In many ways, assemblies are what programmers think of as components. In their on-disk form, they are durable atoms that can move, as needed, from CLI to CLI and from application version to application version. As a key element of the CLI component model, they are the packages within which types are named and implemented, and from which types are extracted. Assemblies also define the unit of isolation for the code access security model, which facilitates safe interactions between independently developed components by enforcing isolation (in conjunction with the execution engine).
Binding to disk-based assemblies is usually name-based, and the namespace used to bind to assemblies provides scoping flexibility as systems evolve over time. While the common path is to load from disk, it is also important for compilers and tools to have the ability to create assemblies on the fly, and dynamic assemblies are supported for this purpose. Dynamic assemblies can be used to create new on-disk assemblies programmatically or create new in-memory assemblies that can be run immediately.
Once an assembly has been loaded into the CLI either dynamically or by using an application domain, its types and security data are ready to be converted from their passive PE format into the runtime structures that drive the CLI. Each type will be loaded and compiled in turn from the assembly on demand, which is the subject of the next two chapters.
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