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Reading the sentence； you get an idea of what is being said； but you are not pletely
sure。 Visual Basic without generics is like this sentence； in that you express your ideas in
code； but some things are not as clear as you would like。
Visual Basic with generics is like this sentence：
Ducks walk in a funny manner due to their flat feet； and when they quack； it is very
loud。
The sentence is clearer and uses more words to describe the same thing。 The reason we
talk using a more sophisticated language is that we want to explain concepts and be under
stood。 If you accept that； then you can accept why there is a need and context for generics。
And if not； feel free to skip this chapter and read the next one。
An example that illustrates how using generics makes your code clearer； as well as
more concise； than code that does not use generics is a container。 A container is a type
that is used to manage other types； lists and collections are examples of containers。 To keep
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286 CH AP T E R 1 1 ■ L E A R N I N G A B OU T 。 N E T G E N E R I CS
things simple； let’s look at a container that manages a single reference。 The following is the less
concise version that uses the Object type。
Public Class Container
Private _managed As Object
Public Sub New（ByVal toManage As Object）
_managed = toManage
End Sub
Public ReadOnly Property Managed（） As Object
Get
Return _managed
End Get
End Property
End Class
In the code； the class Container has a constructor with a parameter and a single property；
Managed； which references the variable _managed。 The idea behind this class is to assign an
object in the constructor that can be referenced using the property。 The class Container does
not know what the variable _managed does or its capabilities。 Container does not care； because
Container is acting like a basket that holds an instance of whatever is given to it。
The Container class could be used as follows：
Dim container As Container = New Container（New MyType（））
TryCast（container。Managed； MyType）。Method（）
When Container is instantiated； the _managed data member is assigned an instance of
MyType。 MyType is a type that is used for illustrative purposes and has a single method； Method（）。
To retrieve and use the managed type； the property Managed is referenced。 However； the
method Method（） cannot be called directly because the property Managed is of type Object； and
thus you need to cast the property to type MyType so that the call to Method（） is legal。
The cast using TryCast will result in a valid instance of MyType or a Nothing value； resulting
in a null object reference exception。 Here’s a safe way of using the Managed property：
Dim container As Container = New Container（New MyType（））
If TypeOf container。Managed Is MyType Then
TryCast（container。Managed； MyType）。Method（）
End If
The bolded code is the addition of the If block to test whether the container references the
type MyType。 Not shown is what the code should do if the property Managed is not MyType。 The
fact that you need to verify that the container references the correct type； and you also need to
think of an alternative plan if the type is incorrect； adds quite a bit of code。 It’s like that sentence
about the duck—sure； you get the general idea of what is being said； but are you 100％ sure of
the meaning？
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CH AP T E R 1 1 ■ L E A R N IN G AB O U T 。 N E T G E N E R I CS 287
Now look at the following code； which implements a container using generics。
Public Class GenericsContainer（Of ManagedType）
Private _managed As ManagedType
Public Sub New（ByVal toManage As ManagedType）
_managed = toManage
End Sub
Public ReadOnly Property Managed（） As ManagedType
Get
Return _managed
End Get
End Property
End Class
You can write code that uses generics and code that provides types based on
generics。 The definition of GenericsContainer demonstrates code that provides types based on
generics。 You’ll see the code that uses generics next。
generics parameters are associated with types； such as classes and interfaces； or with
methods。 In the case of GenericsContainer； the generics parameter ManagedType is a
placeholder for an actual type。
■Note monly； developers use the notation of a single letter when defining a generics parameter。
I am not a fan of that notation； because it tells me nothing； especially when there are multiple parameters。 I
remend using an identifier that describes what the parameter does； appended with the word Type； to
indicate a generics parameter is being defined。
With GenericsContainer； ManagedType is used as an identifier in the place of the identifier
object in the type Container。 This is a rule of thumb。 Whenever you find yourself using an object
generically to define a type； you probably could use generics。 Think of generics
types as general things。
The following code demonstrates how to use GenericsContainer with MyType。
Dim container As GenericsContainer（Of MyType） = _
New GenericsContainer（Of MyType）（New MyType（））
container。Managed。Method（）
When instantiating GenericsContainer； notice how the identifier ManagedType must be
replaced with an identifier that represents an already existing type。 This identifier replacement
results in a new and unique type （also called a concrete type）。 The advantage of generics is
that when you provide a concrete type； you don’t need to check to make sure that everything is
correct。
The runtime will generate a type that has an intent similar to the following code。
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288 CH AP T E R 1 1 ■ L E A R N I N G A B OU T 。 N E T G E N E R I CS
Public Class GenericsContainerMyType
Dim _managed As MyType
Public Sub New（ByVal toManage As MyType）
_managed = toManage
End Sub
Public ReadOnly Property Managed（） As MyType
Get
Return _managed
End Get
End Property
End Class
Visual Basic piles a generics type as an inplete type。 When the inplete
type is concretized； creates a brand…new type； and does this without requiring the devel
oper to do anything in particular。 This means that if you use GenericsContainer with 15 different
types； will generate 15 definitions of GenericsContainer while the program is executing。
ABSTRACTION AND GENERICS
With generics； you can verify and ensure that everything is being said properly； with the cost being
plexity。 Consider the sentence that clearly described the duck。 To produce it； you need to have learned