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Assembly Names

Each assembly has a four-part name that uniquely identifies it. This four-part name consists of the friendly name, culture, developer, and version of the component. These names are stored in the assembly manifest of the assembly itself as well as all assemblies that reference it. The CLR uses the four-part assembly name to find the correct component at load time. The CLR provides programmatic access to assembly names via the System.Reflection.AssemblyName type, which is easily accessed via the System.Reflection.Assembly.GetName method.

The Name property of the assembly name typically corresponds to the underlying file name of the assembly manifest sans any file extension that may be in use. This is the only part of the assembly name that is not optional. In simple scenarios, the Name property is all that the CLR needs to locate the correct component at load time. When one builds an assembly, this part of the name is automatically selected by your compiler based on the target file name.

All assembly names have a four-part version number (Version) of the form Major.Minor.Build.Revision. If you do not set this version number explicitly, its default value will be 0.0.0.0. The version number is set at build time, typically using a custom attribute in the source code. The System.Reflection.AssemblyVersion attribute accepts a variety of string formats, as shown in Table 2.3. When you specify the version number, the Major version number is mandatory. Any missing parts are assumed to be zero. At build time, the Revision can be specified as * (asterisk), and that causes the compiler to use the wall clock to produce a monotonically increasing revision number for each compilation. If an * is specified for the Build number, the number emitted into the assembly manifest is based on the number of days that have elapsed since February 1, 2000, ensuring that each day has its own unique build number but that a given build number will be applied only for a given 24-hour period. You cannot specify an * for the Major or Minor part of the version number. Later, this chapter discusses how the assembly loader and resolver use the Version of the assembly.

Table 2.3 Inside the AssemblyVersion

Attribute

Actual

arameter

Value

1

1.0.0.0

1.2

1.2.0.0

1.2.3

1.2.3.0

1.2.3.4

1.2.3.4

1.2.*

1.2.d.s

1.2.3.*

1.2.3.s

absent>

0.0.0.0


Where d is the number of days since Feb. 1, 2000, and s is the number of seconds since midnight /2

Assembly names can contain a CultureInfo attribute that identifies the spoken language and country code that the component has been developed for. Developers specify CultureInfo using the System.Reflection.AssemblyCulture attribute, which accepts a two-part string as specified by Internet Engineering Task Force (IETF) Request for Comments (RFC) 1766. The first part of the string identifies the spoken language using a two-character lowercase code. The (optional) second part of the string identifies the geographic region using a two-character uppercase code. The string "en-US" identifies U.S. English. Assemblies that contain a CultureInfo cannot contain code; rather, they must be resource-only assemblies (also known as satellite assemblies) that can contain only localized strings and other user-interface elements. Satellite assemblies allow a single DLL containing code to selectively load (and download) localized resources based on where they are deployed. Assemblies containing code (that is, the vast majority of assemblies) are said to be culture-neutral and have no culture identifier.

Finally, an assembly name can contain a public key that identifies the developer of the component. An assembly reference can use either the full 128-byte public key or the 8-byte public key token. The public key (token) is used to resolve file name collisions between organizations, allowing multiple utilities.dll components to coexist in memory and on disk provided that each one originates from a different organization, each of which is guaranteed to have a unique public key. The next section discusses public key management in detail.

Because assembly references occasionally must be entered by hand (for example, for use in configuration files), the CLR defines a standard format for writing four-part assembly names as strings. This format is known as the display name of the assembly. The display name of the assembly always begins with the simple Name of the assembly and is followed by an optional list of comma-delimited properties that correspond to the other three properties of the assembly name. If all four parts of the name are specified, the corresponding assembly reference is called a fully qualified reference. If one or more of the properties is missing, the reference is called a partially qualified reference.

Figure 2.5 shows a display name and the corresponding CLR attributes used to control each property. Note that if an assembly with no culture is desired, the display name must indicate this using Culture=neutral. Also, if an assembly with no public key is desired, the display name must indicate this using PublicKeyToken=null. Both of these are substantially different from a display name with no Culture or PublicKeyToken property. Simply omitting these properties from the display name results in a partially specified name that allows any Culture or PublicKeyToken to be matched.

Figure 5Figure 2.5: Fully Specified Assembly Names

In general, you should avoid using partially specified assembly names; otherwise, various parts of the CLR will work in unexpected (and unpleasant) ways. However, to deal with code that does not heed this warning, the CLR allows partial assembly names to be fully qualified in configuration files. For example, consider the following application configuration file:

<configuration>
 <runtime>
   <asm:assemblyBinding
    xmlns:asm="urn:schemas-microsoft-com:asm.v1"
   >
     <asm:qualifyAssembly partialName="AcmeCorp.Code"
         fullName="AcmeCorp.Code,version=1.0.0.0,
         publicKeyToken=a1690a5ea44bab32,culture=neutral"
     />
    </asm:assemblyBinding>
  </runtime>
</configuration>

This configuration allows the following call to Assembly.Load:

Assembly assm = Assembly.Load("AcmeCorp.Code");

The preceding call behaves identically to a call such as this one:

Assembly assm = Assembly.Load("AcmeCorp.Code,"+
  "version=1.0.0.0,publicKeyToken=a1690a5ea44bab32,"+
  "culture=neutral");

The partialName attribute must match the parameter to Assembly.Load completely; that is, each property specified in the call to Assembly.Load must also be present in the partialName attribute in the configuration file. Also, each property specified in the partialName attribute must be present in the call to Assembly.Load. Later, this chapter discusses how configuration files are located.

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