Thank you for sharing this incredible small piece of software that made my day. I've found and tested a dozen other tools to change brightness with Windows 10 on a Lenovo X1 connected via Thunderbolt to a LG Ultrafine 5K and none of them worked. Your tool works absolutely fine and instantly on model number 27MD5KL-B.AEU.

I used the fact that I found a minor typo in the title (t missing form Ultra)

to say THANK YOU!!!!! <3

I installed the app, I try to use it but I see this:

My LG 27UL850 is connected via DisplayPort and I also have the USB-C cable connected to the Hackintosh.

What am I doing wrong?

I have two LG UltraFine 5K displays (27MD5KL-B) and the software only adjusts brightness for one of them.

This stopped working for me recently. Don't believe I did any recent updates but not sure. Here is the output

PS C:\Program Files> .\LG-UltraFine-Brightness-Control\LG-Ultrafine-Brightness.exe

Unhandled Exception: System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
   at hid_get_feature_report(hid_device_* , Byte* , UInt64 )
   at get_brightness(hid_device_* handle)
   at LGUltrafineBrightness.mainForm..ctor()
   at main(String[] args)
   at mainCRTStartupStrArray(String[] arguments)

Not sure what's the cause.