ASP.NET Core–Reintroduce the startup.cs file

With the release of .NET 6, when you create a new ASP.NET Core application, you no longer have a Startup.cs file. Instead everything should be written inside the Program.cs file:

	var builder = WebApplication.CreateBuilder(args);
	// Add services to the container.
	builder.Services.AddRazorPages();
	var app = builder.Build();
	// Configure the HTTP request pipeline.
	if (!app.Environment.IsDevelopment())
	{
	app.UseExceptionHandler("/Error");
	// The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
	app.UseHsts();
	}
	app.UseHttpsRedirection();
	app.UseStaticFiles();
	app.UseRouting();
	app.UseAuthorization();
	app.MapRazorPages();
	app.Run();

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If you don’t like this and you still want to use a Startup.cs file, this is still possible.

First let’s recreate our good old Startup.cs file:

	public class Startup
	{
	public Startup(IConfiguration configuration)
	{
	Configuration = configuration;
	}
	public IConfiguration Configuration { get; }
	public void ConfigureServices(IServiceCollection services)
	{
	services.AddRazorPages();
	}
	public void Configure(WebApplication app, IWebHostEnvironment env)
	{
	if (!app.Environment.IsDevelopment())
	{
	app.UseExceptionHandler("/Error");
	// The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
	app.UseHsts();
	}
	app.UseHttpsRedirection();
	app.UseStaticFiles();
	app.UseRouting();
	app.UseAuthorization();
	app.MapRazorPages();
	app.Run();
	}
	}

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In a first attempt, I tried the following:

	var builder = WebApplication.CreateBuilder(args);
	// This doesn't work.
	builder.WebHost.UseStartup<Startup>();
	var app = builder.Build();
	app.Run();

view raw DontTryThis.cs hosted with ❤ by GitHub

Unfortunately, this didn’t work and resulted in the following error message:

Exception thrown: 'System.NotSupportedException' in Microsoft.AspNetCore.dll
An unhandled exception of type 'System.NotSupportedException' occurred in Microsoft.AspNetCore.dll
UseStartup() is not supported by WebApplicationBuilder.WebHost. Use the WebApplication returned by WebApplicationBuilder.Build() instead.

I switched to a more manual approach where I create the Startup class myself and invoke the methods directly:

	var builder = WebApplication.CreateBuilder(args);
	var startup = new Startup(builder.Configuration);
	startup.ConfigureServices(builder.Services);
	var app = builder.Build();
	startup.Configure(app, builder.Environment);

view raw Program.cs hosted with ❤ by GitHub

Of course now it doesn’t matter anymore how the Startup.cs file is structured or how you should name the different methods.