C_SHARP_SKILLS/.ai/skills/10-ef-core-data-access.md

Skill 10 - EF Core Data Access

Core Concepts

• EF Core maps entity classes to relational database tables.
• DbContext represents a unit of work and change tracker.
• Providers translate LINQ expressions to database-specific SQL.
• Database First starts from an existing database; Code First starts from C# models and migrations.

DbContext Rules

• Register DbContext with dependency injection using an appropriate lifetime, usually scoped for web apps.
• Keep DbContext short-lived.
• Do not share a DbContext across threads.
• Use AsNoTracking() for read-only queries when tracking is not needed.
• Use transactions when multiple changes must succeed or fail together.

Entity Design Rules

• Entities should protect important invariants, but EF also needs materialization support.
• Avoid exposing EF entities directly from public APIs.
• Keep navigation properties intentional.
• Avoid lazy-loading surprises unless the project deliberately uses lazy loading.

Query Rules

• Filter in the database, not in memory.
• Project only needed columns for read models and API responses.
• Watch for N+1 query problems.
• Use Include intentionally; do not include entire object graphs by habit.
• Use pagination for large result sets.
• Use async EF methods in web applications.

Change Rules

• Validate input before attaching or saving entities.
• Handle concurrency where concurrent updates are possible.
• Do not build SQL by string concatenation with untrusted input.
• Use migrations deliberately and review generated migrations.

Testing Rules

• Use real providers for integration tests where SQL translation matters.
• Avoid assuming the in-memory provider behaves like a relational database.

Repository Pattern

When the project uses a Unit of Work or generic repository abstraction, load .ai/skills/18-unit-of-work-pattern.md for rules on shared DbContext lifetime, repository staging, and single SaveChangesAsync per logical operation.