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ate 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 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 wi