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parameter results in a False value。
Another variation of the If function is a two…parameter version that can be used to test
whether or not an object has a value of Nothing； as follows：
Dim cleanUpList = Function（defaultObject As Object）_
Function（objToTest As Object） If（objToTest； defaultObject）
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The implementation of the lambda expression has an If with two parameters。 The first
parameter is the object to test for Nothing； and the second parameter represents the expression
to execute if the object is Nothing。
Learning More About Collection Types
In the beginning of this chapter； I talked about the different collection types； but not how to use
them。 This section presents examples of using the types。
Using a Plain…Vanilla List
A plain…vanilla list is a list of specific types such as Integer； Double； or some other object type。
Instantiate the list as follows：
Dim lst As List（Of MyType） = New List（Of MyType）（）
The default type List is used for most of your coding situations； but if you are adding and
removing elements frequently； the following code is better。
Dim lst As IList（Of MyType） = New LinkedList（Of MyType）（）
The type LinkedList implements the logic that was presented in the previous chapter；
whereas List manipulates a fixed…length array。 LinkedList has a downside in that it does not
support the IList interface； and the sample code will not pile。 Thus； to make the code
work； you would need to use ICollection：
Dim lst As ICollection（Of MyType） = New LinkedList（Of MyType）（）
Add an element to IList as follows：
lst。Add（New MyType（））
To add elements to your list according to an order； use the following form （note that
ICollection doesn’t have this method）。
lst。Insert（0； New MyType（））
This adds an element to the front of the list。 If you are adding elements at the beginning of
the list or somewhere in the list； it is better to use the LinkedList type； as it is more efficient。
Using the class List incurs an array copy resource penalty。
You can also add one list to another：
Dim lst As List（Of MyType） = New List（Of MyType）（）
Dim lstToBeAdded As List（Of MyType）
lst。AddRange（lstToBeAdded）
lst。InsertRange（0； lstToBeAdded）
The AddRange（） method is used to append the list lstToBeAdded to lst。 The InsertRange（）
method inserts all of the elements in lstToBeAdded to the front of the list lst。
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Delete an element from the list like this：
lst。Remove（existingMyType）
The Remove（） method expects an instance of a type to remove from the list。
To delete a particular element at a particular index； use the following form。
lst。RemoveAt（0）
This code would remove the element at the front of the list。
Using a Key/Value Pair List
A key/value pair list is a list that has a cross…reference。 It is like a dictionary where you have
a word and associated meaning。 In puting terms； the word is a type and its definition is
another type。 The word is a key and the definition is a value。 A key/value pair definition would
be as follows； using the IDictionary interface and Dictionary class。
Dim dictionary As IDictionary（Of String； Object） = _
New Dictionary（Of String； Object）（）
You could also use SortedDictionary； but that implies the elements within the list are sorted。
To add values to the dictionary； use the Add（） method：
dictionary。Add（〃List〃； lst）
dictionary。Add（〃List To Be Added〃； lstToBeAdded）
When working with IDictionary objects； you might want to know whether or not a key is
available。 The following code is used to verify if a key exists。
If dictionary。ContainsKey（〃List〃） Then
。 。 。
End If
If you want to iterate the keys； use this form：
For Each key As String In dictionary。Keys
Next
Iterate the values as follows：
For Each value As Object In dictionary。Values
Next
Using a Stack
A Stack is a special list that behaves like a stack of paper on a table。 When you add three items
on the Stack； the last one added to the Stack is the first one off the Stack。 Here is an example of
using a Stack：
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Dim stack As Stack（Of String） = New Stack（Of String）（）
stack。Push（〃first〃）
stack。Push（〃second〃）
stack。Push（〃third〃）
Dim popped As String = stack。Pop（）
If popped。pareTo（〃third〃） = 0 Then
' This is what we expect
End If
The code demonstrates using the Push（） method to push items on the stack and the Pop（）
method to remove items from the stack。 Remember that Push（） is an explicit addition； and
Pop（） an explicit removal （though a call to Pop（） returns the object removed from the stack so
you can do something with it； as shown in the code）。
If you want to know what is on the top of the stack； use Peek（）； which acts like Pop（）； except
it does not remove the item from the list。
Using a Queue
A Queue is another special type of list that behaves like a queue that you would encounter at
ticket counter。 As people start queuing； the first person to be served is the one at the front of the
line。 Here is an example of using a Queue：
Dim queue As Queue（Of String） = New Queue（Of String）（）
queue。Enqueue（〃first〃）
queue。Enqueue（〃second〃）
queue。Enqueue（〃third〃）
Dim dequeued As String = queue。Dequeue（）
If dequeued = 〃first〃 Then
' This is what we expect
End If
The Important Stuff to Remember
In this chapter； you learned about using delegates； lambda expressions； extension methods；
and lists。 The main items to remember are as follows：
o You are using Visual Basic 2008； and thus you should use the generics…based
collection classes。
o There are many different types of lists。 The main types are the simple object collection；
key/value collection； stack； and queue。
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o generics…based classes are type…safe and have better performance than old…style
collections。
o Delegates help you define a generic method…calling mechanism without needing to
implement an interface。
o Delegates can be shared methods； instance methods； or module methods。 The only
important aspect to the method is to make sure the method signature matches the
delegate declaration。
o Lambda expressions are a specialized form of delegate method that enable you to write
deferred execution code。 The advantage of deferred execution is that the code can contain
a state whenever it is executed。
Some Things for You to Do
The following are some things for you to do to start applying your budding knowledge of soft
ware engineering to improving the code base。
1。 Collection classes before Visual Basic 2005 allowed you to mix types。 With Visual Basic
2005 and later； the generics classes do not allow you to mix types。 Provide a
solution where you could mix types with Visual Basic 2005 and later collections。
2。 Create a list that contains the numbers 1 to 20。 Remove the numbers； 15； 10； and 3 to 7。
3。 Create a list with an object that is defined as follows：
Class MyType
Public Value as String
End Class
4。 Add ten elements to the list； and then sort the list alphabetically from A to Z。 Hint： look
at the method Sort（） and implement a custom Iparer（Of ）。 As part of this exercise；
you need to investigate and figure out how to use Iparer（Of ）。 My suggestion is to
search the MSDN and Code Project web sites。
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Learning About Persistence
Your programs will probably need to read and/or write data to some type of storage device。
That storage device might be a hard disk； USB drive； or even the network。 The key concept is
that you are taking information from memory and transferring it to some other location。 Later；
you will retrieve that information and use it to execute some task。
Taking data from memory and transferring it to another place is referred to as persistence。
Most examples of persistence involve creating an object； and then saving that object via a file
