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that open source projects make multiple versions available。 In the example； you should down
load version 4。23； because 4。29 is a beta that may or may not work。 Version 4。23 is considered
stable and therefore usable。
The open source munity will often use the following terminology when releasing
software。
o Stable： A version that can be used in a production environment and should not crash。
o Unstable： A version that should not be used in production。 It will probably work； but
have some crashes。
o Nightly： A version with all bets off； meaning that the version may or may not work。 The
reason for using a nightly build is to monitor progress and check specific issues。 Such a
version is not intended for consumer consumption； it is intended solely for developers。
o Alpha ： A version that demonstrates the concepts that will make up a future version of the
software。 However； in alpha versions； what was available one day might be gone the
next day。
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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 335
Versioning Assemblies
assembly version numbers are different from those used for open source packages。 The
following is an example of versioning an assembly。
The attributes AssemblyVersion and AssemblyFileVersion can be added anywhere in the
assembly or application。 In Visual Basic Express； most likely the attributes are added to the file
AssemblyInfo。vb。
The versions of the file have four significant parts。 From left to right； these are major
version； minor version； build number； and revision。 The build number can represent a daily
build number； but this is not required。 The revision identifier can represent a random number；
but this is not required either。 For my assemblies； I use a revision number of 0； and consider the
build number as a patch number。
Visual Basic Express has a built…in mechanism that automatically updates the build and
revision numbers。 Here； the asterisk represents the auto…increment：
Alternatively； you can use a versioning tool； or you can increment the numbers manually。
The gacutil tool can be executed multiple times with multiple versions； as shown in Figure 12…4。
■Note For more information about using versioning tools； see this blog entry about auto…incrementing
assembly versions： http：//weblogs。asp/bradleyb/archive/2005/12/02/432150。aspx。
Figure 12…4。 An assembly added three times to the GAC with three different versions
In Figure 12…4； the assembly VersioningAssembly has been added three times to the GAC
with three different versions （1。0。0。0； 1。1。0。0； an d 1。2。0。0）。 With the GAC in this state； an appli
cation or another assembly has the option to reference three different versions of the same
assembly。
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336 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
For an application or assembly to use another assembly； you create a reference。 When the
application or assembly is piled； a specific version number of the assembly is referenced。
For example； if a reference to the version 1。1。0。0 of VersioningAssembly is defined； then version
1。1。0。0 of the assembly is loaded。
Adding an Assembly Redirection to a Configuration File
Let’s say an application or assembly needs to use a new version of the VersioningAssembly
assembly。 To make the application or assembly aware of the new assembly； you update the
application or assembly configuration file that references the old assembly。 The configuration
file update includes an assembly redirection。 Essentially； what the redirection says is that if a
certain version of an assembly is requested； the new version should be loaded。 The following is
an example of an assembly redirection。
This configuration file includes an assemblyBinding XML element that defines a collection
of assemblies that will be affected。 The collection of assemblies is embedded within the
dependentAssembly element。 Within the dependentAssembly element are two child elements：
assemblyIdentity and bindingRedirect。 The assemblyIdentity element is used to identity the
assembly for which a reference will be redirected。
The bindingRedirect element contains two attributes： oldVersion and newVersion。 The
oldVersion attribute identifies the version of the old assembly in the calling assembly or appli
cation。 If the specified version of the old assembly is found； the newVersion attribute is used to
identify which assembly version should be used instead。 In the example； the old version is 1。1。0。0；
and the new version is 1。2。0。0。 The new version has an incremented minor number； indicating
a new version of an assembly。 However； the binding redirection does not care whether the
newVersion attribute references a newer version or an older version。 The version identifiers
identified by the attributes newVersion and oldVersion are just that： identifiers。
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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 337
Implementing a Shared Typed Convention…Based
Architecture
There has been quite a bit of talk in the software munity about convention over configu
ration。 Most of this talk began in earnest with the development of Ruby on Rails （http：//
rubyonrails。org/）。 Ruby on Rails （Rails for short） is a tool that allows people to very
quickly create web sites that provide useful functionality。
Many attribute the success of Rails to its use of convention over configuration。 Some say
it is the Ruby language。 Others say it is because Rails is a professional product。 I believe it’s a
bination of factors； but the convention over configuration angle does play an important role。
Let’s go back to the problem of loading code dynamically； or for that matter； executing
code dynamically。 How much do you expect the programmer to know； and how much do you
expect the programmer to guess？ Consider this code：
Interface IDefinition
End Interface
Sub DoIt（ByVal def As IDefinition）
' Do Something with def
End Sub
In the code； you can see an interface IDefinition and a method DoIt（）； with a parameter
of type IDefinition。 This creates a contract where to call DoIt（）； you need to pass an instance
of type IDefinition。
Is it correct to assume that the dynamic loading of a type can fulfill the