readytorun

ReadyToRun is a feature of the .NET runtime that provides ahead-of-time (AOT) compilation for managed assemblies. It creates ReadyToRun images by precompiling a subset of IL code into native code and embedding it alongside assemblies. The goal is to accelerate startup and reduce the workload on the Just-In-Time (JIT) compiler during application startup.

Origins and tooling: ReadyToRun originated in the .NET Core ecosystem as a mechanism to improve startup performance. It is produced using the crossgen tool, and in newer runtimes it has been extended with crossgen2. The precompiled native code is platform- and runtime-specific. At startup, the runtime will use the precompiled image when it is compatible; if not, it falls back to JIT compilation for the relevant code paths.

Usage and configuration: ReadyToRun images are generated at publish time for selected assemblies, and developers can configure which parts of an application are precompiled. The feature is controlled by publish or project settings and depends on the target runtime identifier (RID). Images may need regeneration after framework updates or changes to the application or its dependencies to remain valid.

Impact and limitations: The primary benefit is reduced startup latency and faster initial JIT warm-up by using precompiled code paths. Trade-offs include increased package size and potential compatibility concerns with certain dynamic code patterns or framework updates. Not all code is necessarily precompiled, and some scenarios such as heavy use of reflection may benefit less or require careful configuration.

Relation to other technologies: ReadyToRun is distinct from Native AOT, which compiles an application to a fully standalone native binary. ReadyToRun serves as an optimization within the runtime, precompiling critical paths to improve startup without replacing the broader JIT and runtime model.