Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using api_chatroom.Hubs;

namespace api_chatroom {
    public class Startup {
        public Startup(IConfiguration configuration) {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services) {
            services.AddCors((options) => {
                options.AddPolicy("DevelopmentCorsPolicy",
                    (builder) => {
                        builder.WithOrigins("http://localhost:4200")
                               .AllowAnyHeader()
                               .AllowAnyMethod()
                               .AllowCredentials();
                    });
                options.AddPolicy("ProductionCorsPolicy",
                    (builder) => {
                        builder.WithOrigins("https://app-chatroom-ng.herokuapp.com")
                               .AllowAnyHeader()
                               .AllowAnyMethod()
                               .AllowCredentials();
                    });
            });

            services.AddControllers();
            services.AddSignalR();
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env) {
            if (env.IsDevelopment()) {
                app.UseDeveloperExceptionPage();
    
                app.UseCors("DevelopmentCorsPolicy");
            } else {
                app.UseCors("ProductionCorsPolicy");
            }

            app.UseDefaultFiles();
            app.UseStaticFiles();
            app.UseRouting();

            app.UseEndpoints((endpoints) => {
                endpoints.MapControllers();
                endpoints.MapHub<ChatHub>("/chathub");
            });
        }
    }
}