Startup.cs

public class Startup
{
  public IConfiguration Configuration { get; set; }

  public Startup(IHostingEnvironment env)
  {
      var builder = new ConfigurationBuilder()
       .SetBasePath(env.ContentRootPath)
       .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
       .AddEnvironmentVariables();

       Configuration = builder.Build();
   }

  // This method gets called by the runtime. Use this method to add services to the container.
  public void ConfigureServices(IServiceCollection services)
  {
    services.AddDbContext<BooksCatalogDbContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
    services.AddMvc();
  }

 // 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())
  {
	app.UseDeveloperExceptionPage();
	app.UseBrowserLink();
  }
  else
  {
	app.UseExceptionHandler("/Error");
  }

  app.UseMvc();
 }  
}