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

Answer 3

IStartupFilter is usually used when you do not want any intervention from app’s author i.e. you don’t want that app.Usexxxx() to get called but a different component should configure that middleware. Now, consider a scenario where you want to execute certain code even if the request pipeline is not executed.
Lets take a URI which does not exist and returns 404 error, in that case request processing pipeline won’t be executed but startup filter will be.

Answer 4

A simple answer is no, it doesn't add any new functionality.
The point behind the IStartupFilter is to run some stuff in the pre-defined order independently on how yours Startup.Configure method changes. It simply creates a sequence of methods they do actions on IApplicationBuilder (WebHostBuilder) to build the middleware pipeline. You can define action that will perform before or after the Startup.Configure method is called.
Your IStartupFilter implementation will look similarly to this:
public class ExampleStartupFilter : IStartupFilter
{
public Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next)
{
return builder =>
{
// This will run before the Startup.Configure
builder.UseMiddleware<PreStartupConfigureRegisteredMiddleware>();
// This is Startup.Configure (or the next IStartupFilter.Configure in the sequence)
next(builder);
// This will run after Startup.Configure
builder.UseMiddleware<AfterStartupConfigureRegisteredMiddleware>();
};
}
}
Arguably, as you have to register your IStartupFilter in the Startup.ConfigureServices method and the order the filters are registered is the order they will be performed, there is not much difference compare to the order of direct registrations of UseMiddleware in Startup.Configure.
The only benefit I can see is that you can register something that has absolutely run the first or the last in the pipeline, no matter what other middlewares you use. Then you don't have to worry about changes in the Configure method. It should, however, be used very scarcely.
Very good blog post explaining how the IStartupFilter works has been written by Andrew Lock, check it out here:
https://andrewlock.net/exploring-istartupfilter-in-asp-net-core/
One thing Andrew points out is that you can register filter to run it prior to the AutoRequestServicesStartupFilter registered automatically by the WebHostBuilder. Not that I would be aware of an example where this would have a practical use.