# Change Tracking

> Implementation details for EF Core change tracking. Use when changing InternalEntityEntry, ChangeDetector, SnapshotFactoryFactory, or related entity state, snapshot, or property accessor code.

- Skill: `dotnet/change-tracking` (Agent Skill)
- Install (CLI): `npx skillmds@latest add dotnet/change-tracking`
- Raw SKILL.md: https://api.skillmd.com/api/skills/dotnet/change-tracking/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: .NET (Microsoft) (https://skillmd.com/u/dotnet)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/dotnet/change-tracking

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# Change Tracking

Manages entity states and detects changes for `SaveChanges()`.

## Core Components

- `StateManager` — central engine, identity maps, tracks all entities
- `InternalEntityEntry` — per-entity state, property flags, snapshots
- `SnapshotFactoryFactory` subclasses build snapshot factories for change detection
- `PropertyAccessorsFactory`, `ClrPropertyGetterFactory` and `ClrPropertySetterFactory` compile property accessors for efficient snapshotting and change detection
  - Ordinals in `indices` parameter specify element at each complex collection depth

## Testing

Unit tests: `test/EFCore.Tests/ChangeTracking/`. Functional tests: `test/EFCore.Specification.Tests/GraphUpdates/`.

## Common Pitfalls

| Pitfall | Solution |
|---------|----------|
| There is a failure when there is shared identity entry (Added and Deleted) | Add code that checks `SharedIdentityEntry` |

## Validation

- `DetectChanges()` identifies modified properties via snapshot comparison
- Setting original values marks properties as modified or unchanged based on comparison with current values