Why is ASP.NET Core's Startup class not an interface or abstract class?

There are several reasons why its done the way its done. One of the more obvious reasons is, because you can inject services into Configure method, such as

public void Configure(IAppBuilder app, IMyService myService)
{
    myService.DoSomething();
}

Obviously, you can't do that with interfaces, abstract classes or inheritence.

The second reason why its done by convention method is, that there is not only Configure/ConfigureServices method, there is an infinite number of environment-dependent configure methods.

public void Configure(IAppBuilder app) { }
public void ConfigureDevelopment(IAppBuilder app) { }
public void ConfigureProduction(IAppBuilder app) { }
public void ConfigureStaging(IAppBuilder app) { }
public void ConfigureSomethingElse(IAppBuilder app) { }

and depending on your environment variable for ASPNET_ENVIRONMENT a different method will be chosen and executed (or the default Configure/ConfigureServices if no matching environment specific method was found).

None of this is possible with traditional OOP (inheritance/interfaces/abstract classes).

The same applies to other parts of ASP.NET Core, like Middlewares and the Invoke Method. The Invoke method can also have dependencies injected into it, but in order to call the next middleware you simply do

await next?.Invoke();

and do not have to worry which dependencies the next middleware requires or may take.

And to be complete, one can also have multiple Startup classes with the default method names (Configure/ConfigureServices) named StartupDevelopment, StartupProduction, Startup (as fallback) and ASP.NET Core will pick up the correct one based on Environment variable set.

Startup class can be inherited from IStartup interface.

// \packages\microsoft.aspnetcore.hosting.abstractions\2.2.0\lib\netstandard2.0\Microsoft.AspNetCore.Hosting.Abstractions.dll
namespace Microsoft.AspNetCore.Hosting
{
 public interface IStartup
  {
   IServiceProvider ConfigureServices(IServiceCollection services);
   void Configure(IApplicationBuilder app);
  }
}

By default wizard doesn't create template file with implementation from IStartup. Why not - probably mistake or influence of non-typed languages..