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Home Interviews What is the use of the Program.cs and Startup.cs files in ASP.NET Core? – MindStick
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1 Answers

Anubhav Sharma
Anubhav Sharma 19 May 2025
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In ASP.NET Core, the Program.cs and Startup.cs files play a crucial role in configuring and bootstrapping your application. Here's a clear explanation of their purposes:

Program.cs – Application Entry Point

Purpose:

  1. Defines the main entry point of the application.
  2. Responsible for creating and running the web host (the environment in which the app runs).

Responsibilities:

  1. Configures the host (web server, logging, etc.)
  2. Calls the Startup class to set up services and middleware.

Example (.NET Core 3.1 style):

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 – App Configuration

Purpose:

  1. Contains methods to configure services and the HTTP request pipeline.

Key Methods:

  1. ConfigureServices(IServiceCollection services) Register app services, e.g., MVC, EF Core, authentication, etc.
  2. Configure(IApplicationBuilder app, IWebHostEnvironment env) Set up the middleware pipeline (e.g., routing, error handling, static files).

Example:

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

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

        app.UseStaticFiles();

        app.UseRouting();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapDefaultControllerRoute();
        });
    }
}

In .NET 6/7+, Program.cs Merges Everything

In .NET 6 and newer, Startup.cs is optional. The Program.cs file uses a minimal hosting model, combining everything in one place:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllersWithViews();

var app = builder.Build();

app.UseStaticFiles();
app.UseRouting();
app.MapDefaultControllerRoute();

app.Run();

Summary

File Purpose Used In
Program.cs Entry point, host setup All ASP.NET Core apps
Startup.cs Configure services and middleware .NET Core 2.x – 5.x (optional in .NET 6+)
Program.cs (Minimal API style) Combines both responsibilities .NET 6 and newer
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