Asp.Net Web Api Fluentvalidation Integration Complete Guide

1. What is FluentValidation in ASP.NET Web API?

Answer: FluentValidation is a popular library for .NET applications that allows you to create highly readable validation rules using a fluent interfaces. In ASP.NET Web API, it can be used to validate models before processing them in your controllers, helping to maintain clean and testable code.

2. How do I install FluentValidation in my ASP.NET Web API project?

Answer: You can easily add FluentValidation to your ASP.NET Web API project via NuGet. Simply open the Package Manager Console within Visual Studio and execute the following command:

Install-Package FluentValidation.AspNetCore

Alternatively, you can use the NuGet package manager GUI to search for FluentValidation.AspNetCore and install it that way.

3. How can I set up a basic model validator with FluentValidation?

Answer: To set up a validator, you need to create a class that inherits from AbstractValidator<T> where T represents the type of the object you want to validate. Here’s an example:

public class MyModel {
    public string Name { get; set; }
    public int Age { get; set; }
}

public class MyModelValidator : AbstractValidator<MyModel> {
    public MyModelValidator() {
        RuleFor(x => x.Name).NotEmpty().WithMessage("Name is required");
        RuleFor(x => x.Age).InclusiveBetween(1, 120).WithMessage("Age must be between 1 and 120");
    }
}

4. How do I integrate FluentValidation with my ASP.NET Web API services?

Answer: To integrate FluentValidation, you need to add it as a service in your Startup.cs file:

public void ConfigureServices(IServiceCollection services) {
    services.AddControllers()
            .AddFluentValidation(fv => {
                fv.RunDefaultMvcValidationAfterFluentValidationExecutes = false;
                fv.RegisterValidatorsFromAssemblyContaining<Program>();
            });
}

The RegisterValidatorsFromAssemblyContaining<Program>() line tells FluentValidation to scan the specified assembly for validators and automatically register them.

5. Can FluentValidation handle complex nested objects?

Answer: Yes, FluentValidation supports validation for nested objects through the SetValidator() method. For example:

public class OrderDetail {
    public Product Product { get; set; }
    public int Quantity { get; set; }
}

public class OrderDetailValidator : AbstractValidator<OrderDetail> {
    public OrderDetailValidator() {
        RuleFor(x => x.Quantity).GreaterThan(0);
        RuleFor(x => x.Product).NotNull();
        RuleFor(x => x.Product).SetValidator(new ProductValidator());
    }
}

public class Product {
    public string Name { get; set; }
    public decimal Price { get; set; }
}

public class ProductValidator : AbstractValidator<Product> {
    public ProductValidator() {
        RuleFor(x => x.Name).NotEmpty();
        RuleFor(x => x.Price).GreaterThanOrEqualTo(0);
    }
}

6. Does FluentValidation work with custom attributes?

Answer: FluentValidation doesn’t support data annotation attributes by default but you can create custom validators that mimic those attributes or adapt your validation logic within the FluentValidation framework. However, using the framework's built-in methods generally results in cleaner and more flexible validation rules.

7. How do I implement custom validation rules with FluentValidation?

Answer: Implement custom validation rules by creating a method within your validator class and then referencing it in your validation chain using the Custom() method. Here’s an example:

public class EmailValidator : AbstractValidator<string> {
    public EmailValidator() {
        Custom((email, context) => {
            if (!email.Contains("@")) {
                context.AddFailure("Not a valid email address.");
            }
        });
    }
}

8. Can I use FluentValidation with other libraries like MediatR?

Answer: Absolutely, FluentValidation can seamlessly integrate with MediatR or any other similar CQRS implementation. When using MediatR, the request models can be validated by applying validators to those request types. This ensures all business rules are applied consistently across your application.

9. How can I handle validation failures in ASP.NET Web API when using FluentValidation?

Answer: FluentValidation automatically attaches validation failures to the model state. If validation fails, the ModelState.IsValid property will be false, and the errors can be accessed via ControllerBase.ModelState. For instance, you might return BadRequest(ModelState) from your action if validation didn't pass.

10. How can I configure FluentValidation to throw exceptions instead of returning model errors on ASP.NET Web API actions?

Answer: By default, FluentValidation doesn't throw exceptions but rather aggregates errors onto the model state. However, if you want to change this behavior, you can use the ThrowOnFailures() method within your validator:

public class MyModelValidator : AbstractValidator<MyModel> {
    public MyModelValidator() {
        CascadeMode = CascadeMode.StopOnFirstFailure;

        RuleFor(x => x.Name).NotEmpty().WithMessage("Name is required")
                             .ThrowOnFailures();

        RuleFor(x => x.Age).InclusiveBetween(1, 120).WithMessage("Age must be between 1 and 120")
                           .ThrowOnFailures();
    }
}

To enforce globally, you can set this in your Startup.cs configuration when registering services: