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Separate products, yes.
Common dependencies, also true.
Plenty of common settings in the registry? Take a guess.
Do you think there exists any third party dependencies that does not work with both versions at the same time?
So while you really can have several versions on your computer, the older ones tend to not work as advertised anymore IMHO. The new ones neither now that I think of it.
My recommendation would always be a reinstall of the computer if you want to have a newer version of VS.
They are separate products, you may not have value in the old version after an upgrade but many other people do.
In C++, each version of VS comes with a huge set of libraries that are vastly different from each other. The libraries are part of the VS installation, not some shared common library. This is good for backwards compatibility etc.
Such as how VS2012 comes with an early implementation of C++11, while VS2015 comes with a C++11/14 almost complete.
Also the compilers are different in each version, and having separate versions on your machines allow you to compile for each one.
In Web development or something a lot of people may not care about previous versions, but VS is an IDE that caters to "most" developers, and upgrading over a previous install isn't always appropriate.
And finally VS2015 fixes most issues mentioned in this post, on installation almost every major feature is now option to set to install. Installing C++ support can be disabled in VS2015 so the install size is gigabytes smaller than before.
I've installed VS 2015 community a couple of months ago for trial purposes, with VS 2010 installed on the system. I had no problems, and could use either.
Now that we've decided to purchase VS 2015 Pro, I've uninstalled VS 2015. It appeared to work, although, as you noted, the uninstallation rocess took quite some time. (~10-15 minutes - I didn't really check)
Then I started VS 2010 again and loaded a solution: unfortunately, the solution wouldn't load: it claimed that a dependency, .NET 4.0 was missing! I checked program installations, but there still was a dotnet 4.0.1 installation that I could see! Even after restarting, dotnet 4.0 was still being shown. Yet VS 2010 kept complaining that it was missing: clearly, the VS 2015 uninstallation must have broken something, although it wasn't clear what.
To cut the long story short: it wasn't .NET 4.0 that was missing, it was the -NET Multi-targeting Pack - whatever that is supposed to be! I got it back by 'repairing' the VS 2010 installation.
tldr; You can install VS 2015 side by side with an earlier version, but if you ever uninstall it (or the previous version), you may need to repair the remaining installation(s).
On a sidenote: the solution in question is 100% C/C++ unmanaged code. no idea why there is any link to .Net at all...
GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)
Does anyone else go through this... my mother never keeps her cell phone on. She turns it off like it's a regular phone and when I want to call her I have to call the old land line first and tell her to turn her cell phone on. Then I can call that.
I know that. My solution is to send her an SMS to "land line" first to tell her to Switch the cell on. I don't know whether it is the same in other countries, but in .ch a SMS send to land line will be delivered... in the way a synthesized voice does read it