Advanced Model Validation (FluentValidation)

Mind Map Summary

• Topic: Advanced Model Validation (FluentValidation)
• Limitations of Data Annotations:
  • Limited Functionality: Not possible to create complex validation rules, such as conditional validation or rules that depend on other services.
  • Coupling: Validation rules are coupled to the model.
  • Testability: Difficult to unit test validation rules.
• FluentValidation:
  • Fluent Interface: Provides a fluent interface for building validation rules.
  • Decoupling: Decouples validation rules from the model.
  • Testability: Easy to unit test validation rules.
  • Integration with ASP.NET Core: Integrates with the ASP.NET Core pipeline to automatically validate models and populate the ModelState.

Practice Exercise

Create a complex model and use FluentValidation to create rules that are not possible with data annotations (e.g., conditional validation, or a rule that depends on another service). Integrate it into the ASP.NET Core pipeline to have validation errors automatically populate the ModelState.

Answer

1. Model and Validator:

public class Customer
{
    public string Name { get; set; }
    public string Email { get; set; }
    public bool IsPreferred { get; set; }
    public string DiscountCode { get; set; }
}

public class CustomerValidator : AbstractValidator<Customer>
{
    public CustomerValidator()
    {
        RuleFor(x => x.Name).NotEmpty();
        RuleFor(x => x.Email).EmailAddress();
        RuleFor(x => x.DiscountCode).NotEmpty().When(x => x.IsPreferred);
    }
}

2. Configure FluentValidation in Program.cs:

using FluentValidation.AspNetCore;

builder.Services.AddControllers()
    .AddFluentValidation(fv => fv.RegisterValidatorsFromAssemblyContaining<CustomerValidator>());

3. Controller:

using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("[controller]")]
public class CustomersController : ControllerBase
{
    [HttpPost]
    public IActionResult Post(Customer customer)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        // ...

        return Ok();
    }
}

Explanation:

• The CustomerValidator defines a conditional validation rule that requires the DiscountCode to be not empty only if the IsPreferred property is true.
• FluentValidation is integrated into the ASP.NET Core pipeline by calling AddFluentValidation() in Program.cs.
• When a POST request is made to the CustomersController, the Customer model is automatically validated. If the validation fails, the ModelState is populated with the validation errors, and the controller returns a 400 Bad Request response.