I don't understand why every longhorn project is based on build 4074 or rarely on build 3718, why for example no use for base build 4042 ? Is quite stable and it has quite nice auro. WinFS in this build seems to be quite functional. It is hard to me to develop in Visual Studio 2005 Beta the longhorn apps and I have no idea how to fix bugs in longhorn. What are your opinions ?

you can NOT fix bugs in longhorn.. without the source code of the OS you cant do anything
all longhorn based projects FAILED
longhorn never made it to beta,all of the builds 3xxx-4xxx are alphas and HIGHLY UNSTABLE

You could fix Bugs by going deep into the system (files), you don't need the source code at all (but it would be very useful).
I think it's time to get over Longhorn. Yes it's great and a nice OS to test for some hours, but it will never be fixed.
Maybe in some time a source Code of LH gets leaked, and you could fix it but there are more then 50 Million lines of code to look at. So forget about it.

Non-Use Information: Vista 70 Million Lines of Code, XP 50 Million Lines of Code , 2000 30 Million Lines of Code, NT4 16 Million Lines of code

Not to mention we'd have to have the same tools and compilers they used which is very important considering the majority of pre-reset Longhorn UX (perhaps some other things too) code was written in .NET, which was still in development at that time.

Most notable custom projects fail... Why should LH be any different?

I just had the idea about this. There is a project like ReactOS (for those who do not know http://en.wikipedia.org/wiki/ReactOS), it is a free operating system written from scratch and binary compatible with windows. The project is developed, it has been operating for many applications. You can take as a basis for ReactOS and recreate the longhorn on it. That will have a free operating system compatible with windows, and contains the original idea longhorn.

4074 is used because it has the most stuff. 4042 generally isn't used because it leaked after most projects started for 4074. 3718 generally isn't used because there's not that much in it.

And this is why Longhorn projects have always been weak - the lack of skilled people able to donate a serious amount of time to it. Obviously, there are limitations, but we could go further than we have, but who's going to do the work? and frankly, so little of the community cares, it's simply not worth the effort. No-one appreciates genuinely interesting finds, it's all about the flashy stuff with the longhorn kids.

I really tried to develop in that VS 2005. But hard to me to get used to the .net 1.2 . I started programming in C# 2.0 and this is quite different. Also Avalon is quite tricky. Anyway, where can be found what every library in system do and it is responsible for ? I am trying to understand the schema of 4042's WinFS because it works better than in 4053.