CiL

CIL, or Common Intermediate Language, is the CPU- and language-agnostic intermediate language used by the Common Language Infrastructure (CLI). It is the low-level, typed instruction set to which developers' high-level languages such as C#, Visual Basic .NET, and F# are compiled. CIL code resides in assemblies along with metadata that describes types, members, references, and other information. The CLI specifies the Common Type System and the metadata format, enabling inter-language interoperability and robust type safety.

CIL is stack-based: instructions generally operate on an evaluation stack and use typed operands. Its instruction set includes arithmetic, control flow, object creation, method calls, boxing/unboxing, and exception handling. Because CIL is portable, compiled code can be executed on different hardware architectures and operating systems through a runtime.

Execution is performed by a CLR or another CLI-compliant runtime. At runtime, CIL is typically compiled to native code by a Just-In-Time compiler, though some environments may interpret or ahead-of-time compile IL. The runtime provides garbage collection, security, reflection, and interop with unmanaged code, and it enforces the Common Type System for language interoperability.

Historically, CIL emerged with the .NET Framework as part of the CLI specification, originally called Microsoft intermediate language (MSIL). It is standardized as part of ECMA-335 and ISO/IEC 23270 and is used by languages such as C#, VB.NET, F#, and many others across the .NET ecosystem, including .NET Framework, .NET Core, and Mono.