.NET Framework to .NET 8 SaaS Platform Migration
Migrating 500,000 lines of .NET Framework 4.5 code to .NET 8 — zero downtime, 60% performance gain, using the Strangler Fig pattern with full CI/CD automation.
The Challenge
The Challenge: 500,000 Lines of .NET Framework Code and Zero Downtime Tolerance
The client's enterprise SaaS platform had been built on .NET Framework 4.5 over eight years, accumulating 500,000 lines of C# code, 200+ SQL Server stored procedures, Windows-specific dependencies (Windows Communication Foundation, Windows Identity Foundation), and a deployment process that required 2-hour scheduled maintenance windows. The platform served enterprise customers with SLAs requiring 99.9% availability. Microsoft's end-of-support for .NET Framework 4.5 was a driver, but the performance gap between .NET Framework and .NET 8 was equally compelling: benchmarks showed ASP.NET Core on .NET 8 delivering 2–3x the throughput of the legacy stack on identical hardware, with 40% lower memory usage. The migration needed to deliver performance improvements, eliminate Windows deployment lock-in (to enable Azure Linux containerisation), and modernise the authentication stack from WIF to MSAL — all without a single extended production outage or customer-facing disruption.
The Solution
Solution: Strangler Fig Pattern with Feature Flags and Blue-Green Deployment
Applied the Strangler Fig migration pattern — progressively replacing legacy .NET Framework components with new .NET 8 services behind an API gateway, routing traffic incrementally from old to new.
Migration Architecture and Dependency Analysis
Conducted a full codebase dependency analysis to identify migration order — which components could be migrated independently and which had tightly coupled dependencies requiring coordinated migration. Categorised all 200+ stored procedures as candidates for migration to EF Core, retention as stored procedures, or elimination. Identified all Windows-specific dependencies (WCF, WIF, Windows Registry access) and designed .NET 8 equivalents.
Strangler Fig Implementation
Deployed Azure API Management as the routing layer between legacy and new services. New .NET 8 microservices were deployed behind the API gateway with feature flags controlling traffic percentage. Starting at 1% of traffic for the first migrated component (user authentication), progressively increased to 100% over 4 weeks as stability was confirmed. The legacy .NET Framework service remained available as the fallback throughout.
Performance and CI/CD Modernisation
Replaced the 2-hour manual deployment with a GitHub Actions CI/CD pipeline delivering blue-green deployments with zero-downtime. Containerised all new .NET 8 services with Docker, deployed to Azure Kubernetes Service. Added comprehensive integration testing with TestContainers (real SQL Server, real Redis), automated performance benchmarking in the CI pipeline, and SonarCloud static analysis on every pull request.
Technology Stack
Tools & Technologies Used
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