Unity Error without specific reason - it works but has red lines on visual studio files

Unknown red lines

As you can see from the picture above, sometimes it happens like that.

Usually,  it happened after unzipping the zipped file and reopening the project.

It's so simple to fix this problem.

Turn off the project you have a problem with.

The file location of your project

Get into the project location and delete "TestDelegate.sln" file.

In Unity

Then, re-open the project from unity and double click on "delegateTest" file.

And it fixes the problem!

Done