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e architecture to
use ponents。 In general； developers consider this approach to ponents as building a plug…in architecture。
Enhancing the Configuration File
The configuration file is a bit of a clumsy definition because all of the available types are
stored in a single key/value pair。 It would be preferable to be able to define custom items in
the configuration file； as follows：
This approach is preferable because it is easier to read and understand； thus reducing the
chances that you will have an error。 Additionally； because the various identifiers are broken up；
you don’t need to build a lengthy single…line string。
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■Note For simplicity； we will have only a single dynamic type； represented by the XML element。
In a plete solution； you would use the same approach with different interfaces and types。
If you were to add the sample XML into the configuration file and run the application；
you would get a configuration initialization failure。 That’s because the XML node is not a
standard configuration item and thus needs custom parsing。 And when you want custom parsing；
you need to tell the configuration infrastructure what code to execute。 The following is
the plete configuration file with custom processing tags。
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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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（_propT