VB2008从入门到精通(PDF格式英文版)-第144章
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Max（） Finds the maximum value of a list。
Min（） Finds the minimum value of a list。
Reverse（） Reverses the order of the list。
Select（） Selects from the iteration being executed。
SelectMany（） Selects many items from a list where the selected items form another list。
Sum（） Calculates the sum of a list。
Union（） Takes the list and the passed…in list and calculates the union of the two lists。 Uses
the equality test as defined in the Distinct（） method。
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408 CH AP T E R 1 5 ■ L E A R N I N G A B OU T L I N Q
■Note With Visual Basic 2008； lists and the manipulation of lists have dramatically changed for the better。
The general structure is to define methods that allow a developer to specify a lambda expression that is then
chained together with other methods。 Take some time to learn about all of the possibilities。
As an example of the power of the various methods； consider the following code； which
pacts the frequency code into a couple of lines of source code。
Function FrequencyOfANumberFunc（ByVal numberToSearch As Integer） As Integer
Return _tickets。SelectMany（ _
Function（ticket） ticket。Numbers）。Where（ _
Function（num） num = numberToSearch）。Count（）
End Function
Here； each ticket is iterated by calling the SelectMany（） method。 This returns an array of
numbers； which represents the drawn numbers。 The purpose of SelectMany（） is to bine
the individual arrays of numbers into a large array of numbers。 The code then calls Where（）
to filter out only those numbers that equal the number to search for； and finally the Count（）
method is called to return the number of found values。
The following sections present examples of using the extension methods with LINQ。 They
are in the LINQExamples project； which is a console application。
■Note In all of the examples； I have taken shortcuts for simplicity。 So you will see some coding practices
that are not remended； such as creating public data members and not using properties。
Selecting and Altering Data
When running a LINQ query； the data that you filter and manipulate does not need to stay in
its original form。 Let’s say that you have a list of customers； and you have identified a set of
customers who deserve more loyalty points。 You want to select those customers； increment
their points； and then return the list of altered customers。 To do that； you can mix LINQ with
the extension methods。
Consider the following simplified customer declaration。
Class Customer
Public Identifier As String
Public Points As Integer
Public Overrides Function ToString（） As String
Return 〃Identifier （〃 & Identifier & 〃） Points （〃 & Points & 〃）〃
End Function
End Class
A list will be created with two customers； where one customer has no points and the other
one does。 Here is the source code to create that list：
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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 409
Private Function CreateList（） As Customer（）
Return New Customer（） ｛ _
New Customer（） With ｛。Identifier = 〃Person 1〃； 。Points = 0｝； _
New Customer（） With ｛。Identifier = 〃Person 2〃； 。Points = 10｝｝
End Function
The customers that have enough points are selected and rewarded with extra points。 To do
that； use the following LINQ statement。
Function Increment（ByVal pCustomer As Customer； ByVal index As Integer） _
As Customer
pCustomer。Points ＋= 5
Return pCustomer
End Function
Sub CountCustomers（）
Dim points = （From customer In CreateList（） _
Where customer。Points 》 5 _
Select customer）。Select（AddressOf Increment）
Console。WriteLine（〃Count is （〃 & points。Count（） & 〃）〃）
End Sub
The LINQ query is bined with a modification operation。 The LINQ statement that uses
From； Where； and Select is not new。 New are the parentheses enclosing the LINQ statement。 By
using a set of parentheses； you are identifying the LINQ statement as an object that references
a result set。
In the example； the method called on the LINQ statement is Select（）。 Using the Select（）
method； each item in the result set is iterated and passed as a parameter （pCustomer） to the
lambda expression （Increment（））。 Passed with the item is the index of the item in the list。 The
role of the lambda expression is to do something with the item and return what should be used as
a basis for another list。 In the example； an instance of the type Customer is passed in to Increment（）；
and an instance of type Customer is passed out。 But before the instance is returned； it is manip
ulated to get an additional five bonus points。
What might concern you is that there is no test to check if the customer actually warrants
earning five bonus points。 That would be a concern if you were not using the LINQ expression。
The LINQ expression is responsible for filtering out only those customers who should get the
additional bonus points。 Thus； when the method Select（） is called； you are 100％ sure that
only the customers who should get bonus points actually get bonus points。 Think of this as
building a pipeline of manipulations。
Selecting with Anonymous Types
The Select（） method and statement are used to generate a result set after finding a particular
element。 As demonstrated in the previous section； a Select statement is used to generate a new
result set。 For example； what if you want to find all of the customers who fulfill a certain crite
rion； but do not want to copy all of the associated data？ You might want only the customer
identifier and accumulated points。 To do that； you could modify the Select（） part of the LINQ
statement to return a new type that you declare dynamically。 The following is the previous
example rewritten to use an anonymous type。
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410 CH AP T E R 1 5 ■ L E A R N I N G A B OU T L I N Q
Function Increment2（ByVal pCustomer As Customer； ByVal index As Integer）
pCustomer。Points ＋= 5
Return New With ｛。identifier = pCustomer。Identifier； _
。points = pCustomer。Points｝
End Function
Sub CountCustomers2（）
Dim points = （From customer In CreateList（） _
Where customer。Points 》 5 _
Select customer）。Select（AddressOf Increment2）
Console。WriteLine（〃Count is （〃 & points。Count（） & 〃）〃）
End Sub
In the example； the Return statement in the Increment2（） method uses the keyword New
without a type identifier； but with the syntax of an object initializer。 This is defining an anony
mous type。 An anonymous type is an object instance that has no explicitly named type。 The
anonymous type has properties； but it does not have methods。
Anonymous types are useful only in the context of the method in which they are declar
