Assemblies form the fundamental unit of deployment, version control, reuse, activation scoping, and security permissions for a .NET-based application. Assemblies take the form of an executable (.exe) file or dynamic link library (.dll) file, and are the building blocks of the .NET Framework. They provide the common language runtime with the information it needs to be aware of type implementations. You can think of an assembly as a collection of types and resources that form a logical unit of functionality and are built to work together.
With Visual Basic, you use the contents of assemblies, and add references to them, in much the same way as you use type libraries with previous versions of Visual Basic. However, what makes assemblies different from .exe or .dll files in earlier versions of Windows is that they contain all the information you would find in a type library, plus information about everything else necessary to use the application or component.
Within every assembly is an assembly manifest. Similar to a table of contents, the assembly manifest contains the following:
The assembly's identity (its name and version).
A file table describing all the other files that make up the assembly, for example, any other assemblies you created that your .exe or .dll file relies on, or even bitmap or Readme files.
An assembly reference list, which is a list of all external dependencies—.dlls or other files your application needs that may have been created by someone else. Assembly references contain references to both global and private objects. Global objects reside in the global assembly cache, an area available to other applications, somewhat like the System32 directory. The Microsoft.VisualBasic namespace is an example of an assembly in the global assembly cache. Private objects must be in a directory at either the same level as or below the directory in which your application is installed.
Because assemblies contain information about content, versioning, and dependencies, the applications you create with Visual Basic do not rely on registry values to function properly. Assemblies reduce DLL conflicts and make your applications more reliable and easier to deploy. In many cases, you can install a .NET-based application simply by copying its files to the target computer.
To use an assembly, you must add a reference to it, as described in How to: Add or Remove References in Visual Studio (Visual Basic). Next, you use the Imports statement to choose the namespace of the items you want to use, as described in References and the Imports Statement. Once an assembly is referenced and imported, all the accessible classes, properties, methods, and other members of its namespaces are available to your application as if their code were part of your source file. A single assembly can contain multiple namespaces, and each namespace can contain a different grouping of items, including other namespaces.
For information on creating assemblies, see How to: Create and Use Assemblies.