Common Language Infrastructure (CLI)

The Common Language Infrastructure (CLI) provides a specification for executable code and the execution environment (the Virtual Execution System) in which it runs. Executable code is presented to the VES as modules. At the center of the CLI is a unified type system, the Common Type System that is shared by compilers, tools, and the CLI itself. It is the model that defines the rules the CLI follows when declaring, using, and managing types. The CTS establishes a framework that enables cross-language integration, type safety, and high performance code execution. This clause describes the architecture of the CLI by describing the CTS. The following four areas are covered in this clause:

• The Common Type System (CTS) - The CTS provides a rich type system that supports the types and operations found in many programming languages. The CTS is intended to support the complete implementation of a wide range of programming languages.
• Metadata - The CLI uses metadata to describe and reference the types defined by the CTS. Metadata is stored (that is, persisted) in a way that is independent of any particular programming language. Thus, metadata provides a common interchange mechanism for use between tools (such as compilers and debuggers) that manipulate programs, as well as between these tools and the VES.
• The Common Language Specification (CLS) - The CLS is an agreement between language designers and framework (that is, class library) designers. It specifies a subset of the CTS and a set of usage conventions. Languages provide their users the greatest ability to access frameworks by implementing at least those parts of the CTS that are part of the CLS. Similarly, frameworks will be most widely used if their publicly exported aspects (e.g., classes, interfaces, methods, and
• fields) use only types that are part of the CLS and that adhere to the CLS conventions.
• The Virtual Execution System (VES) - The VES implements and enforces the CTS model. The VES is responsible for loading and running programs written for the CLI. It provides the services needed to execute managed code and data, using the metadata to connect separately generated modules together at runtime (late binding).

Together, these aspects of the CLI form a unifying infrastructure for designing, developing, deploying, and executing distributed components and applications. The appropriate subset of the CTS is available from each programming language that targets the CLI. Language-based tools communicate with each other and with the VES using metadata to define and reference the types used to construct the application. The VES uses the metadata to create instances of the types as needed and to provide data type information to other parts of the infrastructure (such as remoting services, assembly downloading, and security).