tl:dnr;
using your source (I chose to use a Windows 10 .iso file I had recently created using the Windows Media Creator Tool for another project…) and using an elevated command prompt, point dism at the “online” image and feed in the source (in my case, D drive housed my mounted .iso, so D:\Sources\SXS), thusly: dism /online /enable-feature /featurename:netfx3 /source:D:\Sources\SXS /limitaccess
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The Longer Explanation
Many reasons, actually, might make a person wish to install/enable .NET Framework 3.5; so much so, this is now a “feature enable” option on Windows 10. However, nearly every “how to” do this demonstrates downloading the full (aka “offline”) installer from Microsoft’s download page and then just run the installation. Problem with this is that this kicks off the feature enable, and tries to download a package from Windows Updates to enable the feature. Which means you wasted your time downloading the offline, full installer anyway, but if that works, YAY you! If not… 😦
Usually we get some error, like “Error code 0x800F0954” which never seems to exist when researched but largely boils down to “unable to download/unzip/whatever the package”. In my case, with that error, it was traceable back to my WSUS server not caching the confounded package. No idea why it’s trying to do this, when I’ve downloaded the (full, offline) installer, but whatevs, yo! GRR!
So what to do about it?
As noted at the beginning, we can simply use a cached .iso (or other source) for our installation set and dism. We just need to assemble the correct series of options/parameters, and wham-oh! In like Flynn.
Thank you to Matt Brown for posting this how-to on Spiceworks!
sypak 