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CIL Code, Type Metadata, and the Assembly Manifest

At its core, an assembly contains CIL code, which as you recall is a platform- and CPU-agnostic

intermediate language. At runtime, the internal CIL is compiled on the fly (using a just-in-time [JIT]

compiler) to platform- and CPU-specific instructions. Given this architecture, .NET assemblies can

indeed execute on a variety of architectures, devices, and operating systems. Although you can live

a happy and productive life without understanding the details of the CIL programming language,

Chapter 19 offers an introduction to the syntax and semantics of CIL.

An assembly also contains metadata that completely describes the format of the contained

types as well as the format of external types referenced by this assembly. The .NET runtime uses this

metadata to resolve the location of types (and their members) within the binary, lay out types in

memory, and facilitate remote method invocations. You’ll check out the details of the .NET meta-

data format in Chapter 16 during our examination of reflection services.

An assembly must also contain an associated 

manifest (also referred to as assembly metadata).

The manifest documents each module within the assembly, establishes the version of the assembly,

and also documents any 

external assemblies referenced by the current assembly (unlike legacy

COM type libraries, which did not provide a way to document external dependencies). As you will

see over the course of this chapter, the CLR makes extensive use of an assembly’s manifest during

the process of locating external assembly references.