The Program.cs file will most likely raise the curiosity of most seasoned ASP.NET programmers, as it's not something we usually see in a web application project. First introduced in ASP.NET Core 1.0, the Program.cs file's main purpose is to set up and build the IWebHost.

If we open the Program.cs file, we can easily see that the web host is built in an extremely easy way:

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 WebHost.CreateDefaultBuilder() method is one of the many improvements of ASP.NET Core 2.0 over its 1.x counterpart as it simplifies the amount of source code required to set up basic use cases, thus making it easier to get started with a new project.

To understand this better, let's take a look at the sample Program.cs equivalent, like it was in ASP.NET Core 1.x:

public class Program
{
    public static void Main(string[] args)
    {
        var host = new WebHostBuilder()
            .UseKestrel()
            .UseContentRoot(Directory.GetCurrentDirectory())
            .UseIISIntegration()
            .UseStartup<Startup>()
            .UseApplicationInsights()
            .Build();

            host.Run();
        }
    }

This used to perform the following steps:

• Setting up the Kestrel web server
• Setting the Content root folder, that is, where to look for the appsettings.json file and other configuration files
• Setting up the IIS Integration
• Defining the Startup class to use (usually defined in the Startup.cs file)
• Finally, Build and Run the now configured IWebHost

In .NET Core 1.x, all these steps must be called explicitly here and also manually configured within the Startup.cs file; in .NET Core 2.0, we can still do this, yet using the WebHost.CreateDefaultBuilder() method will generally be better as it will take care of most of the job, also letting us change the defaults whenever we want.