Assemblies in .NET

Assemblies in .NET

Introduction:

1. In the Microsoft .NET framework, an assembly is a partially compiled code library for use in deployment, versioning and security. There are two types: process assemblies (EXE) and library assemblies (DLL). A process assembly represents a process which will use classes defined in library assemblies. .NET assemblies contain code in CIL, which is usually generated from a CIL language, and then compiled into machine language at run time by the CLR just-in-time compiler.
2. An assembly can consist of one or more files. Code files are called modules. An assembly can contain more than one code module and since it is possible to use different languages to create code modules it is technically possible to use several different languages to create an assembly. Visual Studio however does not support using different languages in one assembly.
3. When you compile an application, the MSIL code created is stored in an assembly . Assemblies include both executable application files that you can run directly from Windows without the need for any other programs (these have a .exe file extension), and libraries (which have a .dll extension) for use by other applications. Assemblies can be Process Assemblies i.e exe files or Library Assemblies i.e dll files. Process assemblies are the processes which use library assemblies in their execution.
4. Even though assembly  word can be used for EXE's as well but it mainly refers to Dll's. Assemblies are also called Portable Executable s (PE) as they can be easily carried and executed on any other system having CLR installed.

  

Parts Of An Assembly:

1. Assembly Manifest : It contains information about the assembly itself i.e name of assembly, version number, security permissions, details of other assemblies it refers and strong name for assembly if any. This manifest information is used by the CLR. The manifest also contains the security demands to verify this assembly. It also contains the names and hashes of all the files that make up the assembly. The .NET assembly manifest contains a cryptographic hash of different modules in the assembly. And when the assembly is loaded, the CLR recalculates the hash of the modules at hand, and compares it with the embedded hash. If the hash generated at run time is different from that found in the manifest, .NET refuses to load the assembly and throws an exception this ensures security of an assembly.
2. Type  Meta Data :  Metadata means Data about the data. Metadata yields the types available in that assembly, viz. classes, interfaces, enums, structs, etc., and their containing namespaces, the name of each type, its visibility/scope, its base class, the interfaces it implemented, its methods and their scope, and each method’s parameters, type’s properties, and so on. The assembly metadata is generated by the high-level compilers automatically from the source files. The compiler embeds the metadata in the target output file, a dll, an .exe or a .netmodule in the case of multi-module assembly( multi-module assembly is which contain multiple code files instead of just single file as in case of single assembly).
3. Intermediate Language (IL) :  It is the actual code that is implemented in an assembly i.e the high level code written using any .net language is compiled into Intermediate language which is then executed by Just-In-Time compiler in CLR.
4. Resources : It is the resources used by the assembly like resource files (.resx) for multilingual sites or images or bitmaps used if any. Assembly will refer these resources at run time.

Types Of Assemblies:

1. Private Assembly: An assembly is used only for a particular application. It is stored in the application's directory otherwise in the application's sub directory. There is no version constraint in private assembly.
2. Public/Shared Assemblies : Refers to the assembly that is allowed to be shared by multiple applications. A shared assembly must reside in Global Assembly Cache (GAC) with a strong name assigned to it. For example, imagine that you have created a DLL containing information about your business logic. This DLL can be used by your client application. In order to run the client application, the DLL must be included in the same folder in which the client application has been installed. This makes the assembly private to your application. Now suppose that the DLL needs to be reused in different applications. Therefore, instead of copying the DLL in every client application folder, it can be placed in the global assembly cache using the GAC tool. These assemblies are called shared assemblies.
3. Satellite Assemblies : A Satellite Assembly contains only static objects like images and other non-executable files required by the application. "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 selects to view the application in that language.