Assemblies
An assembly is a collection of one or more files which form a logical unit. An assembly can be a .exe,.dll etc., An assembly can be a managed module or unmanaged module.
An assembly is a smallest unit of reuse, security and versioning. In other words, an assembly is the building block of .NET application.
Assemblies are made of two parts:
• Manifest (Describes the assembly, providing the logical attributes shared by all the modules and all components in the assembly. The manifest contains the assembly name, version number, locale and an optional strong name that uniquely identifies the assembly.)
• MSIL (Microsoft Intermediate Language is a language used as the output of a number of compilers (C#, VB.NET etc.,)
Disadvantages of DLL
There are many problems with conventional dll’s which led to the recommendation to the users by Microsoft. Three basic disadvantages of dll are
• Version Incompatibility
• Location Intransparency
• Language Depandency.
All the above problems are successfully eradicated with assemblies. Though, other technologies like COM and DCOM solves the above problems by using interfaces, they have got their own disadvantages like Performance and Deployment. By using assemblies, we can even solve these problems.
Earlier developers used to share libraries of code through DLL. To use the DLL that is developed by another developer for another application, you have to register that DLL in your machine. In .NET, the assembly is created by default whenever a DLL is built. By using classes located in the System.Reflection namespace, details of the manifest of the assembly can be checked.
Types of assemblies
Assemblies are of two types:
1. Private Assemblies
2. Public/Shared Assemblies
3. Satellite Assemblies
Private Assemblies:
Private assemblies are the ones intended only for one application. The files of that assembly must be in the same folder as the application folder or sub folder. One major advantages of the assembly is the ease of naming on a condition that there is no other assembly has the same name.
Shared Assemblies:
If the assembly is a Shared Assembly, then the naming conventions are very strict since it has to be unique across the entire system. The naming conventions should also take care of newer versions of the component being shipped. These are accomplished by giving the assembly a Shared Name. Then the assembly is placed in the global assembly cache, which is a folder in the file system reserved for shared assemblies.
Satellite Assemblies:
MSDN definition: "A .NET Framework assembly containing resources specific to a given language. Using satellite assemblies, you can place the resources for different languages in different assemblies, and the correct assembly is loaded into memory only if the user elects to view the application in that language."
Satellite assemblies are DLLs that provide culture-specific icons and text strings for your .NET application. Satellite assemblies are used to hold resources (strings, icons, bitmaps, etc.) and generally do not have code.
There are two ways to create a satellite. The first way is to use your language compiler to create a library assembly with an embedded resource and to mention the culture of the assembly using the [AssemblyCulture] custom attribute. This requires that you have at least one code file, but that file does not have to define a type or an entry point. The other way is to use the assembly linker tool, al.exe, and specify the resource and culture on the command line; this does not require a code source file.
GAC (Global Assembly Cache)
GAC stores all the shared assemblies to be used by several applications on the computer. As all COM components must register and store in Windows Registry, shared assemblies must be installed in GAC. Any assemblies to be deployed in Global assembly must have a strong name.
A strong Name key pair has to be created before deploying the assembly into the GAC. Key pair can be created by using a Strong name tool. (sn.exe). Key pair is created with an extension .snk. A strong Name can be created from the .NET Command Prompt from the application folder and typing the following command:
Sn –k <filename>.snk
Once the Key pair is written, the key pair file has to be linked to the application through Assemblyinfo.vb file.
Next step is to install a strong-named assembly into the global assembly cache At the command prompt, type the following command:
Gacutil –i < assembly name>
In this command, assembly name is the name of the assembly to install in the global assembly cache.
After installing successfully, the shared assembly can be checked from command prompt by checking the assembly folder in windows folder.
An assembly contains the version number and public key token. For example, System folder in GAC has a folder with the version number and public key value as follows:
1.0.5000.0_b77a5c561934e089.
1-Major Version
0-Minor Version
5000- Build Number
0- Revision Number
b77a5c561934e089 – Public Key Value.
Any new version of System.dll will be in the System folder with different version number and public keys values. Multiple versions of the same assembly can be installed in Global Assembly cache. Thus, Versioning and deployment problems can be solved with shared assemblies. CLR always looks into GAC for assemblies.
For the Word document version with images please visit http://www.filelodge.com/files/room25/674555/Assemblies.doc
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