The .NET team built a Virtual Monolithic Repository (VMR) that aggregates source code from dozens of standalone repositories into a single git repository for unified builds. The VMR uses a custom two-way synchronization algorithm based on git patches to keep changes flowing between product repositories and the monorepo. The synchronization evolved through three stages: tarball-based Source Build, read-only VMR-lite with one-way sync, and finally a writable VMR with bidirectional flow. The team encountered challenges with conflicting changes, branch lifecycle mismatches, and metadata corruption, ultimately settling on a rebase-based approach that handles conflicts interactively. This infrastructure now powers .NET SDK builds starting from .NET 10 Preview 4.
