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Main method in asp.net core

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In this video we will discuss
  • The significance of Main() method in an ASP.NET Core application
  • What happens behind the scenes when a .NET core application is executed

In an ASP.NET Core project we have a file with name Program.cs. In this file we have a public static void Main() method


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

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>();
}

If you have any experience with previous versions of .NET, a console application has a Main() method and it is the entry point for that console application.

But here, we are creating an asp.net core web application and not a console application. So the obvious question that comes to our mind is why do we have a Main() method.

Well, the important point to keep in mind is that, an asp.net core application initially starts as a console application and the Main() method in Program.cs file is the entry point. 

So, when the runtime executes our application it looks for this Main() method and this where the execution starts.

This Main() method configures asp.net core and starts it and at that point it becomes an asp.net core web application.

So, if you take a look at the Main() method, it calls CreateWebHostBuilder() method passing it the command line arguments.

As you can see, CreateWebHostBuilder() method returns an object that implements IWebHostBuilder.

On this object, Build() method is called which builds a web host that hosts our asp.net core web application.

On the web host Run() method is called, which runs the web application and it begins listening for incoming HTTP requests.

CreateWebHostBuilder() method calls CreateDefaultBuilder() static method of the WebHost class.

CreateDefaultBuilder() method creates a web host with pre-configured defaults. CreateDefaultBuilder() method does several things to create a web host. We will discuss all that the CreateDefaultBuilder() method does in detail in our next video. For now, just understand that the CreateDefaultBuilder() method sets up a web host with certain defaults.

As part of setting up a web host, Startup class is also configured using the UseStartup() extension method of IWebHostBuilder class. If you are new to the concept of extension methods, please check out the following video.

Extension Methods in C#

By convention, the startup class in ASP.NET Core is named Startup. This class has 2 methods.

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    { }

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

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

Startup class does the following 2 very important things
  • ConfigureServices() method configures services required by the application
  • Configure() method sets up the application's request processing pipeline
It is very important we understand what these 2 methods does. We will be revisiting these 2 methods several times as we progress through this course and discuss them in detail.

asp.net core tutorial for beginners

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