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dotnet-onion-architecture

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.Net onion/clean architecture sample

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# Onion Architecture / Clean Architecture

- Onion architecture can solve problem of separation of concern and tightly coupled components from N-layered architecture.
- All layers are depended on inner layer.
- The core of the application is the domain layer.
- Provide more testability than N-layered architecture.

## Layers

### Domain Layer:

This layer does not depend on any other layer. This layer contains entities, enums, specifications etc.  
Add repository and unit of work contracts in this layer.

### Application Layer:

This layer contains business logic, services, service interfaces, request and response models.  
Third party service interfaces are also defined in this layer.  
This layer depends on domain layer.

### Infrastructure Layer:

This layer contains database related logic (Repositories and DbContext), and third party library implementation (like logger and email service).  
This implementation is based on domain and application layer.

### Presentation Layer:

This layer contains Webapi or UI.

## Domain model

Domain model are of 2 types

1. Domain entity (data only)
	- This model contains only fields
    - This is an anti pattern used widely. Read blog from Martin Fowler (<a href="https://martinfowler.com/bliki/AnemicDomainModel.html">here</a>)

2. Domain model (data + behaviour)
	- This model has fields and behaviours. Fields can be modify only within behaviours.
	- Follow Aggregate pattern with Aggregate root, Value object, Entity, Bounded context, Ubiqutous language

## Validations in Domain driven design:

There are 2 types of validations in DDD:
1. Model Field validations
	- Properties having valid length
    - required field validations
    - regex

Model validations can be validated in Application layer or Domain layer.

- Use DataAnnotation (https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations?view=net-7.0)
    - Use Guard pattern (https://github.com/NilavPatel/Guard-Pattern)
        - mostly used when you have single source of truth (validations in aggregate pattern)
    - Use fluent validations pattern (https://docs.fluentvalidation.net/en/latest/aspnet.html)
        - Used when validating models instead of single properties like request models, commands

2. Business validations
	- Balance should be more than Withdraw amount
	- User should be active 
	- User name should not be exist

Business validations can be validated in Applciation layer or Domain layer.

Business validations have two types:

1. Validations in same domain model
        - Balance should be more than Withdraw amount
        - User should be active 
    2. Validations against other domain models
        - User name should not be already exist

For Aggregate pattern add both types of validations inside domain layer.  
  
Problem occurs when validating domain model against other domain models.  
  
- In this case use Func<> methods to pass validations to domain model from Application layer.  
And run this Func<> from domain models.  
````csharp
public WithdrawMoney(double amount, Func<string, bool> isBankAccountActive){
    if(!isBankAccountActive(BankAccountNumber)){
        throw new DomainValidationException("Bank account is not in active state");
    }
}
````

- Otherwise pass domain model of other type as parameter and then validate.
````csharp
public WithdrawMoney(double amount, BankAccount bankAccount){
    if(bankAccount.Status != Active){
        throw new DomainValidationException("Bank account is not in active state");
    }
}
````

**For more details <a target="_blank" href="https://docs.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/ddd-oriented-microservice">read</a>**

### Technologies Used:

- .Net 8
- Rest API
- Entity Framework
- NLog
- Swagger
- Xunit
- Moq
- Generic Repository Pattern
- Specification pattern

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