The following is simply a copy of the README.md from my latest project found here, I hope you find it useful.
It's a babysitter for your WiFi NIC.
This is born out of the need to have the wifi adapter disabled when Ethernet is active, seems like this should be a solved problem but there are no good free tools for this. Some PC vendors produce their own which provide this behavior but they also try to be the one stop shop for all your WiFi management needs. Windows does a fine job managing which hotspots you're connected to, this just fills in one gap.
While running the application watches for IP changed and availability changed events and enables/disables WiFi adapters accordingly. When Ethernet is active, WiFi adapters are disabled if they've made a connection, if WiFi is active but not connected it is left alone, when ethernet gets unplugged or the network availability goes away all WiFi adapters are enabled. That is all it does, there is no configuration, it ignores Bluetooth adapters and MS WiFi Direct Virtual adapters. It compiles to a single .Net 4.0 executable and only makes use of .Net and a few commands present in all standard Windows installs (not tested on Windows embedded).
Roadmap:
Wifi-Sitter can be installed from the command line. First place the exe where you'd like it to stay (if you move it, the service will break), then run like so from an admin shell:
WifiSitter.exe /install
The service is configured to start automatically but will not be started after if you're automating a deployment, you'll need to run net start wifisitter or use your service starting command of choice.
Similarly, uninstall like so:
WifiSitter.exe /uninstall
It can be run as a console application for debugging purposes by running:
WifiSitter.exe /console
There isn't much to configure in WifiSitter but there is one tunable to configure, there may be some network adapters you want ignored complete, Microsoft WiFi Direct for example. Network adapters are named "Ethernet" or "WiFi", names are too generic so the whitelist is made up of the network adapter descriptions. They are string values located at: HKLM\SYSTEM\CurrentControlSet\services\WifiSitter\NicWhiteList
Keys are ignored entirely, they are only used to reference the values and can be anything, incrementing numbers are used by default. Regular expressions were overkill for my needs so matching is done by a case-insensitive .StartsWith(). Note, these values are removed when uninstalling.
Many thanks to Matt Davis for this answer and Samuael Neff for this. They helped a lot with converting to a service.
This repository is also mirrored on my site, here.