Visual Studio Editor does not underline errors anymore

Question

Have you checked Tools→Options...→Text Editor→C#→Advanced→Underline errors in the editor?

I usually like to reset my settings after messing around with plugins, as they tend to mess with settings: Tools→Import and Export Settings...→Reset all settings.

About possible causes.

For VS 2012 and 2013 if you have more than one instance of Visual Studio on different machines binded to one "live" account and have installed ReSharper on one of them, it disables the native IntelliSense and error underlines (to replace by it's own rules) that will be synchronised through your account to another machine without ReSharper.

Found it in Visual Studio 2019 as: Tools > Options > Text Editor > General > Show error squiggles

This is generally called Disable Squiggly or Wavy lines in Visual Studio.

How you will do in Visual studio 2013?

TOOLS -> Options... -> Text Editor -> C/C++ -> Advanced -> Disable Squiggles: True/False (Under IntelliSense) -> Press OK

I know its an old question, and with various solutions, but I have fixed it in different way. I'm working with Unity3D on my C# code using VS2017, when suddenly VS decides to stop underlining error while im typing. However, if I close the file tab and reopen, it suddenly undelines the error.

For example:

class A { 
public int x; 
s;
}

should obvsiouly give an error for that lonely 's' symbol. But, VS doesn't underline it until I close and reopen this file tab.

Solution: Copied the entire Unity Project folder (which is like a regular VS Solution folder basically) and worked with the new folder, which issue was gone there.

Unloading and loading same project again from the solution does the trick. Just right click on the project and click "Unload Project". Once unloaded, again right click the same project and click "Reload Project". Error highlighting will return.

I had the same issue with 2017. There was a 'disable intelisense' option, make sure that is set to false.