VB2008从入门到精通(PDF格式英文版)-第82部分
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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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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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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
additional rules regarding grammar。 When using generics； you need to master creating abstractions。
generics are an abstraction。 Just as interfaces are an abstraction of classes； generics are an
abstraction above interfaces。 Interfaces define an intention； and generics define an abstract implemen
tation of an intention。
What is challenging with generics is getting your thoughts together into an abstract intention
implementation。 It is like writing a document—you write it once； read it over； rewrite it； read it over； and
rewrite it again。 With generics； you are gathering thoughts together into a general plan of action。 This
is why some people are pletely confused and don’t understand generics。 Writing your own
generics code requires some forethought。
The Theory of a Server…Side Spreadsheet
The example in this chapter is a spreadsheet for security traders。 When you trade securities—
whether they are equities； bonds； options； or futures—you will be confronted with information
overload。 You might have seen pictures of traders with desks full of desktop monitors。 A trader
might have seven to eight monitors displaying various bits of information。 A trader is a very
specialized type of domain that requires its own ways of processing information。 One aspect
that makes writing applications for traders difficult is that the nature of the data constantly
changes； and types get more in the way than they help。 As a result； traders adore spreadsheets。
Spreadsheets are useful because they can process large amounts of information in a relatively
ad hoc manner。 However； one downside to spreadsheets is that the processing time can
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dramatically increase due to the constant pushing and pulling of the data to and from the
spreadsheet。 To speed up processing； we will define and implement a spreadsheet that has the
advantages of a traditional client…side spreadsheet。
■Note The theory and solution presented here are specific to the domain of trading； where the cost of hard
ware is well worth the ability to trade properly。 Therefore； specific design aspects assume that you have the
latest and greatest hardware。
An initial attempt at a spreadsheet would be the following code that uses generics：
Class Spreadsheet
Public Cells As Func（Of Object）（；）
Public State As Object（；）
Public Sub New（）
Cells = New Func（Of Object）（10； 10） ｛｝
State = New Object（10； 10） ｛｝
End Sub
Public Sub Execute（）
For col As Integer = 0 To Cells。GetLength（1） 1
For row As Integer = 0 To Cells。GetLength（0） 1
Console。WriteLine（col & 〃 〃 & 〃 〃 & row）
If Cells（row； col） IsNot Nothing Then
State（row； col） = Cells（row； col）（）
End If
Next
Next
End Sub
End Class
The sample spreadsheet is defined using the data members Cells and State。 Both data
members are arrays with two dimensions。 The first dimension represents the rows； and the
second dimension represents the columns。 You could define as many dimensions as you wish；
but for the scope of the server spreadsheet； we take a two…dimensional approach。
The Execute（） method goes through the individual rows and columns of the Cells data
member； calculates the state of the cell； and assigns the state to the State data member。 The
data member Cells represents a function that is executed to generate the result of a particul
