What is the point of IStartupFilter in asp.net core?

Question

IStartupFilter is the basis of a mechanism for libraries to add middleware to the app. According to the Docs "IStartupFilter is useful to ensure that a middleware runs before or after middleware added by libraries at the start or end of the app's request processing pipeline".

Does the mechanism allow you to manipulate the pipeline in any way that can't be done from Startup.Configure()?

If the point is modularity then you just seem to be trading coupling through Startup.Configure() for coupling via the IServicesCollection (a call to DI is required). In the simple case (as per the example) a call to services.AddTransient<IStartupFilter, ...>() can be removed from ConfigureServices() and app.AddMiddleware<MyMiddleware>() can be added to achieve the same functionality with less complexity and magic.

Is the main point of the mechanism to allow the library to apply conditions as to what middleware should be included? If so, it seems to lack asp.net core's customary economy and clarity of design.

Answer 1

In the simple case (as per the example) a call to services.AddTransient<IStartupFilter, ...>() can be removed from ConfigureServices() and app.AddMiddleware() can be added to achieve the same functionality with less complexity and magic.

That's not true.

There is a big difference between using IStartupFilter and using a middleware. A middleware is part of the request pipeline, which means it gets executed on every request. On the other hand, IStartupFilter gets executed just once when the application starts (not on every request).

Answer 2

To answer my own question, I think that the main use case is to enable the framework to encorporate assemblies that were not known at build time. The docs cover this in the aspnetcore/Fundamentals/Host nhance an app from an external assembly in ASP.NET Core with IHostingStartup. (Although the documentation makes no mention of IStartupFilter the associated StartupDiagnostics sample project does.