.NET Framework to Modern API Layer Modernization

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Context

An enterprise software vendor maintained a revenue-critical product on .NET Framework. The product worked, but the team could not adopt modern libraries, hiring was slow, and operational costs had grown over time. A “big rewrite” had been considered and rejected as too risky.

Problem

• The monolith was tightly coupled across UI, business logic, and persistence.
• Test coverage was concentrated in low-risk areas; the most critical workflows were almost untested.
• Operational tooling did not match modern observability standards.
• Customers were beginning to ask about integration capabilities the platform could not deliver.

Constraints

• Continuous delivery to existing customers had to continue.
• No regression in functional behaviour was acceptable.
• Existing engineers had to remain productive throughout.

Approach

1. Carve a clean API boundary in front of the existing monolith.
2. Introduce a modern .NET LTS API project alongside, sharing the database initially.
3. Move read endpoints first, behind feature flags, with shadow comparison against the legacy code path.
4. Migrate write workflows in priority order, with explicit regression tests before each cutover.
5. Gradually retire the equivalent code paths from the monolith.

Deliverables

• Modular API layer on modern .NET LTS
• Shared regression test suite covering both code paths during transition
• Feature flag infrastructure with safe cutover patterns
• Updated CI/CD pipelines
• Operational dashboards aligned with the new runtime

Measurable outcomes

• Endpoints migrated to modern .NET: [Replace with verified metric]
• Cycle time on the modernized surface: [Replace with verified metric]
• Memory footprint at equivalent load: [Replace with verified metric]
• New integrations enabled by the API layer: [Replace with verified metric]

Technologies

.NET Framework, .NET LTS, REST APIs, feature flags, contract testing, observability tooling.

Lessons learned

The decisive move was not the code migration — it was establishing a clean boundary in front of the monolith and using it as the point of comparison. Once shadow traffic existed for every migrated endpoint, the team could make small, safe cutovers without ever staking the business on a single switch.