4d-schema-migrationlisted

# 4D to .NET/SQL Server Migration > "Data is a precious thing and will last longer than the systems themselves." > -- Tim Berners-Lee This skill guides migration from 4D (4th Dimension) applications to modern .NET 10 + SQL Server 2022 + Blazor architecture. ## Core Philosophy 4D is **not just a database** — it is a complete application development platform encompassing a relational database engine, a visual forms designer, a proprietary programming language, triggers, and a built-in user/group security model. Migrating from 4D is fundamentally harder than a typical database-to-database migration because you are migrating an *entire runtime*: data types, business logic, UI forms, validation rules, and security semantics all at once. A naive "export tables, import rows" approach will fail. 4D has data types with no direct SQL Server equivalent (multi-value fields, subtables, Pictures stored as proprietary binary), stores dates and times as separate platform-specific values, has NULL semantics that differ from SQL Server, and uses internal record numbers that can be reused after deletion. Treat the migration as a full-stack platform transition — data types first, then schema, then business logic, then forms — with validation gates between every phase. The full domain-principle set, AI discipline rules, anti-pattern catalog, and knowledge-base query-to-phase lookup live in `references/common-4d-patterns.md`; per-phase validation checklist and error-recovery procedures live