How and when does ngen.exe work?

Question

I want to know the benefit of pre-JIT compilation (ngen.exe). What is the role of the Native Image Generator (NGen) process and why is it required?

Please provide an

Answer 1

When a .NET compiler compiles C# or VB.NET code it half compiles them and creates CIL code. When you run this half-compiled .NET EXE file the JIT runs in the background and compiles the half CIL code in to full machine language. This mode is termed as normal JIT.

You can also go the other way around saying you do not want runtime compilation by running a full compiled EXE file. This compilation is done by using negen.exe. In this scenario the JIT does not participate at runtime. This is termed as pre-JIT mode.

If you want to see how they affect performance you can see this YouTube video which demonstrates normal-JIT and pre-JIT mode of compilation:

Explain JIT, Ngen.exe, Pre-jit, Normal-Jit and Econo-Jit.? (.NET interview questions)