Implementing FluentValidation with ASP.NET Core

FluentValidation for me is a validation framework that enables readability and more cleaner code into our application. There are a predefined set of rules that already predefined from the framework using Lambda Expression which makes the framework more readable and very neat.

For this post, we will create ASP.NET Core WebApi Project.

Configure The Project

Installing FluentValidation.AspNetCore Nuget using this command prompt in the Package Manager Console:

Install-Package FluentValidation.AspNetCore -Version 8.5.0

Creating Our First Validator

Let’s say we will create a Validator that will restrict Customers to had the same email address. For that purpose, I will create the first mock of DataContext and the Validator.

    public class Customer
        public string Name { get; set; }
        public string Email { get; set; }

    public interface IDataContext
        List<Customer> Customers { get; set; }

    public class DataContext : IDataContext
        public List<Customer> Customers { get; set; }

        public DataContext()
            Customers = new []
                new Customer { Name = "Temmy Wahyu Raharjo", Email = ""},
                new Customer { Name = "Inge Agustiningrum", Email = ""},
                new Customer { Name = "Feronic Oktarini", Email = ""},


  public class CustomerValidator : AbstractValidator<Customer>
        public CustomerValidator(IDataContext dataContext)
            RuleFor(e => e.Name).NotEmpty();
            RuleFor(e => e.Email).NotEmpty();
            RuleFor(e => e.Email)
                .Must(email => !dataContext.Customers.Any(c => c.Email == email))
                .WithMessage(customer => $"Customer using {customer.Email} already exists");

Wrapping It Up

We will wrap all the works with a Dependency Injector. For this sample, we will use ASP.NET Core Dependency Injector.


    public class Startup
        public Startup(IConfiguration configuration)
            Configuration = configuration;

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
            services.AddSingleton<IDataContext, DataContext>();
            services.AddTransient<IValidator<Customer>, CustomerValidator>();

            services.Configure<ApiBehaviorOptions>(opts =>
                opts.InvalidModelStateResponseFactory = (context) =>
                    var errors = context.ModelState.Values.SelectMany(a => a.Errors.Select(e => e.ErrorMessage)).ToList();
                    var result = new
                        Code = "err",
                        Message = "Validation Errors",
                        Errors = errors

                    return new BadRequestObjectResult(result);

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
            if (env.IsDevelopment())
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see


In line 13, we adding Dependency Injection for IDataContext. This means that if got a class that needs IDataContext, we will substitute it with DataContext. The other setting is we just want to create 1 single implementation of this class per App (Singleton Pattern).

In line 14, we let the services know that we will add FluentValidation.

In line 16, we register our validator. If got validation for the Customer, we will invoke CustomerValidator.

In Line 18 – 32, we configure if got validation errors, we will throw a new object with 3 attributes (Code, Message, and Errors).



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