Can't install .net 3.5 on Windows 10

Please open a command prompt (cmd.exe) as admin and run the following command to install .Net 3.5:

DISM /Online /Enable-Feature /FeatureName:NetFx3 /Source:d:\sources\sxs /All /LimitAccess 

(replace D: with your mounted Win10 ISO)

In some environment, DISM fails to fetch the file (Proxy, network policies). The DISM command takes the files from the DVD and you're fine.

If you are MSDN subscriber you can also download the Windows 10 Features on Demand ISO, mount the ISO, copy the Microsoft-Windows-NetFx3-OnDemand-Package.cab to C:\Features (or a different folder) and those commands:

DISM.EXE /Online /Add-Capability /CapabilityName:NetFx3~~~~ /Source:C:\Features

If this fails, run this:

DISM.EXE /Online /Add-Package /PackagePath:C:\Features\Microsoft-Windows-NetFx3-OnDemand-Package.cab

Windows Side by Side (WinSxS) location is probably empty in the registry.

From the GUI called from APPWIZ.CPL this results in error code: 0x800F081F

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Servicing]
"LocalSourcePath"="d:\\sources\\sxs"

Check/add the subkey 'Servicing', and the value of 'LocalSourcePath'. It should contain a correct string (e.g. 'd:\sources\sxs'), data type (REG_SZ). Where "d:" represents your Windows media drive letter.