What is the Role of Program.cs and Startup.cs in ASP.NET Core?

🔧 Technical Explanation

`Program.cs` contains the `Main()` method and sets up the application’s host environment via `CreateHostBuilder()` or `WebApplication.CreateBuilder()` in .NET 6+.

`Startup.cs` contains two main methods: `ConfigureServices()` for registering services and dependencies, and `Configure()` for setting up the middleware pipeline.

In .NET 6 and later, both responsibilities can be combined into `Program.cs` using the minimal hosting model, but the traditional separation is still common.

💻 Real Code Example

Program.cs (.NET 5)

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>();
            });
}
            

Startup.cs

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers();
    }

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