The difference between IApplicationBuilder and IWebHostBuilder in ASP.NET Core lies in when and how they are used during the application lifecycle. They serve different roles in configuring and initializing an ASP.NET Core application.

IWebHostBuilder – Builds and configures the web host

When it's used:

• Before the application runs.

• It sets up the infrastructure required to run the application (Kestrel, IIS integration, etc.).

Purpose:

• Creates an instance of IWebHost which represents the web server and application host.

• Used to configure:

  • Server (e.g., Kestrel)

  • Logging

  • Dependency Injection (DI) container

  • Configuration (appsettings.json, environment variables, etc.)

  • Hosting environment

Common Methods:

• UseKestrel(): Use Kestrel web server.

• UseStartup<T>(): Specifies the Startup class.

• ConfigureAppConfiguration(): Add custom configuration sources.

• ConfigureServices(): Register services in DI container.

• Build(): Creates the IWebHost.

Think of it as:

Bootstrapping the application. It prepares everything needed before the request pipeline is constructed.

IApplicationBuilder – Builds the HTTP request pipeline

When it's used:

• During application startup.

• Passed into the Startup.Configure() method.

Purpose:

• Configures the HTTP request pipeline — middleware components that handle requests/responses.

• Does not handle the infrastructure setup.

Common Methods:

• UseRouting()

• UseAuthentication()

• UseAuthorization()

• UseEndpoints()

• UseStaticFiles()

• Run() / Map()

Think of it as:

Laying down the roads for HTTP requests. It defines how the application should respond to HTTP traffic.

Summary Table:

Typical Flow in ASP.NET Core:

1. Program.cs:

   csharp

   public static void Main(string[] args) {
CreateHostBuilder(args).Build().Run(); }
public static IHostBuilder CreateHostBuilder(string[] args) => Host.CreateDefaultBuilder(args).ConfigureWebHostDefaults(webBuilder => { webBuilder.UseStartup<Startup>();
// IWebHostBuilder in action });

2. Startup.cs:

   csharp

   public class Startup {
public void Configure(IApplicationBuilder app, IWebHostEnvironment env) {
app.UseRouting();
// IApplicationBuilder in action
app.UseEndpoints(endpoints => { endpoints.MapControllers(); });
}
}