In the interest of saving time and bandwidth, i've downloaded an "ESD" version of Windows 10 Pro Build 10240 (x64).
While tried to Enabled .Net Framework 3.5 Features used DiSM Code:

Code:

Dism /online /enable-feature /featurename:NetFX3 /All /Source:X:\sources\sxs /LimitAccess >NUL

and get Error: 0x800f081f.
The source files could not be found. Use the "Source" option to specify 
the location of the files that are required to restore the feature. 
For more information on specifying a source location, 
see http://go.microsoft.com/fwlink/?LinkId=243077. 
The DiSM log file can be found at C:\Windows\Logs\DISM\dism.log.

So used ".NET Framework 3.5 Standalone Installer for Windows 10" and integrated manually before Slimmingdown with WinReducer EX-100 as a follow:
- Extract dotNetFx35_W10_x86_x64.exe to Local HDD "eg. NetFx35DiR".
- Copy paste CMD CODE below:

Code:

REM Mount and Modify Windows 10 'install.wim' image...
SET WiMFiLE=%WiN10DiR%\sources\install.wim
SET MOUNT=%TEMP%\MOUNTDiR
MD "%MOUNT%"
%DiSMW10%" /Mount-Image /ImageFile:"%WiMFiLE%" /Index:1 /MountDir:"%MOUNT%"

REM Enabled .Net Framework 3.5 Features Windows 10...
SET NetFx35DiR=%TEMP%\dotNetFx35_W10_x86_x64\x64
SET SCRATCH=%TEMP%\SCRATCHDiR
MD "%SCRATCH%"
"%DiSM10%" /Image:"%MOUNT%" /NoRestart /Add-Package /PackagePath:"%NetFx35DiR%\update.mum" /ScratchDir:"%SCRATCH%"

REM Saving Modified 'install.wim' image...
"%DiSM10%" /Commit-Image /MountDir:"%MOUNT%" /CheckIntegrity

REM Unmounts Modified 'install.wim' image...
"%DiSM10%" /Unmount-Image /MountDir:"%MOUNT%" /Discard

REM Cleaning Temporary all Mounted WiM image, folder and files...
"%DiSM10%" /Cleanup-Mountpoints >NUL
RD /S /Q "%MOUNT%" >NUL
RD /S /Q "%SCRATCH%" >NUL
RD /S /Q "%NetFx35DiR%" >NUL

* iMPORTANT
- It's applied once times only, otherwise will get "Error: 14081".
- If used to integrate with WinReducer EX-100 will get message "This folder is incorrect !".

For more detail (full) info: "http://forums.mydigitallife.info/threads/63835-NET-Framework-3-5-Standalone-Installer-for-Windows-10"

Working PC Description: :


 Hope that help Some !!! Smile

 _________________
 home7ech
 pS : Creative Learning by Doing, Slowly but Do iT Cool's