.NET Learning Notes: How to use FluentValidation and Custom attributes to check parameters

Reference:
https://docs.fluentvalidation.net/en/latest/installation.html
https://learn.microsoft.com/en-us/aspnet/core/mvc/models/validation?view=aspnetcore-9.0

Thanks for providing learning code from Zack.Yang

FluentValidation is a .NET library for building strongly-typed validation rules.

The FluentValidation.AspNetCore package is no longer being maintained and is now unsupported. We encourage users move away from this package and use the core FluentValidation package with a manual validation

1.Install NuGet package:

FluentValidation

if you want to let FluentValidation automatically find all the validators in a specific assembly using an extension method. You should install another NuGet package:

FluentValidation.DependencyInjectionExtensions

2.Register validators

services.AddScoped<IValidator<Person>, PersonValidator>();

Or (DependencyInjectionExtensions)

services.AddValidatorsFromAssemblyContaining<UserValidator>();

3.Define a set of validation rules for a class(e.g. Person) by inheriting from AbstractValidator

public class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public int Age { get; set; }
}

public class PersonValidator : AbstractValidator<Person>
{
    public PersonValidator()
    {
        RuleFor(p => p.Id).NotNull();
        RuleFor(p => p.Name).Length(5, 10);
        RuleFor(p => p.Email).EmailAddress();
        RuleFor(p => p.Age).InclusiveBetween(18, 60);
        // if the field is a complex properties, you can set another  
        // validator for it
        // RuleFor(p => p.Address).SetValidator(new AddressValidator)
    }
}

4.Manual Validation

[HttpPost]
public IActionResult Validate([GuidValidator]Guid guid, Person person)
{
    var result = validator.Validate(person);
    if (!result.IsValid)
    {
        result.AddToModelState(this.ModelState);
        return BadRequest(ModelState);
    }
    return Ok("Validate person ok");
}

Besides, For scenarios that the build-in validation attributes don't handle, you can create custom validation attributes.
1.Create a class that inherits from ValidationAttribute, and override the IsValid method

public class GuidValidatorAttribute : ValidationAttribute
{
    public const string DefaultErrorMessage = "The {0} field is required and not Guid.Empty";
    public GuidValidatorAttribute() : base(DefaultErrorMessage) { }
    public override bool IsValid(object? value)
    {
        if (value is not null && value is Guid)
        {
            Guid guid = (Guid)value;
            return guid != Guid.Empty;
        }
        else
        {
            return false;
        }
    }
}

2.Add this attribute to parameter

public IActionResult Validate([GuidValidator]Guid guid, Person person)
{
}