VB2008从入门到精通(PDF格式英文版)-第94部分
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The bolded section represents a new part of the configuration file that could be called
“configuring the configuration。” Any XML node that is within the section is
used to customize the configuration…parsing infrastructure。 In the example； the XML node
has two attributes that refer to what the configuration infrastructure should do
when it encounters a specific XML node。 The attribute name specifies the name of the XML
node； and the attribute type references a type （Definitions。LoaderSection） and assembly
（Definitions） that will process the XML node。
Step back and think about what is happening here。 The XML node is used to
configure the configuration processor using dynamically loaded types。 This demonstrates that
the process of dynamically loading and executing pieces of code is not rocket science； but a
daily part of developing ponents in 。
Marshaling the Configuration File Data
When you want to use a custom node in a configuration file； you need to implement a class that
subclasses System。Configuration。ConfigurationSection。 The purpose of the custom class is to
process and structure the data in the configuration file into something that the application can
manipulate。 In other words； we are marshaling the data from the configuration file to the
environment。 Here’s the start of our class； which belongs in the Definitions assembly：
Public Class LoaderSection
Inherits ConfigurationSection
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328 CH AP T E R 1 2 ■ L E A R N I N G A B OU T A PP L I CA TI O N CO N F I G U R AT IO N AN D D Y N A M I C L O AD I N G
The infrastructure helps you implement the marshaling routines by providing class
types that allow you to explicitly associate an XML attribute or keyword with some variable。 For
example； suppose you want to cross…reference the XML attribute easyname with a type。 To
do so； first define the nature and type of the property using the type ConfigurationProperty
（this code goes in the LoaderSection class）：
Dim _propEasyName As ConfigurationProperty = _
New ConfigurationProperty（〃easyname〃； GetType（String）； Nothing； _
ConfigurationPropertyOptions。IsRequired）
This example uses the ConfigurationProperty constructor that has four parameters：
o 〃easyname〃： Identifies the XML attribute identifier that will be used。
o GetType（String）： Identifies the type of the attribute； which could also include Double
and Integer to indicate a number。
o Nothing： Identifies the default value of the attribute if the attribute is not present in the
XML configuration file。
o ConfigurationPropertyOptions。IsRequired： Identifies some characteristics of the
attribute； in the case of this example； the attribute is required。
■Note There are multiple constructors for ConfigurationProperty。 For more information； check out the
MSDN documentation （http：//msdn2。microsoft。/en…us/library/system。configuration。
configurationproperty。configurationproperty。aspx）。
Once you have defined a property； you need to cross…reference it with a property and the
base class ConfigurationSection （again； this code goes in the LoaderSection class）：
_
Public ReadOnly Property EasyName（） As String
Get
Return CStr（MyBase。Item（_propEasyName））
End Get
End Property
Whenever you reference the property EasyName； the attributes associated with the property
provide a cross…reference with the configuration file。 Whenever the property is called； the base
indexer with data member that references ConfigurationProperty is called and marshals the
data from the configuration file to the type。
The plete implementation used to process the XML node is as follows （added
to the Definitions assembly）：
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CH AP T E R 1 2 ■ L E AR N IN G AB O U T AP P L I CAT I ON CO N F IG U R AT IO N A N D D Y N A M IC L O AD IN G 329
Imports System。Configuration
Public Class LoaderSection
Inherits ConfigurationSection
Private Shared _propAssemblyName As ConfigurationProperty = _
New ConfigurationProperty（〃assemblyname〃； GetType（String）； Nothing； _
ConfigurationPropertyOptions。IsRequired）
Private Shared _propEasyName As ConfigurationProperty = _
New ConfigurationProperty（〃easyname〃； GetType（String）； Nothing； _
ConfigurationPropertyOptions。IsRequired）
Private Shared _properties As ConfigurationPropertyCollection = _
New ConfigurationPropertyCollection（）
Private Shared _propTypeName As ConfigurationProperty = _
New ConfigurationProperty（〃typename〃； GetType（String）； Nothing； _
ConfigurationPropertyOptions。IsRequired）
Shared Sub New（）
_properties。Add（_propAssemblyName）
_properties。Add（_propEasyName）
_properties。Add（_propTypeName）
End Sub
_
Public ReadOnly Property AssemblyName（） As String
Get
Return CStr（MyBase。Item（_propAssemblyName））
End Get
End Property
_
Public ReadOnly Property EasyName（） As String
Get
Return CStr（MyBase。Item（_propEasyName））
End Get
End Property
_
Public ReadOnly Property TypeName（） As String
Get
Return CStr（MyBase。Item（_propTypeName））
End Get
End Property
End Class
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330 CH AP T E R 1 2 ■ L E A R N I N G A B OU T A PP L I CA TI O N CO N F I G U R AT IO N AN D D Y N A M I C L O AD I N G
The LoaderSection class does two jobs： it tells the underlying configuration infrastructure
what LoaderSection is interested in； and it provides an easy…to…use API to the configuration
data。 The first job is nice to have done； but the second is absolutely essential。
To understand what happens； let’s see an example of retrieving the values from a configu
ration file。
Reading the Enhanced Configuration File
As you’ve seen； the element declares a custom XML node that； when encoun
tered； instantiates a specific type— LoaderSection is instantiated in this example。
The values are retrieved from the custom configuration as follows：
o The configuration infrastructure reads the various attributes； such as
。
o The configuration infrastructure does a cross…reference of which attributes exist and
if they are required。 Think of this step as the bookkeeping of the custom configuration
information。
o The configuration infrastructure waits for the application code to call the properties
EasyName； TypeName； or AssemblyName。 In the implementation of the individual properties；
the MyBase indexer （such as MyBase。Item（_propTypeName） from LoaderSection。TypeName）
is referenced； and the value of the configuration item is retrieved using the property
descriptors declared as shared members。
We can use the following code to read the custom configuration item （added to the
Definitions assembly）。 This code is an example of replacing the previously declared Load（）
functionality。
Public Sub Load（）
Dim loader As LoaderSection = _
CType（ConfigurationManager。GetSection（〃loader〃）； LoaderSection）
If loader IsNot Nothing Then
Dim configInfo As ConfigurationInfo = _
New ConfigurationInfo（） With ｛ _
。EasyName = loader。EasyName； _
。TypeName = loader。TypeName； _
。AssemblyName = loader。AssemblyName｝
_availableTypes。Add（loader。EasyName； configInfo）
End If
End Sub
The only major difference between this code and the previous Load（） code is the
ConfigurationManager reference。 In the previous Load（） implementation； it was assumed that
all configuration settings were stored in the section。 This time； we retrieve the
section； which is defined to be t
