db4o (database for objects) is the open source object database for .NET. It is a fast, lean and easy to use database for native .NET objects.

It is free software available under the GPL but also under an optional commercial license.

It can persist any native .NET object with a single line of code, minimizing persistence hassle and allowing you to experience the gains of true object-orientation.

Key Features:
- Simplicity (one line of code to store objects)
- Native .NET objects (no interface, inheritance, pre or post-processing required)
- Class model == database schema
- Smooth production process
- Automatic schema versioning
- Query-by-Example
- S.O.D.A. active queries
- 300kB footprint

Key benefits:
- Greatly reduced development time and costs
- Higher quality, less error-prone OO code
- Embedded, web or mobile device deployment
- No need for training and administration
- Build lean, structured and manageable code
- Better refactorability and re-use
- Avoid writing object mapping code
- Simple installation and deployment
- No language conversion overhead
- No problems with debuggers
- No time-consuming build process
- Use existing .NET class libraries without modification
- Run your own .NET code within queries
- Benefit from fast serverside query execution
- Be ready for next generation applications
- Use database technology where your competition needs to store binary

Free Community-Driven Resources and Support:
- Step-by-step Interactive (!) Tutorial
- Community Newsgroup
- Knowledgebase-driven FAQ
- Bug Tracking
- Online API Documentation
- CVS Access
- Direct No-Hassle Download