How to structure an application by defining its main layers?

In a typical C# application, several main layers are distinguished to improve support and scalability:

• Presentation Layer (UI) — responsible for user interaction, such as a web interface or desktop application.
• Business Logic Layer (BLL) — contains business rules and data processing logic.
• Data Access Layer (DAL) — responsible for interacting with the database or other data sources.
• Domain Layer — models of the subject area, describing the main entities.

This separation helps isolate changes, facilitates testing, and reusing code.

Example project structure:

MyApp
├── MyApp.UI
├── MyApp.Business
├── MyApp.Data
└── MyApp.Domain

Each layer is implemented in a separate assembly or folder, which simplifies navigation and maintenance.