What is assembly?

Assemblies are the building blocks of .NET Framework applications; they form the fundamental unit of deployment, version control, reuse, activation scoping, and security permissions. An assembly is a collection of types and resources that are built to work together and form a logical unit of functionality.

An assembly gives the common language runtime with the information it requires to be aware of type implementations. To the runtime, a type does not exist outside the context of an assembly. Assemblies are a fundamental part of programming with the .NET Framework. An assembly performs the following functions.

It contains code that the common language runtime implements. Microsoft intermediate language (MSIL) code in a portable executable (PE) file will not be executed if it does not have an associated assembly manifest. Note that each assembly can have only one entry point (that is, DllMain, WinMain, or Main). 

It forms a security boundary. An assembly is the unit at which permissions are requested and granted. It forms a type boundary. Each type's identity contains the name of the assembly in which it resides. A type called MyType loaded in the scope of one assembly is not the similar as a type called MyType loaded in the scope of another assembly.