So I've been struggling with this myself and no solution I've found on the internet helped. Turn Windows Features On or Off, install through CMD, powershell, in safe mode, removing certain windows updates; all those didn't work. I managed to figure out a way to install it myself so I wanted to let others know how they might be able to fix it.
1. Download .NET Framework 3.5 installer
https://www.microsoft.com/en-us/download/details.aspx?id=21
Also, download the netfx_setupverifier from here:
http://blogs.msdn.com/b/astebner/archive/2008/10/13/8999004.aspx
Under the Download Location, download from the second link
2. Once downloaded, drag the netfx_setupverifier application to your desktop. Press "windows key+x" and open Command Prompt (Admin). Type this code in:
Start C:\Users\yourusername\Desktop\netfx_setupverifier.exe /q:a /c:"setupverifier2.exe /a"
Type in your username where it says "yourusername". At the end where you see the two quotation marks, leave those. Launching the application this way through CMD allows you the option to verify all existing versions of the framework, even if they are not installed on your system.
3. Select .NET Framework 3.5 to verify. Click Verify Now and wait. Once it's finished verifying, select View Log. A log should pop up showing you what files are installed and what files are not. Keep this log open as you will need it for the next steps.
4. Assuming that you do not have the framework installed at all, the whole log should show only warnings, with file paths saying that certain files are not installed. Start at the top of the list and read the file path. Open that in the File Explorer and you should indeed see that the file it says that should be installed, is not in that folder.
Now launch the .NET Framework 3.5 setup. Begin installation. What should happen, as I've noticed when doing this, is it will say "Installing" for awhile before giving you the error that the framework could not be installed. However, what I've noticed, is that at some point during the installation, all the files for the framework in fact do install and they appear in their folders. But as quickly as they came, when the installer gives you the error, all the files vanish. So to start, have open any one of the folders listed in the log that say a file is missing from there, and keep it open while the framework is installing. At some point into the installation process, the file that was supposedly missing appears. Seeing that one file has appeared, that means that every other file for the framework has now been installed. Knowing this, you need to go back a few folders, and copy the folder which contains framework files, and then paste it to your desktop. This way, when the installation gives you the error and the files disappear, you will have a copy of them on your desktop to replace.
For example, many files of the framework are found in C:\Windows\Assembly\xxx. So what I would do, is just copy the Assembly folder so that everything within is copied and you can just replace it afterwards. Now obviously, just about all the framework files are located within the Windows folder, but because there's too much stuff in that folder, copying it would take far too long and you would get the installation error before it can finish copying. So take it one step at a time. And when you copy and paste the folders to your desktop, don't delete them.
5. When the installation finishes and gives you the error, copy (don't drag) the folders from your desktop and paste them back to where they belong. Now do another verification of .NET Framework 3.5 and view that log. Now you should see that the files in the folders you copied and pasted are now installed, but you still have other files missing.
Now basically, it's just rinse and repeat. Launch the installer again, wait until you see the files appear and copy more folders and paste to the desktop only to paste back once the installation gives the error. But make sure you don't delete them once they're on your desktop, because after the installation gives you the error, it will still remove the files from the folders you already copied and pasted. So you basically have to keep replacing the folders until you've replaced all of them.
Hope this works!
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