorted within the group by first name.
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Using the Count and First Extension Methods
As we’ve mentioned, LINQ operates on
IEnumerable objects.
The IEnumerable interface contains the overloaded
method GetEnumerator, which returns an object
that can be used to iterate over a collection of data.
Though IEnumerable declares only the overloaded
GetEnumerator method, you can actually call many
other methods on an IEnumerable object.
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In the IDE, when you type the name of an IEnumerable
object (such as an array or the result of a LINQ query) then
type the dot (.) separator, IntelliSense shows you a list of
the methods and properties that can be used with that
object.
Some of the methods are so-called extension methods.
Extension methods allow you to enhance the functionality
of an existing class or interface without modifying the
class’s or interface’s source code.
In fact, Microsoft has defined most LINQ capabilities as
extension methods for the IEnumerable interface and
you can call these directly on any IEnumerable object.
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For example, if you have an array of Doubles called
numbers and you want to calculate the average of its
values, you can simply call the Average extension
method, as in numbers.Average().
You can also create your own extension methods,
which is beyond the scope of this book; so, we show
how to do this at
www.deitel.com/books/vb2010htp/.
Some of IEnumerable’s 45 extension methods are
shown in Fig. 11.5.
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Line 51 introduces the Count extension method, which
returns the number of elements in the result of the query at
lines 43–45.
The First extension method (line 52) returns the first
element in the result—an Employee object in this
example.
If the result does not contain any elements, method First
throws an InvalidOperationException.
For this reason, we use extension method Count to ensure
that the query result contains at least one item (line 51)
before calling First.
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You might wonder why Microsoft chose to use extension
methods rather than simply including these methods
directly in the IEnumerable interface.
There are 45 extension methods for interface
IEnumerable, so it would be costly to implement the
interface for every class that wants to be an
IEnumerable when you only need a few of the methods
for your class.
Separately, the extension methods for IEnumerable are
implemented in such a manner that they are defined once
and can be used on any IEnumerable object, so you do
not need to customize these methods for your own classes.
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Selecting Specific Properties of an Object
◦ Line 60 uses the Select clause in a new way.
◦ Rather than selecting the range variable, we select each
Employee’s LastName property.
◦ In this case, the query result is a collection of only of the last
names—which are String objects, rather than complete
Employee objects.
◦ The Distinct clause (line 61) prevents duplicate values from
appearing in the query results.
◦ In this case, it eliminates duplicate last names—this occurs
because the clauses in a LINQ query are applied in the order in
which they appear, each using the results of the previous clause
in the query.
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Creating Objects of Anonymous Types
◦ The last LINQ query in the example (lines 73–74) selects only
the properties FirstName and LastName from each
Employee object.
◦ You can select portions of matching objects by specifying the
properties to select in a comma-separated list.
◦ Only the selected properties can be accessed when iterating
over the query results.
◦ When you select a portion of an object’s properties, the
compiler creates a new class containing those properties—
FirstName and LastName in this example—and the
methods that are inherited by all classes from class Object.
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The new class does not have a name and cannot be
used by you to create new objects—such classes are
called anonymous types.
In the anonymous type, the compiler overrides the
inherited ToString method to return a String
representation of the object in the form:
{ propertyName1 = value1, propertyName2 =
value2, … }
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Local type inference allows you to use the anonymous
type—the compiler can infer the anonymous type from
the query in lines 73–74.
In addition, the IDE provides IntelliSense support for
the compiler-generated anonymous types.
In the loop at lines 79–82, the compiler infers that the
control variable’s type is the anonymous type
containing the FirstName and LastName properties
and the methods inherited from class Object.
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If you type a dot (.) separator after the control
variable’s name (element) in line 81, the IDE
displays the IntelliSense window (Fig. 11.6) showing
the anonymous type’s properties and methods.
When you execute this program, line 81 implicitly calls
the anonymous type’s ToString method to get the
String representation of each element.
You can see in the program’s output that the
ToString method of an anonymous type returns the
property names and their values, enclosed in braces.
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You can create your own anonymous types, which is
beyond the scope of this book; so we show how to do
this at www.deitel.com/books/vb2010htp/.
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LINQ uses a technique called deferred execution—a
query executes only when you iterate over the results,
not when the query is defined.
This allows you to create a query once and execute it
many times.
This is similar to how methods are used.
If you make any changes to the data in a LINQ query’s
data source, the next time you iterate over the query’s
results, the query will process the current data in the
data source.
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Figure 11.7 filters an array of Strings by searching
for those that begin with "r".
Initially the array (lines 8–9) contains two such
Strings.
Later in the program we modify the array then
reexecute the LINQ query to demonstrate deferred
execution.
This example also demonstrates how to transform the
items that match the Where clause—in this case, each
matching String is converted to uppercase in the
query result.
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Calling a Method of an Object in a LINQ Query’s Where
Clause
The query in lines 13–16 filters the colors array by using
String method StartsWith in the Where clause (line 14)
to determine whether each item in the array starts with the
letter "r".
Method StartsWith uses a case sensitive comparison to
determine whether a String starts with the String in
the method’s argument.
If the String starts with "r", the StartsWith method
returns True and the element is included in the query
results.
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Transforming the Results
◦ In Section 11.2, we mentioned that the Select clause can
transform the selected items.
◦ The Select clause (line 16) transforms the Strings in the
result by converting each String to uppercase using
String method ToUpper.
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Modifying the Array’s Contents and Reexecuting the Query
◦ We created the query only once (lines 13–16), yet iterating over the
results (lines 19–21 and 30–32) gives two different lists of colors.
◦ This demonstrates LINQ’s deferred execution.
◦ The first line of output (produced by lines 19–21) shows the two
Strings that begin with "r" in the original array.
◦ Lines 26–27 replaced the Strings "blue" and "orange" in the
original array with the Strings "ruby" and "rose".
◦ Notice that the second line of output (produced by lines 30–32)
shows four matching Strings rather than two—the changes to the
data source were included when we reexecuted the query.
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This chapter introduced LINQ’s basic capabilities and
syntax.
We use more advanced features of LINQ in later chapters.
We’ve created an extensive LINQ Resource Center that
contains many links to additional information, including
blogs by Microsoft LINQ team members, sample chapters,
tutorials, videos, FAQs, resource sites and webcasts.
Browse the LINQ Resource Center
(www.deitel.com/LINQ/) to learn more about this
exciting technology.
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