crossgen2

Crossgen2 is a tool in the .NET runtime toolchain used to ahead-of-time compile managed assemblies into native-ready code, creating ReadyToRun images that improve startup performance by reducing the JIT compilation workload at application start. Introduced as the successor to the legacy crossgen, crossgen2 is designed to work with the CoreCLR runtime across Windows, Linux, and macOS and to better support large framework dependencies and multi-targeting scenarios.

It analyzes the intermediate language, metadata, and commonly used code paths to emit architecture-specific native code and metadata in a form that the runtime can consume at load time. The produced ReadyToRun images are stored alongside the assemblies and are loaded by the runtime during startup, reducing the time spent to JIT compile hot methods.

Crossgen2 is typically used as part of the application's publish or prebuild process, rather than at runtime. It is the preferred path when targeting startup time improvements for client applications and when shipping frameworks with ReadyToRun-compiled images. It is closely related to the ReadyToRun feature and to the broader family of ahead-of-time options. It is separate from NativeAOT, which compiles to a fully native executable rather than to ReadyToRun images.

Implementation and scope: Crossgen2 is part of the dotnet/runtime toolchain and is maintained as open source by the .NET community. While it can improve startup, it does not guarantee universal improvements, as some code patterns or dynamic features may be less amenable to pre-compilation. Developers should test broadly to ensure compatibility across runtimes and updates.