VB2008从入门到精通(PDF格式英文版)-第143章
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used。 The From part is the _tickets variable itself。 The Where part is the method Where（）； and the
Select part is a default selection of the currently selected node。
To specify an action with Where（）； you use a lambda expression； which has two parame
ters： the object and the index of the object。 The lambda expression expects that you return a
Boolean value indicating whether the ticket item should be added to a returned list。
LINQ is a syntax that wraps SQL…like text。 LINQ is much easier to understand and program
than using the method…call syntax of the previous example。 Using the methods gives you more
flexibility； but they also are more plicated to write。
For example； if you wanted to find the frequency of two numbers in a list； you could use
this code：
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406 CH AP T E R 1 5 ■ L E A R N I N G A B OU T L I N Q
Function FrequencyOfTwoNumbersList（ByVal number1ToSearch As Integer ； _
ByVal number2ToSearch As Integer） As Integer
Dim query = _tickets。Where（ _
Function（ticket； index） _
ticket。Numbers（0） = number1ToSearch _
Or ticket。Numbers（1） = number1ToSearch _
Or ticket。Numbers（2） = number1ToSearch _
Or ticket。Numbers（3） = number1ToSearch _
Or ticket。Numbers（4） = number1ToSearch _
Or ticket。Numbers（5） = number1ToSearch）。Where（ _
Function（ticket； index） _
ticket。Numbers（0） = number2ToSearch _
Or ticket。Numbers（1） = number2ToSearch _
Or ticket。Numbers（2） = number2ToSearch _
Or ticket。Numbers（3） = number2ToSearch _
Or ticket。Numbers（4） = number2ToSearch _
Or ticket。Numbers（5） = number2ToSearch）
Return query。Count（）
End Function
In the code； the bolded line demonstrates how the output of one method can serve as the
input for another method。 This chaining of methods works because the list method returns
other lists。 Thus； you could add multiple criteria by concatenating multiple Where（） method calls。
The methods are used to filter or manipulate the set where the details of the method are
provided by a lambda expression。 Table 15…1 briefly describes some of the useful methods that
you can use to filter and manipulate a list。 The best way to learn about all of the methods is to
use Visual Basic Express； declare a list； and use IntelliSense to discover the different methods
available。 Also； see http：//msdn2。microsoft。/en…us/vbasic/bb688088。aspx for many exam
ples that demonstrate the various list…manipulation methods。
Table 15…1。 Some Methods for Filtering and Manipulating Lists
Method Description
Aggregate（） Returns a fact about the list。 A fact could be how many even numbers there are
or the frequency of a particular number。 All of the elements in the list are iterated
and returned as a single fact； not as a list。
All（） Iterates all elements of the list and tests according to a lambda expression；
where a True or False is returned。 For example； the test could be to find out if all
objects have a value greater than 10。 The test needs to return only a True or
False value for the individual object； where the All（） method will correlate the
results and return a True or False。
Any（） Like All（）； except that the question is changed to test if any of the objects meet
the criteria； such as having a value greater than 10。 If so； then a True value is
returned； otherwise； a False value is returned。
Average（） Calculates the average of a sequence of values。 The average value returned is
a numeric Decimal value。 This method is a bit odd； because to calculate an
average； you need numbers； even though the lambda expression could calcu
late the average of objects。
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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 407
Table 15…1。 Some Methods for Filtering and Manipulating Lists （Continued）
Method Description
Cast（） Returns a list where each item is converted from the list type to another type。
This is a good method to use when you need to perform bulk conversions of
instance types in a list。
Concat（） Concatenates two lists。
Contains（） Verifies whether an item is present in the list。 The method uses a lambda
expression to determine whether the specified item is in the list； and returns
True or False accordingly。
ConvertAll（） Returns a list where each item is converted from the list type to another type。
This is a good method to use when you need to perform bulk conversions of
instance types in a list。
Distinct（） Removes all duplicates from a list。 By default； the implementation of Distinct（）
checks for equality by calling GetHashCode（） first； and then calling Equals（） if
necessary。 A variation of the Distinct（） method is to supply an IEqualityparer
interface instance that can be used to determine whether two types are equal。
However； a better approach would be to implement GetHashCode（） and
Equals（）。
Except（） Takes the current list and a passed…in list and performs a difference between
the two sets； which is returned to the caller as a new dataset。 The equality tests
are identical to Distinct（）。
Find（） Finds an element of a particular list。 Note that the lambda expression you use
when it has found an element will cause the Find（） method to stop processing
the list and return what you marked as found。
FindAll（） Like Find（）； except you can find multiple elements in a list。 This is like the
Where（） method。
FindLast（） Like Find（）； except the search starts at the end of the list。
ForEach（） An iterator that uses a lambda expression to process each element。 The
ForEach（） method is a simplification of the code illustrated in Chapter 9。
GroupBy（） Takes a list and splits it into specific groupings as per the provided lambda
expression。 For example； you could use it to split the earnings of individuals
into brackets。
Intersect（） Takes the current list and a provided list and determines the elements that are
mon to both lists。 Uses the same equality tests as Distinct（）。
Max（） Finds the maximum value of a list。
Min（） Finds the minimum va
