More Info Here: https://stackoverflow.com/questions/21098333/visual-studio-compiles-fine-but-it-still-shows-red-lines Visual Studio 2019 is showing red underline all the time, even when it compiles fine!

Closing Visual Studio and removing the .vs folder located in the solution directory worked for me.

This folder has a hidden attribute. You may need to change settings in folder options to show hidden files.