Why does order between UseStaticFiles and UseDefaultFiles matter?

Question

I understand that the order of registration for middleware may matter. However, it's not given that it's necessarily the case.

I noticed that UseDefaultFiles() needs to precede UseStaticFiles() (which can neatly be circumvented by UseFileServer()).

What I don't understand is why. How do they collide?!

I've googled the issue but got zero motivation on why the order is significant in this particular case. Only that it is of importance...

Answer 1

Taken from the documentation on Static files in ASP.NET Core (under Serve a default document, there's an important note).

UseDefaultFiles must be called before UseStaticFiles to serve the default file. UseDefaultFiles is a URL rewriter that doesn't actually serve the file. Enable Static File Middleware via UseStaticFiles to serve the file.

Based on this, it's important to first setup the URL rewriter (UseDefaultFiles) before serving the actual file (UseStaticFiles).
If you don't, the UseStaticFiles middleware will kick in first, but a request to the root of the application won't tell the middleware which 'file' to serve. When you make sure the rewrite is in place first, a request to the root of the application will have been rewritten to be a request for (one of the) default file(s).