What should I do if Win8 has 0x800F081F error when installing .NET 3.5?

Cause Analysis:
In Windows 8 and Windows Server 2012, .Net Framework 3.5 is an on-demand feature. Metadata based on functionality is included with Windows 8 and Windows Server 2012. However, binaries and other files associated with the feature are not included. When this feature is enabled, Windows will attempt to download the missing information to install the Windows update for that feature, and an error may occur.
Network configuration and computer installation of updates in this configuration environment can affect this process, so you may encounter errors when installing this feature for the first time.
Solution:
This error code may occur if you specify a different installation source and one of the following conditions is true:
· The location specified by the path does not contain the files needed to install the feature.
· The user who tries to install the feature does not have at least the location to read access, as well as the file.
· The installation fileset is corrupt, incomplete, or invalid for the version of Windows that you are running.
To resolve this issue, make sure the full path to the source is correct (x:sourcessxs) and that you have at least read access locations. To do this, try to access the source directly from the affected computer. Verify that the installation source contains a valid and complete set of files. If the problem persists, try using a different installation source.