Enforcing Modular Monolith Boundaries in .NET: NDepend, Parallel Pipelines, and the Architecture That Holds

A modular monolith without enforcement is not an architecture — it is a monolith with good intentions. The Problem Most teams skip the modular monolith and jump straight to microservices. The ones that do attempt a modular monolith rely on convention — "don't cross module boundaries" — which fails the moment deadlines hit. The difference between a well-structured modular monolith and a mess is whether boundaries are maintained by tooling or by convention. The Solution Structure Each module is a pair of .NET projects: src/Modules/ Orders/ YourApp.Orders/ ← internal: domain, application, infrastructure YourApp.Orders.Contracts/ ← public: DTOs, interfaces, events Payments/ YourApp.Payments/ YourApp.Payments.Contracts/ The rule : modules may only reference each other's *.Contracts projects. The compiler enforces this physically — no project reference means no type access. Four Layers of Enforcement Compiler — project references prevent cross-module type access NetArchTest — architecture tests fail the build on namespace-level violations NDepend CQLinq — catches dependency cycles and coupling the compiler can't see Quality Gates — block PRs that introduce new boundary violations Module-Scoped Data Each module owns a dedicated DbContext with a schema prefix ( orders.* , payments.* ). No module queries another module's tables. Cross-Module Communication Modules communicate via MediatR in-process events. Orders publishes OrderPlaced ; Payments subscribes — without Orders knowing Payments exists. This is also the extraction seam: when you eventually extract a module into a service, MediatR becomes a message broker. The event contract stays the same. Parallel CI strategy : matrix : module : [ Orders , Payments , Inventory ] fail-fast : false Each module's tests run in parallel. CI time scales with the slowest module, not the total count. The Extraction Path When a module genuinely needs independence: Add outbox table → publish to real broker Replace MediatR handlers with brok