Instead of generating instructions that a processor can interpret directly, the C# compiler generates instructions in an intermediate language, the Common Intermediate Language(CIL). A second compilation step occurs, generally at execution time, converting the CIL to machine code that the processor can understand. Conversion to machine code is still not sufficient for code execution, however. It is also necessary for a C# program to execute under the context of an agent. The agent responsible for managing the execution of a C# program is the Virtual Execution System(VES), generally more casually referred to as the runtime. (Note that the runtime in this context does not refer to a time, such as execution time; rather, the runtimethe Virtual Execution Systemis an agent responsible for managing the execution of a C# program.) The runtime is responsible for loading and running programs and providing additional services (security, garbage collection, and so on) to the program as it executes.
Note the similarity between these two acronyms and the names they stand for. Take care to understand these upfront to avoid confusion later on.
The specification for the CIL and the runtime is contained within an international standard known as the Common Language Infrastructure(CLI). This is a key specification for understanding the context in which a C# program executes and how it can seamlessly interact with other programs and libraries, even when they are written in alternate languages. Note that the CLI does not prescribe the implementation for the standard, but rather, identifies the requirements for how a CLI platform should behave once it conforms to the standard. This provides CLI implementers with the flexibility to innovate where necessary, while still providing enough structure that programs created by one platform can execute on a different CLI implementation, and even on a different operating system.
Contained within the CLI standard are specifications for the following:
This chapter broadens your view of C# to include the CLI, which is critical to how C# programs operate and interact with programs and with the operating system.