Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;

namespace FluentValidationSample
{
    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.AddMvc(options =>
            {
                options.Filters.Add(typeof(ValidateModelStateAttribute));
            }).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
            services.Configure<ApiBehaviorOptions>(options =>
            {
                options.SuppressModelStateInvalidFilter = true;
            });
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            app.UseHttpsRedirection();
            app.UseMvc();
        }
    }
}