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in which the console application is CallRuntimeImplementation。exe。 The configuration file is
CallRuntimeImplementation。exe。config。 A configuration file and the assembly that it relates to
are located in the same directory。
Figure 12…3。 A console application and its associated configuration file
■Note Naming the configuration file using a bination of the assembly’s application name with the
。config extension is an example of a convention architecture。
In this section； you’ll see how to write a configuration file and then reference it within an
application。 This setup will use the configuration file whenever a request for a certain piece of
functionality is made and supply the exact location of the assembly。
Creating an XML…Based Configuration File
The configuration file contains Extensible Markup Language （XML）。 XML is a way of structuring
a text file。
Consider the following text structure using spaces。
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320 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
First Element
Second Element
Third Element
First Element is a parent； and Second Element and Third Element are subelements of the
parent。 The structuring of the parent and child elements is fragile； to say the least。 We need a
more robust way of structuring the data； which is where XML es in。 Think of XML as having
the ability to define folders and folders within folders。
Let’s define the XML that makes up a application configuration file。 All applica
tion configuration files can be created using Visual Basic Express； as follows：
1。 Right…click the CallRuntimeImplementation project and select Add New Item。
2。 Select Text File。
3。 Name the file App。config and click Add。 The App。config file is created。
It is fine to leave the name as is； because when your application is piled； the
application configuration file will be renamed and placed beside the application。
■Note application configuration files apply to assemblies。 Thus； if an assembly uses the config
uration API； the configuration file applied will be the one associated with the calling application。
Open the new App。config file and add the following XML：
The first line is called an XML declaration； and it can specify the encoding that will be used
（plus other information； which we won’t need to concern ourselves with here）。 The second and
third lines define a root XML node （also called an XML element）。 Think of a root XML node as
analogous to a root folder where all of the other folders will be stored。
The root node is started with the identifier configuration surrounded by angle brackets。
The node is terminated by the same identifier and angle brackets； with the addition of a slash
prefix to the identifier。 The only place where another XML node can be placed is between the
beginning and ending XML nodes。 For example； this XML is invalid：
In the example； is started inside the declaration of ； but its termi
nation； ； is outside the declaration of 。
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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 321
Adding the Dynamic Loading Configuration Items
After you’ve added a configuration file； the next step is to add the elements that will be used by
the dynamic loader。 The idea will be to define some abstract identifier that will be cross…referenced
to a type and assembly。 Thus； to load Implementations1。Implementation； we’ll use the identifier
Impl1； type Implementations1。Implementation； and assembly Implementations1。dll。 Similarly；
Impl2 will cross…reference to the type Implementations2。Implementation and the assembly
Implementations2。dll。
Modify the configuration file as shown here； adding the bolded code。
The XML node ； which you could think of as adding the folder appSettings；
contains XML nodes with the identifier 。 This node defines a section in the configuration
file that contains application settings expressed as key/value pairs。 Each key/value pair is defined
in the XML node using the XML attributes key and value。
Notice that the XML node has a starting element； but not an ending element。 This is
because the node is terminated with an angle bracket prefixed with a slash。 This means
that the ending element is not necessary。
The configuration file defines a key Assemblies and a value that contains a buffer of ma
separated identifiers。 The identifiers represent the identifier； type； and assembly of supported
dynamically loaded assemblies。
The configuration file is plete； but will not influence the behavior of the application；
because source code must be added to the application that reads the configuration file settings；
as described in the next se