crossgen

Crossgen is a tool in the .NET ecosystem used to precompile managed assemblies into native images to improve startup performance. By translating intermediate language (MSIL) to platform-specific native code, Crossgen yields images (often with a .ni.dll extension) that the runtime can load directly, reducing just-in-time compilation during application startup.

Originating in the .NET Framework and continuing into .NET Core, Crossgen served as a practical means to accelerate application startup for large or performance-sensitive deployments. In the era of .NET Core 3.x, Crossgen and related ReadyToRun techniques were employed to generate precompiled images for assemblies and their dependencies. In newer developments, Crossgen has been augmented or superseded by CrossGen2, which provides a more scalable and flexible approach to precompilation across modern runtimes and platforms.

How Crossgen works at a high level: it analyzes a target assembly and its referenced assemblies, compiles the managed code into native machine code for the target architecture, and stores the results in native images that are loaded by the runtime in place of the standard MSIL-based assemblies. This can improve startup time and reduce CPU overhead during the initial phase of execution. The tool may also generate debugging metadata and must be used with compatible platform targets and reference assemblies.

Use and considerations: Crossgen is typically employed in controlled deployment scenarios where startup latency is critical and where updates to precompiled images can be tightly managed. It requires careful handling of versioning, as updates to managed assemblies can necessitate recomputation of native images. Dynamic code generation or heavy use of reflection can limit or negate the benefits.

See also: ReadyToRun, CrossGen2, NGen, .NET runtime.