Introduction to Repository Pattern (Chat GPT):

The Repository Pattern in ASP.NET Core is a design pattern used to separate business logic from data access logic by providing an abstraction layer over database operations. This pattern improves maintainability, testability, and flexibility in applications by encapsulating database operations in dedicated repository classes.

Why Use the Repository Pattern?

Pros:

Abstraction from ORM (Entity Framework Core)
- Prevents direct dependency on EF Core, making it easier to swap out the data access layer in the future.
Better Code Organization
- Separates concerns by keeping data logic in repositories and business logic in services/controllers.
Improved Testability
- Makes it easier to mock repositories in unit tests.
Encapsulation of Queries
- Common queries can be abstracted, reducing repetition.
Centralized Data Access Logic
- Ensures a single location for handling CRUD operations.

Comparison: Repository Pattern vs. Direct DbSet Operations

Feature	Using Repository Pattern	Using DbSet Directly in Controllers
Separation of Concerns	✅ Maintains separation	❌ Business and data access logic mixed
Testability	✅ Easy to mock and test	❌ Harder to mock DbContext
Code Reusability	✅ Common operations are encapsulated	❌ Repetitive DbSet calls
Flexibility	✅ Can switch database providers easily	❌ Tightly coupled to EF Core

Implementation of the Repository Pattern

We will create:

IRepository (Generic repository interface)
Repository (Generic repository implementation)
Entity-specific repositories (e.g., ICustomerRepository and CustomerRepository)

Introduction to Unit of Work Pattern (Chat GPT):

What is the Unit of Work Pattern?

The Unit of Work (UoW) pattern is a centralized mechanism to manage database transactions and ensure that multiple repository operations are treated as a single unit of execution. It acts as a wrapper around multiple repositories to coordinate their changes and commit them in one go.

Advantages of Unit of Work

1. Single Transaction for Multiple Operations

If you're performing multiple database operations across different repositories, Unit of Work ensures atomicity.
If one operation fails, everything is rolled back (when using explicit transactions).

2. Better Performance

Without UoW: Every repository would call SaveChangesAsync() separately, causing multiple round trips to the database.
With UoW: All changes are saved at once, reducing the number of database calls.

3. Maintains Consistency

When multiple repositories modify related entities, UoW ensures that all changes are either committed or discarded together.

4. Improves Testability

Unit of Work allows you to mock database changes and write unit tests efficiently without worrying about inconsistent data states.

5. Prevents Partial Updates

If multiple repositories handle different entities in the same operation, calling SaveChangesAsync() in individual repositories could lead to partial updates if one operation succeeds and another fails.

Why Should `SaveChanges()` NOT Be in the Repository?

1. Each Repository Should Not Control Transactions

If each repository calls SaveChangesAsync(), you lose control over transactions.

Example Problem (Without UoW)

Imagine you have two repositories: CustomerRepository and OrderRepository.
If you try to add a customer and add an order separately, each calling SaveChangesAsync():

var customer = new Customer { Name = "John Doe" };
await _customerRepository.AddAsync(customer);
await _customerRepository.SaveChangesAsync(); // ❌ First database call

var order = new Order { CustomerId = customer.Id, TotalAmount = 100 };
await _orderRepository.AddAsync(order);
await _orderRepository.SaveChangesAsync(); // ❌ Second database call

What happens if the second SaveChangesAsync() fails?

The customer has already been saved, but the order is missing.
Your database is left in an inconsistent state!

2. Database Round Trips (Performance Issue)

If each repository calls SaveChangesAsync(), you end up with multiple database calls instead of batching them into a single transaction.

await _customerRepository.SaveChangesAsync(); // ❌ DB call
await _orderRepository.SaveChangesAsync(); // ❌ Another DB call

Using Unit of Work, all changes can be saved in one go:

await _unitOfWork.SaveChangesAsync(); // ✅ One database call

This reduces network latency and improves database performance.

3. Promotes Separation of Concerns

Repositories should focus on CRUD operations (data retrieval and manipulation).
Unit of Work should manage transactions.
This makes the code cleaner and easier to maintain.

Key Takeaways

✔ Unit of Work ensures all database operations are part of a single transaction.
✔ Repositories should NOT call SaveChangesAsync() to avoid multiple transactions.
✔ EF Core tracks changes, so calling SaveChangesAsync() once is enough.
✔ Using UoW improves performance, consistency, and maintainability.
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