(edited to add this note)
NOTE: I already tried DISM, sfc, the .NET Framework repair tool, turning the .NET Framework features off and on, restoring from a restore point, and resetting the system.
(end edit)
In my home we have Windows 10 Home on two identical HP Envy m6 Notebooks, and by comparing their Windows\Microsoft.NET folders I see that dozens of files are missing from mine.
Five days ago, this message came up when I tried to start Duplicate Cleaner 5:
Duplicate Cleaner Pro 5.19.0
Error starting up Duplicate Cleaner: Could not load file or assembly ‘System.Data, Version =4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089’ or one of its dependencies. The system cannot find the file specified. DataPath=C:\Users\geotalk\AppData\Roaming\DigitalVolcano\DuplicateCleaner
After a lot of searching, wading through a lot of developer talk that's way over my head, and some discussion with the developer, I discovered that these two folders on my computer were empty:
C:\Windows\Microsoft.NET\assembly\GAC_32\System.Data\v4.0_4.0.0.0__b77a5c561934e089 C:\Windows\Microsoft.NET\assembly\GAC_64\System.Data\v4.0_4.0.0.0__b77a5c561934e089
I looked in the same folders on the other computer and found a System.Data dll in both of them. Before I copied them to mine, I tried to use PowerShell, and the same "Could not load" message came up for the same dll, System.Data. After I copied those dll files to my Windows\Microsoft.NET folder, I was able to use PowerShell and Duplicate Cleaner. Since then I haven't had any other problems with Windows or any apps, but I'm concerned about all those files missing from the Windows\Microsoft.NET folder. Before I discovered those two empty folders, I tried DISM, sfc, the .NET Framework repair tool, and turning the .NET Framework features off and on, and none of that restored the missing files. I also tried restoring from a restore point, and resetting the system, but both of those failed.
My problem with PowerShell and Duplicate Cleaner was solved by copying the System.Data dll from the other computer, but I'm concerned about all those other missing files. I don't want to copy them all from the other computer, because I'm afraid it might break something, even it is the same Windows system on two identical computers. I also don't want to do a clean install of Windows if there's any way I can avoid that. It isn't worth all the time and trouble as long as I'm not having any other problems.
Does anyone have any other ideas about how I can restore files that are missing from the Windows\Microsoft.NET folder?