ASP.NET Core - Startup Class

Here, we will have an overview of Startup class contained in Startup.cs in the root folder of the project.

ASP.NET Core application must include Startup class. It is like Global.asax in the traditional .NET application. As the name suggests, it is executed first when the application starts.

The startup class can be configured using UseStartup<T>() method at the time of configuring the host in the Main() method of Program class as shown below.

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

    public static IWebHost BuildWebHost(string[] args)
    {
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>()
            .Build();
    }
}

The name "Startup" is by ASP.NET Core convention. However, we can give any name to the Startup class, just specify it as the generic parameter in the UseStartup<T>() method. For example, to name the Startup class as MyStartup, specify it as .UseStartup<MyStartup>().

Open Startup class in Visual Studio by clicking on the Startup.cs in the solution explorer. The following is a default Startup class in ASP.NET Core 2.x.

As you can see, Startup class includes two public methods: ConfigureServices and Configure.

The Startup class must include a Configure method and can optionally include ConfigureService method.

ConfigureServices()

The Dependency Injection pattern is used heavely in ASP.NET Core architecture. It includes built-in IoC container to provide dependent objects using constructors.

The ConfigureServices method is a place where you can register your dependent classes with the built-in IoC container. After registering dependent class, it can be used anywhere in the application. You just need to include it in the parameter of the constructor of a class where you want to use it. The IoC container will inject it automatically.

ASP.NET Core refers dependent class as a Service. So, whenever you read "Service" then understand it as a class which is going to be used in some other class.

ConfigureServices method includes IServiceCollection parameter to register services to the IoC container. Learn more about it in the next chapter.

Configure()

The Configure method is a place where you can configure application request pipeline for your application using IApplicationBuilder instance that is provided by the built-in IoC container.

ASP.NET Core introduced the middleware components to define a request pipeline, which will be executed on every request. You include only those middleware components which are required by your application and thus increase the performance of your application.

The following is a default Configure method.

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    app.Run(async (context) =>
    {
        await context.Response.WriteAsync("Hello World!");
    });
}

As you can see, the Configure method includes three parameters IApplicationBuilder, IHostingEnvironment, and ILoggerFactory by default. These services are framework services injected by built-in IoC container.

At run time, the ConfigureServices method is called before the Configure method. This is so that you can register your custom service with the IoC container which you may use in the Configure method.

Learn more about the Configure method in the Middleware chapter.