CIL
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The Common Intermediate Language (CIL, formerly MSIL) is a byte code intermediate language used in the .NET Framework. Code which compiles to CLI byte code is considered 'managed code' and can be created by any CLI compatible compiler (such as the C# Compiler, and executed by the Virtual Execution System (VES). It is CPU and platform independent.
Most of the time, developers will not be programming directly in CIL but use a higher level language such as C#, C++/CLI, Visual Basic .NET and J#. However, CIL has its own assembly language which can be assembled by ILAsm (ilasm.exe). To display the byte code in assembly language, the CIL disassembler ILDAsm (ildasm.exe) is used. However, some specialized tools such as .NET Reflector can represent the byte code as any of the supported higher level languages, such as C#, including the classes, methods and other metadata.
Safe and unsafe code
CIL byte code can either be considered safe or unsafe. By default, all C# is safe, which means that the execution system knows where all the data is, all the time. Therefore it is not possible to use pointers in safe code. Thanks to this property, the most common security and stability problems are eliminated, such as buffer overflows and dangling pointers. A garbage collector is employed to use the memory as efficient as possible.
Unsafe code
Although pointers are not possible in safe code, CIL supports unsafe code which does have support for pointers. Unsafe code is contained in blocks of code which is marked 'unsafe', and in C# it uses the same syntax as C and C++. Objects can be pinned so that the garbage collector doesn't move the data around and pointers remain valid. Using unsafe code is perfectly legal, although discouraged in normal application programming, and may be used to write kernel-level methods.
Executing CIL Code
CIL code is executed by a Virtual Execution System much like Java byte code. Usually this is in the form of a Just In Time compiler, which translates the byte code into a native machine code, usually utilizing optimizations specific to that processor (for example SSE if available). Mono is an open-source .NET project, which consists of an CIL JIT'er (and interpreter for unsupported architectures) as well as a C#->CLI compiler. The JIT'er and can be embedded in to any program or stand alone. The basic JIT'er and compiler is written in C and only requires the C Standard Library to run.
Metadata
The CIL byte code is only half of an .NET assembly. The other half is metadata, which is used by the VES to find which method needs to be called. This makes it possible to inherit or extend classes and override methods while the correct overridden method will be called at run-time without recompiling the calling assembly.