In Windows 8, is .Net 4.5 an optionnal component (in othher words, can we uninstall it)?
My first question is closely connected to the second one: what is ".Net Framework 4.5 Advanced Services" that we can find in "Windows features"?
Well, here's what it is on my copy of Windows 8 Enterprise
Also, .NET 4.5 comes built-in to Windows 8. .NET 3.5 comes built-in to Windows 7. By built-in I mean I don't have to take an extra step to install them after installing the OS.
Notice that there's no ".NET Framework 4.5" here to uncheck.
There's no supported method to remove it, and because the 4.5 framework shares common components with the WinRT API used for the Metro interface a brute force attempt to remove it (ex by manually deleting files) would likely break WinRT. Since not all windows components have classic versions in Win8; even if you use desktop applications exclusively you would probably run into broken components.
Under Net Framework 4.5 Advanced Services are two items. "ASP.NET 4.5" and "WCF Services". The former is used to create and run web applications using .net for server side logic. WCF is an API for sharing data in client-server applications. Based on it's location under advanced services, that it can be installed separately from the main framework, and several blog posts discussing it I believe that item consists of the server side portion of WCF, and not what would be used in end user applications.
As for why these components are separated from the rest of the .net framework I think there are two related major reasons; the commonality between the two is that very few users will need them installed; mostly software developers writing those sorts of applications. Splitting them off reduces the attack surface for anyone trying to write malware targeting Windows client machines. Also by splitting them off they reduce the disk size footprint which is important on computers with limited storage capability like current Windows RT tablets. For people who want to install .net 4.5 on Vista/Win7 machines this also reduces the download size.