Understanding The Role of Startup.cs File In ASP.NET Core Project

The DNX executes Main, and by convention calls ConfigureServices and Configure methods . The purpose of the ConfigureServices method is to configure dependency injection. Dependency injection in previous versions of .NET was optional. In ASP.NET 5, it is integrated. The dependency injection mechanism depends on an IoC (Inversion of Control) container. Typically when an application starts up, types, like a class, are registered in the container. In dependency injection terms, these types are called services. Once registered, other types can ask the container for an instance of that type. Also, during registration, you can determine what the lifetime should be of that instance.

The lifetime of an object is managed by the container. A transient lifetime means a new instance of the type is created every time it is asked for. A scoped lifetime means the instance will live until the web request is completely handled. And a singleton lifetime means once an instance is created, the same instance will be supplied every time until the app closes.

Although you can still easily plug in your own dependency injection framework like Ninject or Autofac or Unity, ASP.NET 5 comes with a default dependency injector out of the box. The container is invisibly created at application startup and filled with ASP.NET 5's services. At application startup, the hosting layer of ASP.NET gives you an opportunity to register your own types in the container by calling the ConfigureServices method in the Startup class. By supplying an IServiceCollection object as a parameter of ConfigureServices method, you get the API to do the registering.

Let's come to the next method in Startup class: Configure

You can see it accepts an interface object of another service called ApplicationBuilder. This object is injected by the dependency injector. The Configure method configures the HTTP request pipeline of ASP.NET. The pipeline specifies how the application should respond to HTTP requests. When your application receives a request from the browser, the request goes through the pipeline and back.

When there's nothing in the pipeline, nothing happens. So you need to plug in stuff you need in the pipeline. The individual parts that make up the pipeline are called middleware. For example, the MVC framework can be plugged in as middleware. But you might do some authentication first. In that case, configure a piece of middleware before MVC. Even the ability to serve static files containing JavaScript and CSS have to be configured as middleware. While travelling through the pipeline, data about the request travels through the pipeline and gets read and manipulated by the middleware and eventually the result is the response.

Using the pipeline you only plugin what you need. This is much more efficient than the old system, web approach, and it will surprise you how performant and resource friendly it is.

Thanks for reading. Hope this post helped you understand the role of Startup class in ASP.NET Core project.

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