Startup.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

using Microsoft.Extensions.FileProviders;
using System.IO;
using Microsoft.AspNetCore.SpaServices.ReactDevelopmentServer;

namespace Kvin.Short
{
    public class Startup
    {
        public static IConfiguration StaticConfig { get; private set; }
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
            StaticConfig = 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.AddCors(options =>
            {
                options.AddDefaultPolicy(
                    builder =>
                    {
                        builder
                        .AllowAnyOrigin()
                        .AllowAnyMethod()
                        .AllowAnyHeader();
                    });
            });

            services.AddControllers();

            services.AddSpaStaticFiles(configuration =>
            {
                configuration.RootPath = "reactapp/build";
            });
        }

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

            app.UseHttpsRedirection();

            app.UseStaticFiles(new StaticFileOptions
            {
                FileProvider = new PhysicalFileProvider(
                    Path.Combine(env.ContentRootPath, "reactapp/build")),
                RequestPath = ""
            });
            app.UseSpaStaticFiles();

            app.UseRouting();

            // app.UseCors();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });

            app.UseSpa(spa =>
            {
                spa.Options.SourcePath = "reactapp";
                
                if(env.IsDevelopment())
                {
                    spa.UseReactDevelopmentServer(npmScript: "start");
                }
            });
        }
    }
}