The Startup class is where:

• Services required by the app are configured.
• The app's request handling pipeline is defined, as a series of middleware components.

Here's a sample Startup class:

public class Startup
{
public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<RazorPagesMovieContext>(options =>
        options.UseSqlServer(Configuration.GetConnectionString("RazorPagesMovieContext")));

    services.AddControllersWithViews();
    services.AddRazorPages();
}

public void Configure(IApplicationBuilder app)
{
    app.UseHttpsRedirection();
    app.UseStaticFiles();

    app.UseRouting();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapDefaultControllerRoute();
        endpoints.MapRazorPages();
    });
 }
}

The Startup class configures services and the app's request pipeline.

• Optionally includes a ConfigureServices method to configure the app's services. A service is a reusable component that provides app functionality. Services are registered in ConfigureServices and consumed across the app via dependency injection (DI) or ApplicationServices.
• Includes a Configure method to create the app's request processing pipeline.

ConfigureServices and Configure are called by the ASP.NET Core runtime when the app starts:

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

public IConfiguration Configuration { get; }

public void ConfigureServices(IServiceCollection services)
{
    services.AddRazorPages();
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();

    app.UseRouting();

    app.UseAuthorization();

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

The preceding sample is for Razor Pages; the MVC version is similar.

The Startup class is specified when the app's host is built. The Startup class is typically specified by calling the WebHostBuilderExtensions.UseStartup method on the host builder:

public class Program
{
public static void Main(string[] args)
{
    CreateHostBuilder(args).Build().Run();
}

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.UseStartup<Startup>();
        });
}

The host provides services that are available to the Startup class constructor. The app adds additional services via ConfigureServices. Both the host and app services are available in Configure and throughout the app.

Only the following service types can be injected into the Startup constructor when using the Generic Host (IHostBuilder):

• IWebHostEnvironment

• IHostEnvironment

• IConfiguration

   public class Startup
   {
     private readonly IWebHostEnvironment _env;
  
     public Startup(IConfiguration configuration, IWebHostEnvironment env)
     {
       Configuration = configuration;
       _env = env;
     }
  
     public IConfiguration Configuration { get; }
  
     public void ConfigureServices(IServiceCollection services)
     {
       if (_env.IsDevelopment())
       {
       }
       else
       {
       }
     }
   }  
  

Most services are not available until the Configure method is called.