Overview

• ASP.NET Core uses a startup class, which is named **Startup** by convention.
• It optionally includes a **ConfigureServices** method to configure the app's services.
• It must include a **Configure** method to create the app's request processing pipeline.
• The **ConfigureServices** and **Configure** are called by the runtime when the app starts.
• Specify the Startup class with the **WebHostBuilderExtensions UseStartup<TStartup>()** method:

public class Program
{
    public static void Main(string[] args)
    {
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>()
            .Build();
    }
}

• The **Startup** class constructor accepts dependencies defined by the host.
• A common use of dependency injection into the **Startup** class is to inject:
  • **IHostingEnvironment** to configure services by environment.
  • **IConfiguration** to configure the app during startup.

public Startup(IHostingEnvironment env, IConfiguration config)
{
    HostingEnvironment = env;
    Configuration = config;
}

• An alternative to injecting IHostingEnvironment is to use a conventions-based approach.
• The app can define separate Startup classes for different environments (for example, StartupDevelopment).
• The class whose name suffix matches the current environment is prioritized.

The ConfigureServices Method

• This method is called by the web host to configure the app's services. The web host provides some services that are available to the Startup class constructor.
• The app adds additional services via ConfigureServices.
• Both the host and app services are then available in Configure and throughout the application.
• The services are resolved via dependency injection or from **IApplicationBuilder.ApplicationServices**.
• The web host may configure some services before Startup methods are called.
• For features that require substantial setup, there are Add[Service] extension methods on IServiceCollection.