Install dot net 3.5 on Windows Server 2012/Windows 8 - 0x800F081F error

When installing dot net 3.5 from the command prompt/powershell in Windows Server 2012 (R2) or Windows 8(.1), you need to have access to a folder (specifically the SXS folder) from the original install media. Whether you extract the contents of an ISO and copy the SXS folder locally or simply mount the image, it doesn't make much difference when you point dism at the folder and then encounter this:

This is a fairly common error and one for which there seems to be a whole range of random ways to fix, depending on your situation. Personally speaking, it was the uninstall of the Updates which solved it but anecdotally speaking, a lot of these have helped various folk:

1] If present, uninstall updates KB2966826, KB2966827 and KB2966828

2] Check the syntax is correct:

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

In case anyone needs it, here's what the commands actually mean:

• /Online targets the operating system you're running (instead of an offline Windows image).
• /Enable-Feature /FeatureName:NetFx3 specifies that you want to enable the .NET Framework 3.5.
• /All enables all parent features of the .NET Framework 3.5.
• /LimitAccess prevents DISM from contacting Windows Update.
• /Source specifies the location of the files needed to restore the feature (in this example, the D:\sources\sxs directory).

3] Check the extracted/mounted ISO isn't corrupt - i.e. download another copy if possible

4] If the server has internet access, drop the /LimitAccess from the syntax