// the find
dotnet/runtime
.NET is a cross-platform runtime for cloud, mobile, desktop, and IoT apps.
This is the official .NET runtime repository — the CLR, JIT compiler, base class libraries, and the dotnet host muxer all live here. It's where .NET itself is built, not where you build things on top of .NET. If you're curious how the GC works, want to trace a performance regression to the JIT, or want to contribute a BCL API, this is the source.
The Book of the Runtime docs (docs/design/coreclr/botr/) are genuinely good — clr-abi.md, garbage-collection.md, and the JIT overview are some of the best internals documentation in any open-source runtime. The agentic workflow setup (.github/skills/, .github/agents/) is unusually mature for a project this size — automated issue triage, CI failure scanning, and JIT regression testing are all codified and running. The JIT's optimizer documentation (JitOptimizerTodoAssessment.md, GuardedDevirtualization.md) gives honest assessments of what's working and what's still rough, which is rare. Cross-platform devcontainer coverage is solid: separate containers for WASM, Android, and libraries work so you can build any target without polluting your local machine.
Build times are brutal — building the full runtime from scratch takes 20-40 minutes on a fast machine, and the build system (ARCADE + MSBuild + native CMake) has enough layers that debugging a build failure is a project in itself. The repo is enormous and undifferentiated from a contributor perspective; finding where a BCL type actually lives versus its reference assembly versus its test requires knowing the layout conventions, which aren't obvious. The JIT source under src/coreclr/jit/ is dense C++ with minimal comments — useful if you already understand SSA and liveness analysis, hostile if you don't. The managed threading and async models are documented in the BOTR but the interaction between the GC's cooperative mode, thread suspension, and async state machines is a frequent source of subtle bugs that even experienced contributors hit.