The Module Writer is the component that compiles back the modifier Code Object Model into a binary assembly. 

PostSharp generates MSIL assembly code and uses ildasm to assemble it into a binary module.

The compilation step is optional in PostSharp (it is useless, for instance, if the assembly was not modified). It is implemented as a standalone task named Compile and implemented by the CompileTask class. By setting proper parameters of this task, plug-in developers can cause PostSharp not to delete the intermediate MSIL file, which can be useful during debugging.