Intermediate Language (IL)

When you click "Build" in Visual Studio, your C# compiler (csc.exe) does NOT create machine code. It creates MSIL (Microsoft Intermediate Language).

• Why? This allows the binary to be platform-independent. The same .dll can contain instructions that can be understood by a 32-bit, 64-bit, or even ARM processor, provided there is a CLR for that platform.

Study Deep: The Three Types of JIT Compilers

The CLR actually supports three ways to perform JIT compilation:

1. Normal JIT (Default): Compiles methods only when they are called and caches the result in RAM. Fast and efficient.
2. Econo-JIT: Compiles methods when called but doesn't cache them. Good for low-memory devices (rarely used now).
3. Pre-JIT (Ngen.exe): Compiles the entire assembly into native code before execution (at install time). This speeds up application startup but sacrifices platform independence for that specific binary.

JIT (Just-In-Time) Compiler

Since the CPU cannot understand IL directly, someone needs to translate it into the native language of the CPU (0s and 1s specific to the chip structure). This is the job of the JIT Compiler.

Why "Just-In-Time"? It doesn't compile the entire program at once (which would slow down startup). It compiles methods only when they are called.

Types of JIT Compilers