I'm trying to understand the #line directive. I read in this documentation and I was able to understand the applicability of #line hidden but not #line [any_numer].
Taking as an example a App.g.i.cs file that is doing the following code snippet in InitializeComponent() method:
#line 5 "..\..\App.xaml"
this.Startup += new System.Windows.StartupEventHandler(this.Application_Start);
#line default
#line hidden
You're saying that the this.Startup += new System.Windows.StartupEventHandler(this.Application_Start); code will appear to be on line 5 in the ..\..\App.xaml file, no matter what line actually is in the file, or what file. Then it returns to normal numbering.
Any error or warning that the compiler issues will have this information. At the time of debugging the code will show this too.
Its use is very rare and almost always linked to generators. This is usually necessary when the code is produced by some generator and not written by the programmer. It may contain some things that need to be shown to the programmer differently than if presented really, even hidden. Without this some information would be strange or misleading.
It is also used in meta-programming, something that is little encouraged, even in C # (tends to be more used, is the future of programming for some things). But it's a much more advanced topic. The reason for not being mainstream is that few programmers can do it right. Since this paradigm generates a lot of code, it is a means of informing a line number that makes sense to the programmer who only sees the written code rather than the generated one, which obviously changes the numbering of the lines.
Almost nobody needs to know this. But it is a useful resource for the cases cited.