Common Intermediate Language

Common Intermediate Language (CIL, pronounced either "sill" or "kill") (formerly called Microsoft Intermediate Language or MSIL) is the lowest-level human-readable programming language in the Common Language Infrastructure and in the .NET Framework. Languages which target the .NET Framework compile to CIL, which is assembled into bytecode. CIL resembles an object-oriented assembly language, and is entirely stack-based. It is executed by a virtual machine. The primary .NET languages are C#, Visual Basic .NET, C++/CLI, and J#.

CIL was originally known as Microsoft Intermediate Language (MSIL) during the beta releases of the .NET languages. Due to standardization of C# and the Common Language Infrastructure, the bytecode is now officially known as CIL. Because of this legacy, CIL is still frequently referred to as MSIL, especially by long-standing users of the .NET languages.

General information

During compilation of .NET programming languages, the source code is translated into CIL code rather than platform or processor-specific object code. CIL is a CPU- and platform-independent instruction set that can be executed in any environment supporting the .NET framework. CIL code is verified for safety during runtime, providing better security and reliability than natively compiled binaries.

Just-In-Time (JIT) Compilation

Just-in-time compilation involves the byte-code being turned into code immediately executable by the CPU. The conversion is performed gradually during the program's execution. JIT compilation provides environment-specific optimization, runtime type safety, and assembly verification. To accomplish this, the JIT compiler examines the assembly metadata for any illegal accesses and handles violations appropriately.

NGEN (Native Image Generator) Compilation

NGEN produces a native binary image for the current environment. The byte-code is either skipped entirely or converted into native CPU instructions completely before runtime. This eliminates the JIT overhead at the expense of portability; whenever an NGEN-generated image is run in an incompatible environment, .NET framework automatically reverts to using JIT.

Once NGEN is run against an assembly, the resulting native image is placed into the Native Image Cache for use by all other .NET assemblies. It is advised that NGEN is run during applications' deployment.

Metadata

Metadata is information about the compiled classes. It serves the same purpose as a type library in COM. Metadata enables applications to support and discover the interfaces, classes, types, methods, and fields in the assembly. The process of reading metadata is called reflection.

Executing CIL

Source code is converted to Common Intermediate Language and an assembly is created.
Upon execution of a .NET assembly, its CIL is passed through the Common Language Runtime's JIT compiler to generate native code. (NGEN compilation eliminates this step at run time.)
The native code is executed by the computer's processor.

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