Lesson 102 +10 XP

Scripting Backends and IL2CPP

Unity compiles your C# code using one of two scripting backends. The backend determines how your code runs and what optimizations are available.

Mono Backend

Mono is the original cross-platform .NET runtime Unity uses.

C# compiles to Intermediate Language (IL), which is interpreted by the Mono runtime at runtime.
Faster iteration: Changes in code compile quickly during development.
Lower performance: The JIT (Just-In-Time) compiler is less aggressive than AOT.
Available on: All platforms as a development option. Not available on iOS App Store builds.

IL2CPP Backend

IL2CPP (Intermediate Language To C++) converts IL bytecode into C++ source code, which is then compiled to native machine code by the target platform's C++ compiler.

Higher performance: Fully native code with AOT (Ahead-Of-Time) optimizations.
Platform requirement: Required for iOS (Apple does not allow JIT), WebGL, and most console platforms.
Slower build times: Requires compiling C++ output.
Smaller binaries on some platforms.

Selecting the Backend

Edit > Project Settings > Player > Other Settings > Scripting Backend:

Choose Mono for fast development iteration.
Choose IL2CPP for release builds and required platforms.

Managed Code Stripping

IL2CPP enables Managed Code Stripping, which removes unused C# code from the build to reduce binary size:

Strip Level	Description
Disabled	No stripping. Safest but largest binary.
Low	Removes unreachable types. Recommended default.
Medium	Removes more aggressively. May require link.xml.
High	Strips most unused code. Requires thorough testing.

Use a link.xml file to preserve types that stripping incorrectly removes:

<linker>
  <assembly fullname="MyAssembly">
    <type fullname="MyNamespace.ImportantClass" preserve="all"/>
  </assembly>
</linker>

Scripting Restrictions with IL2CPP

IL2CPP does not support:

Runtime code generation (e.g., System.Reflection.Emit).
dynamic keyword in C# (limited support).
Some advanced reflection patterns.

TL;DR

Mono uses JIT compilation. Fast iteration, lower performance. Good for development.
IL2CPP converts to C++ and compiles to native code. Required for iOS, WebGL, consoles.
IL2CPP builds are slower but produce faster, smaller runtime executables.
Use Managed Code Stripping with IL2CPP; protect needed types with link.xml.

Scripting Backends and IL2CPP