Legacy desktop application modernization

01The context

A business-critical Windows Forms application with 200+ screens served more than 500 daily users. It was slow, untested, and hard to change — but too important to replace in one go.

02The challenges

• Tightly coupled legacy code

  Business logic was embedded in the UI layer across 200+ screens, making a big-bang rewrite far too risky.

• Performance bottlenecks

  N+1 queries, unoptimized stored procedures, and no caching meant some reports took more than 30 seconds to load.

• No test coverage

  With zero automated tests, any refactoring risked breaking behavior users depended on.

03What we did

1. 01

   Strangler-fig migration

   New features were built in Angular and .NET using Clean Architecture and CQRS, while legacy screens were migrated one module at a time. Shared authentication let users move between old and new seamlessly.

2. 02

   Query optimization and caching

   Critical stored procedures were rewritten, N+1 queries eliminated, and Redis caching added for reference data, along with pagination and virtual scrolling for large datasets.

3. 03

   Test-first migration

   Before each module moved, integration tests captured the legacy behavior and became the acceptance criteria for the new implementation.

04Key features

• Incremental migration with zero downtime
• Shared authentication across legacy and modern systems
• Responsive Angular Material UI
• Azure DevOps CI/CD pipeline with feature flags for gradual rollout