Programming Visual Basic .NET

Intermediate Language (IL) and Just-In-Time (JIT) Compilation

All compilers that target the CLR compile source code to Intermediate Language (IL), also known as Common Intermediate Language (CIL). IL is a machine language that is not tied to any specific machine. Microsoft designed it from scratch to support the CLI’s programming concepts. The CLI specifies that all CLR implementations can compile or interpret IL on the machine on which the CLR is running. If the IL is compiled (versus interpreted), compilation can occur at either of two times:

• Immediately prior to a method in the application being executed

• At deployment time

In the first case, each method is compiled only when it is actually needed. After the method is compiled, subsequent calls bypass the compilation mechanism and call the compiled code directly. The compiled code is not saved to disk, so if the application is stopped and restarted, the compilation must occur again. This is known as just-in-time (JIT) compilation and is the most common scenario.

In the second case, the application is compiled in its entirety at deployment time.

IL is saved to .exe and .dll files. When such a file containing IL is executed, the CLR knows how to invoke the JIT compiler and execute the resulting code.

Note that on the Microsoft Windows platforms, IL is always compiled—never interpreted.