Shared Source CLI Essentials

Application Domains

Application domains (also frequently called “app domains”) are critical to understanding assembly loading within the execution engine. They tend to be a bit mysterious and are often described in terms of their similarity to process address spaces, since they scope the visibility of components and resource handles, as well as provide a security and fault isolation barrier. But from our component model implementation point of view, they are not mysterious at all; application domains are the architectural elements that are responsible for loading and unloading assemblies into the execution engine. In addition, while assemblies are resident in memory, application domains provide for isolation on their behalf.

Although the isolation provided by application domains may bear some passing similarities to an operating system address space, they actually coexist within a single address space for a process. Because of this, all domains in a process share execution engine services such as the garbage collector. Application domains provide the means for externalizing references to their components, which means that their components can set up channels of communication between one another under a programmer’s control. Because component instances can pass such externalized references among themselves, threads of execution can traverse app domain boundaries; the execution engine carefully monitors these transitions to maintain isolation.

Assemblies are always loaded within ...