How interface method called in c#?

Question

I am working on .NET Core.

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

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            });
}

The CreateHostBuilder method returns IHostBuilder interface and is calling the Build method of that. So is it possible to implement code inside that interface or is it any design pattern?

Answer 1

CreateHostBuilder method return type which implement interface IHostBuilder and Build method on that returned type is getting called.

Answer 2

The class isn't sealed, but Build isn't virtual, so it is a bit of a challenge to extend it. But you can always wrap the object and implement the interface in the wrapper, passing through control or overriding or extending whatever behavior you want.

public class MyHostBuilder : IHostBuilder
{
    protected readonly IHostBuilder _inner;

    public MyHostBuilder(IHostBuilder inner)
    {
        _inner = inner;
    }

    public IHost Build()
    {
        DoSomethingCustom();
        return _inner.Build();          //Do something custom then pass through
    }

    public void ConfigureContainer<TContainerBuilder>(Action<HostBuilderContext,TContainerBuilder> action)
    {
        _inner.ConfigureContainer(action);  //Pass through without extending
    }
}